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The use of SiO2/4H–SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) can be problematic due to high interface state density (Dit) and low field-effect mobility (μfe). Here, we present a tetra-ethyl-ortho-silicate (TEOS)-based low-pressure chemical vapor deposition (LPCVD) method for fabricating the gate oxide of 4H–SiC MOSFETs using nitric oxide post-deposition annealing. SiO2/4H–SiC MOS capacitors and MOSFETs were fabricated using conventional wet and TEOS oxides. The measured effective oxide charge density (Qeff) and Dit of the TEOS-based LPCVD SiO2/4H–SiC MOS capacitor with nitridation were 4.27 × 1011 cm−2 and 2.99 × 1011 cm−2eV−1, respectively. We propose that the oxide breakdown field and barrier height were dependent on the effective Qeff. The measured μfe values of the SiO2/4H–SiC MOSFETs with wet and TEOS oxides after nitridation were, respectively, 11.0 and 17.8 cm2/V due to the stable nitrided interface between SiO2 and 4H–SiC. The proposed gate stack is suitable for 4H–SiC power MOSFETs.

In this study, degradation phenomena due to gate bias stress at elevated temperature for the reliability test of punch-through IGBTs, which are widely used in DC circuit systems for automotive applications, were investigated. For the reliability tests of various temperatures, 4000 times temperature cycle stress was applied in IGBTs between −40°C and 200°C to accelerate the degradation process using the wide range temperature. And the delamination of the passivation layer and the metal layer in IGBTs was observed using the scan acoustic microscopy. In addition, the positive bias stress with 48 V at elevated temperature of 150°C for 60 min was applied to the gate, the threshold voltage increased up to 6.07 V due to charge trapping at gate oxide. We also compared the recovery of the devices, but the IGBTs that stressed with high DC field and elevated temperature slowed down the recovery process after 24 hours without reverse voltage. This work was focused to find the device recovery considering real conditions. Therefore, the various temperature stress and high electric field stress are important in the reliability test for IGBTs, it was reported the IGBT degradation process and failure symptoms using the accelerated and various reliability tests.

In this study, high electron mobility transistor (HEMT) device was used as an immuno biosensor to measure concentration of a stress hormone, cortisol, by using selective binding on cortisol monoclonal antibody (c-Mab). Also, the HEMT sensor was enhanced in its sensitivity through light illumination to generate photocurrent. The optical pumping could assist the biosensor to discriminate more detailed change, which could result in an increment of limit of detection (LOD) to 1.0 pM cortisol level. It was the lowest level of detection with semiconductor device-based cortisol biosensors and the enhancement of surface potential sensitivity was induced by laser light (532 nm). Output current amplificated by photocurrent was higher than dark original current at about 3.39% when gate voltage is applied with −3 V. Since the device could be applied to not only standard cortisol solution but also real human salivary sample, it is expected to apply for in vitro direct diagnosis of point-of-care test (POCT).

High-power switching applications, such as thyristor valves in a high-voltage direct current converter, can use 4H-SiC. The numerical simulation of the 4H-SiC devices requires specialized models and parameters. Here, we present a numerical simulation of the 4H-SiC thyristor on an N+ substrate gate current during the turn-on process. The base-emitter current of the PNP bipolar junction transistor (BJT) flow by adjusting the gate potential. This current eventually activated a regenerative action of the thyristor. The increase of the gate current from P+ anode to N+ gate also decreased the snapback voltage and forward voltage drop (Vf). When the doping concentration of the P-drift region increased, Vf decreased due to the reduced resistance of a low P-drift doping. An increase in the P buffer doping concentration increased Vf owing to enhanced recombination at the base of the NPN BJT. There is a tradeoff between the breakdown voltage and forward characteristics. The breakdown voltage is increased with a decrease in concentration, and an increase in drift layer thickness occurs due to the extended depletion region and reduced peak electric field.

Novel phosphorescent emitters, FIr4mpic and FIr4tpic were designed and prepared synthetically to improve the chromaticity and emission efficiency of FIrpic, one of the main blue phosphor. Apparently, methyl and tert-butyl on 4-position of picolinic acid of FIrpic were installed attaining FIr4mpic and FIr4tpic, respectively. With these new ancillary ligand-tuned cyclometalated iridium(III) complexes, phosphorescent organic light-emitting diodes (PHOLEDs) devices were constructed. The ultraviolet–visible (UV–vis) and photoluminescence (PL) spectra of FIr4mpic and FIr4tpic were similar to those of FIrpic; however, in terms of their electroluminescent properties, higher luminance was observed with FIr4mpic device. FIr4mpic and FIr4tpic exhibited deeper blue light emission than that of FIrpic. Once optimized, a maximum external quantum efficiency (EQE) of 18.77% and a maximum current efficiency (CE) of 36.17 cd/A were achieved with the FIr4mpic-based PHOLED. Along with the experimental information we also investigated the electronic properties by means of density functional theory (DFT) calculations being keen to explain the improved quantum yield of FIr4mpic over the other emitters.

The forward current–voltage characteristics of p-type diamond pseudo-vertical Schottky barrier diodes are investigated via numerical simulation. Impact ionization decrease the hole concentration of the p− drift layer from 1015 to 1014 cm–3 at 300 K, thereby increasing the forward voltage drop and on-resistance. When we consider an incomplete ionization with increasing temperature, the increase in the hole concentration is more dominant than the enhanced phonon scattering, thereby resulting in an increasing forward current. We modified the Ohmic contact for both metallic conduction at the p+ layer and incomplete ionization at the p– drift layer. The Baliga figure-of-merit of the device with and without incomplete ionization is 25 and 192 MW cm−2, respectively. Incomplete ionization should be considered in the numerical study of diamond power devices.

We propose a new multi-floating-zone junction-termination-extension (MFZ-JTE) using an exponentially modulated space and width for 4.5 kV SiC power devices to effectively spread out the electric field in reverse blocking mode and improve the tolerance against imprecise process control. A modified MFZ-JTE design with an exponential model was also proposed considering a stable patterning process. The blocking characteristics of the proposed MFZ-JTE using various exponential coefficients (α) in the model were evaluated for undesirable variations in the fabrication processes through TCAD simulations. The modified design for a 4.5 kV MFZ-JTE with α = 0.046 exhibited a low breakdown voltage deviation of 0.13% when the space variation ranged from –0.2 to +0.2 μm and 0.14% when the variation in the surface charge density at the SiO2/SiC interface was within the range of 0 to –1 × 1012 cm–3.

In recent years, the on-resistance, power loss and cell density of Si power devices have not exhibited significant improvements, and performance is approaching the material limits. GaN is considered an attractive material for future high-power applications because of the wide band-gap, large breakdown field, high electron mobility, high switching speed and low on-resistance. Here we report on the Ohmic contact resistance and reversebias characteristics of AlGaN/GaN Schottky barrier diodes with and without annealing. Annealing in oxygen at 500℃ resulted in an increase in the breakdown voltage from 641 to 1,172 V for devices with an anode-cathode separation of 20 μm. However, these annealing conditions also resulted in an increase in the contact resistance of 0.183 Ω-mm, which is attributed to oxidation of the metal contacts. Auger electron spectroscopy revealed diffusion of oxygen and Au into the AlGaN and GaN layers following annealing. The improved reverse-bias characteristics following annealing in oxygen are attributed to passivation of dangling bonds and plasma damage due to interactions between oxygen and GaN/AlGaN. Thermal annealing is therefore useful during the fabrication of high-voltage GaN devices, but the effects on the Ohmic contact resistance should be considered.

We demonstrated 1.2-kV SiC trench metal–oxide semiconductor field-effect transistors with a double p-base structure via technology computer-aided design simulations (TCAD) and verified their breakdown characteristics by performing experiments. Two-different p-bases including a shallow junction and a deep junction were designed via Monte Carlo ion-implantation and two-dimensional device simulations with consideration of the punch-through behavior across the n-source/p-base/drift regions and the suppression of the electric-field crowding at the gate oxide near the bottom corner of the trench. Additionally, the trench depth and the distance between the edge of the deep p-base and the trench sidewall (DPG) were varied for a comparative analysis. When DPG was reduced, the breakdown voltage increased owing to the suppression of the electric field at the gate oxide, while the on-state drain current decreased owing to the increase in the JFET resistance.

Effects of junction profiles in bottom protection p-well (BPW) on electrical characteristics of 1.2 kV SiC trench-gate MOSFETs were investigated using simulation methods. Breakdown mechanisms of BPW in the device were also elucidated by energy-band diagram and electric-field distribution across trench-gate. Monte Carlo Al-implantation simulation on the trench structure for BPW formation was carried out with variations in peak depth (DBPW), concentration (NBPW), and thickness of SiO2 spacer (Tspacer) on trench sidewall. The SiC trench-gate MOSFETs with deep DBPW, high NBPW, and thin Tspacer are suitable for high drain voltage due to a shielded trench gate by BPW. However, specific on-resistance (Ron,sp) increased because of laterally penetrated Al into p-base and accumulation regions during ion implantation for BPW formation. In case of shallow DBPW, low NBPW, and thick Tspacer, however, the gate oxide at trench bottom corner is considerably vulnerable to the dielectric breakdown due to fully depleted BPW near trench bottom corner and electric-field crowding at gate oxide.

We report on a low specific on-resistance ( R on,sp ) of 3.58 mΩ-cm 2 and a high breakdown voltage of 1.4 kV in a CMOS-compatible AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistor (MOS-HEMT). The MOS-HEMT is on a Si substrate and uses TaN electrodes and a HfO 2 -gate insulator. The sputtered TaN – a substitute for Au that has low resistivity, high work function, and thermal stability – was applied at room temperature to the gate, source, and drain. In order to obtain a low R on,sp and high breakdown voltage, sputtering power and post-annealing temperature were optimized by measuring the characteristics of TaN. Using optimized conditions, a sputtering power of 50 W, and an annealing temperature of 880 °C, we successfully achieved a high on/off current ratio of 6 × 10 9 for the proposed AlGaN/GaN MOS-HEMT at a gate–drain distance of 10 μm. These results indicate that the TaN process is a promising technique for power-switching GaN devices with CMOS compatibility.

High-voltage AlGaN/GaN Schottky barrier diodes (SBDs) were fabricated using on a silicon (1 1 1) substrate, and a post-process O2 treatment was carried out to reduce the leakage current and increase the breakdown voltage. Time-of-flight secondary ion mass spectroscopy revealed that, following the post-process O2 treatment, oxygen diffused into an AlGaN barrier and AlO was generated. A significant suppression of the leakage current (of approximately 6 orders of magnitude) occurred in the buffer isolation structures following the O2 treatment. Our method also resulted in suppression of the surface leakage current through the mesa-etched surface. A virgin GaN SBD exhibited a leakage current of 1.76 × 10−2 A/cm2, whereas the equivalent O2-treated device had a leakage current of 1.75 × 10−4 A/cm2 (the anode–cathode distance was LAC = 10 μm, and the applied bias was −100 V). This reduction in the leakage current was caused by surface passivation at the anode and cathode. The post-process O2 treatment also increased the breakdown voltage from Vb = 808 V to Vb = 1590 V for a device with LAC = 10 μm. GaN SBDs with and without the post-process O2 treatment exhibited low specific on-resistance of Ron,sp = 2.51 mΩ cm2 and Ron,sp = 2.48 mΩ cm2, respectively, with LAC = 10 μm. Devices with the post-process O2 treatment exhibited a figure of merit of Vb2/Ron,sp = 1006 MW/cm2, whereas devices without the O2 treatment exhibited a figure of merit of Vb2/Ron,sp = 263 MW/cm2. These high-voltage GaN SBDs employing the post-process O2 treatment are suitable for applications including DC–DC converters, inverters, and power factor correction circuits, where high voltage operation is required with low leakage currents.

The effect of sweeping direction on the capacitance–voltage (C–V) behavior of sputtered SiO2/4H-silicon carbide (SiC) metal-oxide semiconductor capacitors was investigated. Nitric oxide post-deposition annealing was conducted for the sputtered SiO2 on 4H-SiC. The sweeping direction of the measurement changed the C–V behavior and effective oxide charge density (Q eff) because of trapped electrons at the accumulation and detrapped electrons at the depletion. The nitrogen atoms near interface between SiO2 and 4H-SiC as a result of nitric oxide post-deposition annealing shifted the flat-band voltage in the negative direction. When the C–V was measured from depletion to accumulation, the absolute value of the Q eff after 60 min long annealing, 3.465 × 1010 cm−2 was less than that of the Q eff after 30 min long annealing, −2.912 × 1011 cm−2. The mechanisms of the nitric oxide post-deposition annealing on sputtered SiO2 on 4H-SiC are film densification and nitrogen passivation of the defects.

We investigated the effects of oxygen annealing on electrical characteristics of AlGaN/GaN heterostructures grown on Si by metalorganic chemical vapor deposition. It was shown that such treatment suppresses parasitic tunneling leakage current due to surface states or dislocations, decreases the threshold voltage corresponding to depletion of the two-dimensional electron gas at the AlGaN interface, and increases the effective interdevice isolation resistance. The suppression of the leakage current is explained by oxygen passivation of the empty states at the surface of the AlGaN barrier. Oxygen diffusion along dislocations and passivation of dislocation related states also could play a role. The decreased threshold voltage is attributed to the increased concentration of possibly oxygen related deep traps near the AlGaN/GaN interface. The increase in the GaN buffer resistivity after annealing is ascribed to the annihilation of the 0.38 eV deep electron traps believed to be introduced at the surface of GaN during mesa formation by dry etching.

� Abstract�-�WeproposedAlGaN/GaNhighelectronmobilitytransistors�(HEMTs)�usingthermal� oxidationforNiOxpassivation.�Augerelectronspectroscopy,�secondaryionmassspectroscopy,�and� pulsedIVwereusedtostudyoxidationfeatures.�TheoxidationprocessdiffusedNiandOintothe� AlGaNbarrierandformedNiOxonthesurface.�Thebreakdownvoltageoftheproposeddevicewas� 1520�Vwhilethatoftheconventionaldevicewas�300�V.�Thegateleakagecurrentoftheproposed� devicewas�3.5�5A/mmandthatoftheconventionaldevicewas�1116.7�5A/mm.�Theconventional� deviceexhibitedsimilarcurrentinthegateanddrainpulsedIVanditsdrainpulsedcounterpart.�The� gateanddrainpulsedcurrentoftheproposeddevicewasabout�56�%�ofthedrainpulsedcurrent.�This� indicatedthattheoxidationprocessmayformdeepstateshavingalowemissioncurrent,�whichthen� suppressestheleakagecurrent.�Ourresultssuggestthattheproposedprocessissuitableforachieving� highbreakdownvoltagesintheGaNbaseddevices.� �

We have proposed and fabricated high performance AlGaN/GaN high-electron-mobility transistors (HEMTs), of which the breakdown voltage is considerably high and the leakage current is efficiently suppressed employing H 2 O annealing. We also investigated the effects of H 2 O annealing on the formation of GaO x film on the surface of AlGaN/GaN HEMT as well as reverse blocking characteristics including the breakdown voltage and the leakage current. We compared the reverse blocking characteristics of the AlGaN/GaN HEMTs fabricated by the proposed H 2 O annealing treatment with those of the HEMT by widely used O 2 annealing method. Also, we investigated the reverse blocking characteristics of HEMT fabricated without any annealing treatment. We used the smaller molecular size, higher permeation probability, and the lower activation energy of H 2 O than O 2 . The breakdown voltage of AlGaN/GaN HEMT with the L GD of 20 μm employing H 2 O and O 2 annealing was 1674 and 1512 V, respectively. The drain leakage current of these devices using H 2 O and O 2 annealing was 13.1 and 60.1 nA/mm under V DS of 100 V and V GS of −10 V, respectively. A conventional device without any annealing process exhibited the breakdown voltage of 598 V and the leakage current of 666.0 μA/mm. In order to investigate the amounts of deep traps in the interface of AlGaN/GaN HEMT, we measured the pulsed I – V . A low drain current of the device under pulsed bias indicated that the GaO x , which was formed by H 2 O annealing, induced the deeper level and longer emission time of traps compared to the shallow ones. These may be related to the surface leakage current in the AlGaN/GaN HEMTs.

We have proposed and fabricated AlGaN/GaN metal-oxide-semiconductor-high-electron-mobility transistors (MOS-HEMTs) on Si substrate employing RF-sputtered HfO2 gate insulator for a high breakdown voltage. The HfO2 sputtering conditions such as a sputtering power and working pressure have been optimized in order to improve reverse blocking characteristics. We obtained the high breakdown voltage of 1524 V, the low drain leakage current of 67 pA/mm when VDS= 100 V and VGS= –10 V, and on/off current ratio of 2.37×1010 at sputtering power of 50 W and working pressure of 3 mTorr. In addition, we also discussed the mechanism of breakdown voltage improvement and investigated HfO2/GaN interface in the proposed devices by measuring the leakage current, capacitance-voltage characteristics, and X-ray diffraction (XRD).

The effects of post-oxidation on the leakage current of high-voltage AlGaN/GaN Schottky barrier diodes (SBDs) on Si(1 1 1) substrates were investigated. Auger electron spectroscopy was used to study post-oxidation mechanisms. The group-III oxides, AlOx or GaOx, were formed as indicated the oxygen on the AlGaN layer after post-oxidation. The formation of group-III oxides resulted in passivation. The diffusion of Ni into the AlGaN layer was also detected owing to the post-oxidation annealing shallow states near the Schottky contact. When post-oxidation was performed at 600 °C, all Ni at the Schottky contact was combined with oxygen. This high post-oxidation temperature was therefore unsuitable because the Schottky interface changed from Ni/AlGaN to Au/AlGaN. Leakage current until breakdown was considerably decreased after post-oxidation at 500 °C. The breakdown voltage of GaN SBD was increased from 351 to 524 V at a drift length of 20 μm by the post-oxidation. The figure-of-merit (BV2/Ron,sp) of identical device was also improved from 29.6 to 53.1 by the post-oxidation. Our results suggest that post-oxidation is suitable for GaN power switch fabrication.

The annealing technique has considerably suppressed the leakage current of GaN power devices, but this forms NiO at Ni-based Schottky contact with increasing on-resistance. The purpose of the present study was to fabricate 1.5 kV GaN Schottky barrier diodes by improving -annealing process and GaN buffer. The proposed annealing performed after alloying ohmic contacts in order to avoid NiO construction. The ohmic contact resistance () was degraded from 0.43 to after annealing at . We can decrease RC by lowering temperature of annealing. The isolation resistance of test structure which indicated the surface and buffer leakage current was significantly increased from to due to annealing. The improvement of isolation resistance can be caused by formation of group-III oxides on the surface. The leakage current of GaN Schottky barrier diode was also suppressed from to A/mm at -100 V by annealing. The GaN Schottky barrier diodes achieved the high breakdown voltage of 700, 1400, and 1530 V at the anode-cathode distance of 5, 10, and , respectively. The optimized annealing and -thick C-doped GaN buffer obtained the high breakdown voltage at short drift length. The proposed annealing is suitable for next-generation GaN power switches due to the simple process and the low the leakage current.

High-voltage AlGaN/GaN Schottky barrier diodes (SBDs) are fabricated on Si substrates by oxidizing Ni-based Schottky contacts. Oxidation successfully suppresses the isolation-leakage current by about three orders of magnitude because GaOx and AlOx formed at the mesa-etched and active regions with suppressing surface conduction. The oxidation decreased the reverse leakage current of the devices. The reverse leakage current of the oxidized device was less than 10 mA/cm2 at −100 V. The Schottky contact was changed from a Ni/Au to a Ni/Ni-Au complex/Au/NiOx structure after the oxidation. Ni was diffused into AlGaN during the oxidation, which improved the Schottky interface. When the anode-cathode distance (DAC) was 20 µm, the forward voltage drop, the on-resistance, and the breakdown voltage of the oxidized device were 1.82 V, 5.11 mΩ-cm2, and 606 V, respectively. The breakdown voltage was further increased up to 1.1 kV by inserting 2 floating metal rings at a 50-µm-long drift length. The oxidation process is suitable for high-voltage GaN devices with Ni-based Schottky contacts.

1 kV high-voltage GaN Schottky diode is realized using GaN-on-Si template by oxidizing Ni-Schottky contact. The Auger electron spectroscopy (AES) analysis revealed the formation of NiO x at the top of Schottky contact. The Schottky contact was changed to from Ni/Au to Ni/Ni-Au alloy/Au/NiO x by oxidation. Ni diffusion into AlGaN improves the Schottky interface and the trap-assisted tunneling current. In addition, the reverse leakage current and the isolation-leakage current are efficiently suppressed by oxidation. The isolation-leakage current was reduced about 3 orders of magnitudes. The reverse leakage current was also decreased from 2.44 A/cm 2 to 8.90 mA/cm 2 under -100 V-biased condition. The formed group-III oxides (AlO x and GaO x ) during the oxidation is thought to suppress the surface leakage current by passivating surface dangling bonds, N-vacancies and process damages.Key Words : GaN, AlGaN, Schottky diode, Power device, Oxidation†교신저자, 정회원 : 전자부품연구원 선임연구원, 공학박사E-mail : isobar@keti.re.kr* 펠로우회원 : 서울대학교 전기공학부 교수, 공학박사**정 회 원 : 전자부품연구원 책임연구원, 공학박사 접수일자 : 2011년 9월 8일 최종완료 : 2011년 11월 19일

We designed and fabricated aluminium gallium nitride (AlGaN)/GaN high electron mobility transistors (HEMTs) with stable reverse blocking characteristics established by employing a selective fluoride plasma treatment on the drainside gate edge region where the electric field is concentrated. Implanted fluoride ions caused a depolarization in the AlGaN layer and introduced an extra depletion region. The overall contour of the depletion region was expanded along the drift region. The expanded depletion region distributed the field more uniformly and reduced the field intensity peak. Through this field modulation, the leakage current was reduced to 9.3 nA and the breakdown voltage (V(BR)) improved from 900 V to 1,400 V.

We have investigated the effects of SiO2 passivation on the oxygen annealed AlGaN/GaN HEMTs. DC and pulsed I-V characteristics are analyzed to investigate the variation of trap level caused by SiO2 passivation and oxygen annealing. Both of oxygen annealing and SiO2 passivation are found to be effective methods to suppress the surface leakage current and increase the breakdown voltage of the AlGaN/GaN HEMTs. After an oxygen annealing, the leakage current is decreased from 621 μA/mm to 1.7 nA/mm when -5 V of VGS and -50 V of VDS are applied. However, the leakage current is increased to 5.7 μA/mm after SiO2 passivation on the oxygen annealed AlGaN/GaN HEMTs. The electron trapping through the deep trap is found to be dominant mechanism of the suppressed leakage current of the AlGaN/GaN HEMTs due to the low probability of the de-trapping from the deep trap to conduction band. SiO2 passivation suppresses the electron trapping through deep trap, which is produced by oxygen annealing. The breakdown voltage of conventional device without any treatment is 180 V. After oxygen annealing, the breakdown voltage is increased to 830 V while the AlGaN/GaN HEMT employing both of SiO2 passivation and oxygen annealing is 650 V.

We use a triangular-pulse measurement technique to obtain the hysteretic electrical characteristics of flexible graphene field-effect transistors (GFETs). To minimize charge trapping, the gate-voltage scanning rate was controlled (up to 2 V/μs) by varying the triangular-pulse rise and fall times. This method makes it possible to measure the intrinsic-like transfer characteristics of chemical vapor deposition graphene devices. The maximum electron (hole) mobility measured by a dc measurement is ~ 4800 (5200) cm2/Vs, whereas the maximum electron (hole) mobility measured by the triangular-pulse technique with a gate-voltage scanning rate of 0.4 V/μs is ~ 10600 (8500) cm2/Vs. For measurements with a triangular gate pulse, the shift of the Dirac voltage is less than that measured by the dc method. These results indicate that the triangular-gate-pulse measurement is a promising technique with which to determine the intrinsic properties and ambipolar transfer characteristics of GFETs.

The effects of a trench profile and self-aligned ion implantation on the electrical characteristics of 1.2 kV 4H-SiC trench MOSFETs employing a bottom protection p-well (BPW) were investigated to improve blocking capability by simulation studies. The trench profile and thickness of a SiO2 spacer during self-aligned ion implantation for BPW affect electrons flow through a trench gate as well as E-field concentration at the gate insulator on a trench bottom. At trench angle higher than 84° and a SiO2 spacer thicker than 0.2 µm showed that the Al concentration penetrated into the trench sidewall during ion implantation is less than 0.3% in comparison with the background doping concentration in a drift region. Under the optimum conditions with a trench angle of 90° and 0.2-µm-thick SiO2 spacer, a high breakdown voltage of 1.45 kV with a low E-field peak in the gate insulator was achieved.

A new fault current-sensing scheme employing the floating p-well for fast protection of the insulated gate bipolar transistor (IGBT) from the short-circuit faults is proposed and verified by employing 2D mixed mode simulation, based on the previous experimental results. The proposed floating p-well current-sensing scheme detects not the normal operating current but the fault current of the main IGBT by using the diode connected MOSFET and a resistor, when the short-circuit fault occurs. The diode-connected MOSFET eliminates the degradation of the forward voltage drop, because the floating p-well current does not flow under the normal operating condition due to the threshold voltage of the diode connected MOSFET. The proposed current sensor increases the protection speed without any additional delay time by the external blanking filter.

The leakage current of AlGaN/GaN high-electron-mobility transistors employing the additional Schottky gate before and after SiO2 passivation is investigated. The SiO2 passivation successfully decreases the drain leakage current from 1.70 µA to 88.05 nA due to the suppressed conduction of surface trap states. The leakage current injection from additional-gate to main-gate is observed in the unpassivated device while no leakage current injection at additional-gate is observed in the passivated device. The measured leakage current at additional-gate in the unpassivated device and passivated one are 15.58 µA and 7.10 pA respectively. The measured leakage current at main-gate in those devices are -50.72 µA and -91.10 nA respectively. The SiO2 passivation for AlGaN/GaN HEMT employing additional Schottky gate successfully decreases the leakage current through surface trap states as well as the undesirable leakage current between Schottky contacts.

A new silicon-on-insulator (SOI) lateral insulated gate bipolar transistor (LIGBT) and lateral diode employing the separated Schottky anode (SSCA) is proposed and verified by the two dimensional numerical simulation based on experimental results. The Schottky barrier between the aluminum and lightly doped n-type silicon (Al/n-Si) of SSCA provides the potential difference between the n- anode contact and p+ anode to enhance the hole injection of p+ anode, which improves the forward current–voltage (I–V) characteristics. The anode region of SSCA-LIGBT is successfully decreased without negative differential resistance (NDR) regime and sacrificing the switching speed by employing contact. The SSCA structure applied to the diode also considerably improves the reverse recovery compared to the conventional p–i–n diode and reverse blocking capability compared to conventional Schottky diode.

A new field plate structure for suppressing the leakage current of AlGaN/GaN high electron mobility transistors (HEMTs) is proposed. The proposed field plate structure consists of a gate field plate and an additional field plate. We fabricated AlGaN/GaN HEMTs employing the new field plate structure. The proposed field plate expands the depletion region and reduces the electric field concentration at the gate edge compared with the conventional single field plate. Experimental results show that the leakage current of the proposed device is 287.9 µA/mm, which is 42% less than that of a conventional one without field plate.

AlGaN/GaN high electron mobility transistors (HEMTs) have been reported for microwave applications and high power devices due to its wide bandgap, high output power and high breakdown voltage. The electron injections into the surface trap states decrease a forward drain current and breakdown voltage of AlGaN/GaN HEMTs. Various passivations such as Si3N4, SiO2, benzocyclobutene and Sc2O3 have been reported to suppress the surface states of AlGaN/GaN HEMTs so that passivation should be required in order to improve electric characteristics of AlGaN/GaN HEMTs. However, the electric reliability of AlGaN/GaN HEMTs before and after SiO2 passivation has been scarcely reported. The purpose of our work is to report hot carrier stress effects of AlGaN/GaN HEMTs before and after SiO2 passivation. AlGaN/GaN HEMTs with platinum Schottky gate were fabricated. A 400 nm-thick SiO2 passivation of AlGaN/GaN HEMTs was performed by using inductively coupled plasma-chemical vapor deposition at 300 C. Hot carrier stress with VDS=10 V, VGS=0 V and stress time of 50000 sec was applied to HEMTs before and after SiO2 passivation. Experimental results show that SiO2 passivation suppresses the injections of electrons into the surface states during the hot carrier stress. The SiO2 passivation successfully suppresses the degradation due to the hot carrier stress compared with unpassivated device. Fig. 1 shows measured transfer characteristics of SiO2 passivated and unpassivated device after the stress. After SiO2 passivation, the leakage current is decreased from 10 to 10 A due to the suppression of surface leakage current. After the stress, the positive shift of threshold voltage (VTH) is observed in both SiO2 passivated and unpassivated devices due to trapped electrons in AlGaN under Schottky gate. The ∆VTH of SiO2 passivated and unpassivated devices after the stress are 0.14 and 0.22 V respectively. It clearly shows that SiO2 passivation of AlGaN/GaN HEMTs is critical for the hot carrier stress. The SiO2 passivation suppresses VTH shift of AlGaN/GaN HEMT from the hot carrier stress. Hot carrier stress decreases the forward drain current of AlGaN/GaN HEMTs due to the increased VTH, electron injections into surface states and generated interface states. After applying hot carrier stress with VDS=10 V and VGS=0 V, the forward drain current of SiO2 passivated device is decreased by 17.9 mA/mm while that of unpassivated one is decreased by 22.4 mA/mm. This experimental result shows that SiO2 passivated device is more tolerable to the hot carrier stress compared with unpassivated one because SiO2 passivation suppresses the electron injections into the surface states during the hot carrier stress. That also decreases the channel depletion due to the hot carrier stress. The leakage currents of SiO2 passivated and unpassivated device are decreased after the hot carrier stress. In the reverse bias, stressed devices has more depletion regions due to the generated interface states and negatively charged surface states. The leakage current at VGS=-6 V of SiO2 passivated device after the stress is 2.09 nA while that before stress is 4.72 nA. Fig. 2 shows measured transconductance of SiO2 passivated and unpassivated device after the hot carrier stress. The transconductance is shifted positively due to the increase of VTH after the hot carrier stress. Measured gm.max of SiO2 passivated device decreases from 116.8 mS/mm to 114.2 mS/mm due to the decreased mobility and enhanced scattering in channel. The mechanism of the decreased forward drain current after the hot carrier stress is investigated. The difference between ∆IDS (decrease of drain current) and gm∆VTH (increase of VTH) would be attributed to generated states during the stress. Fig. 3 shows the measured -∆IDS and gm∆VTH of SiO2 passivated and unpassivated device before and after the stress. In both SiO2 passivated and unpassivated device, -∆IDS and gm∆VTH are not identical. It is noted that the interface states are generated and the electrons are injected into the surface states during the hot carrier stress. Measured ∆IDS and gm∆VTH of SiO2 passivated device are less than those of unpassivated one. Our experimental results show that SiO2 passivated AlGaN/GaN HEMT are more robust to the hot carrier stress compared with unpassivated one. We have reported the hot carrier stress effects of AlGaN/GaN HEMTs before and after SiO2 passivation. After the hot carrier stress with VDS=10 V and VGS=0 V, ∆VTH and ∆IDS of SiO2 passivated device are 0.14 V and 17.9 mA/mm while those of unpassivated one are 0.22 V and 22.4 mA/mm respectively. The SiO2 passivation suppresses the electron injections into the surface states during the hot carrier stress. The interface states between AlGaN and GaN are generated during the hot carrier stress and those are evidenced by measuring -∆IDS and gm∆VTH. The SiO2 passivation may be very effective to improve not only electric characteristics but also reliability of AlGaN/GaN HEMTs. [1] B. M. Green, IEEE Electron Device Lett., 21, pp. 268, 2000. [2] S. Arulkumaran, Appl. Phys. Lett., 84, pp. 613, 2004. [3] W.-K. Wang, IEEE Electron Device Lett., 25, pp.763, 2004. [4] J. K. Gillespie, IEEE Electron Device Lett., 23, pp.505, 2002. [5] G.Meneghesso, J. Appl. Phys., 82, pp. 5547, 1997.

A new AlGaN/GaN high electron mobility transistor (HEMT) employing Ni/Au Schottky gate oxidation and benzocyclobutene (BCB) passivation is fabricated in order to increase a breakdown voltage and forward drain current. The Ni/Au Schottky gate metal with a thickness of 50/300 nm is oxidized under oxygen ambient at 500 ∘ C and the highly resistive NiO is formed at the gate edge. The leakage current of AlGaN/GaN HEMTs is decreased from 4.94 μA to 3.34 nA due to the formation of NiO. The BCB, which has a low dielectric constant, successfully passivates AlGaN/GaN HEMTs by suppressing electron injection into surface states. The BCB passivation layer has a low capacitance, so BCB passivation increases the switching speed of AlGaN/GaN HEMTs compared with silicon nitride passivation, which has a high dielectric constant. The forward drain current of a BCB-passivated device is 199 mA /mm, while that of an unpassivated device is 172 mA /mm due to the increase in two-dimensional electron gas (2DEG) charge.

A new GaN Schottky barrier diode employing a trench structure, which is proposed and fabricated, successfully decreases a forward voltage drop without sacrificing any other electric characteristics. The trench is located in the middle of Schottky contact during a mesa etch. The Schottky metal of Pt/Mo/Ti/Au is e-gun evaporated on the 300 nm-deep trench as well as the surface of the proposed GaN Schottky barrier diode. The trench forms the vertical Au Schottky contact and lateral Pt Schottky contact due to the evaporation sequence of Schottky metal. The forward voltage drops of the proposed diode and conventional one are 0.73 V and 1.25 V respectively because the metal work function (5.15 eV) of the vertical Au Schottky contact is considerably less than that of the lateral Pt Schottky contact (5.65 eV). The proposed diode exhibits the low on-resistance of 1.58 mΩ cm2 while the conventional one exhibits 8.20 mΩ cm2 due to the decrease of a forward voltage drop.

A post-annealing method employing excimer laser pulses is proposed to improve the transfer characteristics and the breakdown voltage of unpassivated AlGaN/GaN heterostructure field-effect transistors (HFETs) for switching devices. XeCl excimer laser pulses with a wavelength of 308 nm anneal the unpassivated AlGaN/GaN HFET after Schottky gate metallization. The interface defects between the Schottky gate metal and the GaN layer are decreased by the lateral heat diffusion of the laser pulses. The temperature of the device during the laser pulse is analysed by the one-dimensional heat diffusion equation. Our experimental results show that the drain current and the maximum transconductance of the AlGaN/GaN HFET after 10 laser pulses are 496 and 134 mS mm−1, while a virgin device shows 434 and 113 mS mm−1, respectively. The measured leakage current of the AlGaN/GaN HFET at VG=−6 V is decreased from 1.95 to 1.53 mA mm−1 after 600 laser pulses. The breakdown voltage of the AlGaN/GaN HFET is increased due to the decreased leakage current.

A new inductively coupled plasma–chemical vapor deposition (ICP–CVD) SiO2 passivation for high-voltage switching AlGaN/GaN heterostructure field-effect transistors (HFETs) is proposed to increase the breakdown voltage and forward drain current. The AlGaN/GaN HFETs are fabricated and measured before and after the passivation. The measured off-state breakdown voltage of the passivated device is 455 V, whereas that of the unpassivated device is 282 V. The forward drain currents of the passivated devices are increased by 20–35% because the two-dimensional electron gas (2DEG) concentration is increased and the electron injections to the surface states are decreased.

A new junction termination method employing shallow trenches filled with silicon dioxide and field limiting rings (FLRs) for 1200 V power device applications is proposed to decrease the junction termination area without sacrificing the breakdown voltage and any additional fabrication processes. The proposed structure was analyzed and verified by numerical simulations and experiments. The two-dimensional numerical simulation results show that two electric field peaks between the field limiting rings appear in the proposed structure due to the shallow trench filled with silicon dioxide while the conventional FLR structure shows a single electric field peak between the field limiting rings. The proposed structure achieves a uniform electric field distribution and increases a breakdown voltage. The junction termination area of the proposed structure is decreased at the same breakdown voltage due to the low dielectric constant of the silicon dioxide in the trench. The experimental results show that the junction termination length of the proposed structure is 15–21% lower than that of the conventional FLR. The proposed structure achieves the high breakdown voltage, which is 80% of the ideal parallel-plane junction breakdown voltage.

Proposed is a new extended gate towards a source in AlGaN/GaN metal-oxide-semiconductor-high-electron-mobility transistors (MOS-HEMTs) in order to increase breakdown voltage and reduce on-resistance. The TaN gate was isolated from the source by a 15 nm-thick RF-sputtered HfO 2 gate insulator. A high breakdown voltage of 1410 V was measured as a result of the successfully blocked gate leakage current and surface passivation by the HfO 2 gate insulator. The extended gate towards the source was an effective method to improve the on-resistance and drain current density by eliminating the gate-source space. The proposed device with the extended gate exhibited low specific on-resistance of 2.28 mΩΩcm 2 while that of the MOS-HEMT with the conventional structure was 2.91 mΩΩcm 2 . Also, maximum drain current density at the V GS of 2 V was increased from 332 to 420 mA/mm by the proposed extended TaN gate.

A trench punchthrough (PT) insulated gate bipolar transistor (IGBT) employing a curved p-body junction and a wide cell pitch, which improves the short circuit immunity without an increase of the saturation voltage, is proposed and verified by 2D numerical simulation. The curved p-body junction and the carrier stored layer (CSL) increase the spreading angle of the electron current injected from the channel and also enable the highly resistive area under the p-body junction to shift toward the surface below the shallow p++ well so that the saturation voltage of the proposed IGBT does not increase while the channel density is reduced by using the wide cell pitch. The proposed IGBT exhibits 1.6 times higher short circuit withstanding time than the conventional one.

P-type diamond devices have high potential for power semiconductors due to their high critical field, hole mobility, and thermal conductivity. The electrical characteristics of p-type pseudovertical diamond Schottky barrier diodes (SBDs) were investigated by numerical simulation. The impact ionization coefficients were required to obtain the breakdown voltage. They were revised to satisfy a parallel-plane breakdown field of 10 MV/cm. The doping concentration and thickness of a low-doped drift layer were key parameters in determining the parallel-plane breakdown voltage. The p-type pseudovertical diamond SBDs exhibited lower breakdown voltage than the parallel-plane breakdown voltage because field crowding occurred at the edge of the cathode. When the doping concentration and thickness of the p− drift layer were 1016 cm−3 and 4 µm, respectively, the breakdown voltage of the p-type pseudovertical diamond SBD was 961 V, which was considerably less than the parallel-plane breakdown voltage of 3646 V.

A new protection circuit employing the collector to emitter voltage (VCE) sensing scheme for short-circuit withstanding capability of the insulated gate bipolar transistor (IGBT) is proposed and verified by experimental results. Because the current path between the gate and collector can be successfully eliminated in the proposed protection circuit, the power consumption can be reduced and the gate input impedance can be increased. Previous study is limited to dc characteristics. However, experimental results show that the proposed protection circuit successfully reduces the over-current of main IGBT by 80.4% under the short-circuit condition.

We have proposed and fabricated AlGaN/GaN high electron mobility transistors (HEMTs) which increase the gate-drain breakdown voltage by employing a floating gate and a field plate. Experimental results show that the breakdown voltage of the proposed devices was successfully increased compared with that of the devices which employ only the floating gate in our previous report. High breakdown voltage of 484 V is obtained while the breakdown voltage of the conventional devices is 250 V. The leakage current is reduced considerably from 88 to 8.5 μA by employing the additional field plate. Measurement of dynamic characteristics shows that the proposed devices operate under high frequency inductive load switching without any current dispersion.

We have proposed and fabricated AlGaN/GaN MOSHEMTs employing an atomic-layer-deposited (ALD) Al2O3 gate insulator. A 10-nmm Al2O3 served as passivation layer as well as gate dielectric, which results in stable blocking characteristics. The drain leakage current of the proposed device was decreased by five orders. The leakage current of the conventional device (MESFET) with LGD of 15 μm was 87 μA/mm while that of the proposed device with the same LGD was 0.2 nA/mm. The ratio of the gate leakage current to the total leakage (IG/IDSS) was decreased to 0.1 while that of the conventional one was over 0.8. It was found that ALD-Al2O3 gate insulator made a significant contribution to the device blocking capability by suppressing gate leakage effectively. The measured breakdown voltage (VBR) of the MOSHEMTs with LGD of 20 μm was 1980 V while the VBR of the conventional HEMTs device with the same LGD was 1310 V. In addition, the MOS-HEMTs with 3-μm gate exhibited good dc and pulse characteristics due to passivation effect of the ALD-Al2O3 gate insulator.

Normally-off AlGaN/GaN MOS HEMTs were successfully fabricated and investigated by simple KOH wet etch and rf-sputtered HfO2 as a gate insulator. The proposed KOH wet etch resulted in an adequate recess-depth and smooth etched surface. The gate-recessed HEMT exhibits threshold voltage (Vth) shifts from -3 to 1.5 V after 150 s KOH-wet etch. The breakdown voltage of 1580 V and Ron, sp of 8.09 mΩ·cm2 was measured in the AlGaN/GaN HEMT with the gate-drain distance of 20 μm-long. The high FOM (figure of merit) of 308 MW/cm2 was achieved. Our experimental results indicate that the proposed simple KOH wet etching and rf sputtered HfO2-gate insulator may be promising for the normally-off AlGaN/GaN MOS HEMTs fabrication.

We have proposed and fabricated a new lateral double diffused metal–oxide–semiconductor field effect transistor (LDMOSFET) entitled conductivity modulated (CM)-LDMOSFET which increases the saturation current and decreases the on-state voltage drop. The current capability of the proposed device was improved due to conductivity modulation by employing a buried p region and a p+ drain junction. Experimental results showed that the on-state voltage drop of the CM-LDMOSFET was lower than that of the conventional LDMOSFET when the current density was higher than 200 A/cm2 and the saturation current of the CM-LDMOSFET at the gate voltage of 15 V was 50% greater than that of the conventional LDMOSFET. The fabrication process of the CM-LDMOSFET is not complicated and is compatible with the conventional LDMOSFET and lateral insulated gated bipolar transistor (LIGBT) processes.

A new junction termination method employing shallow trenches filled with oxide, which successfully decreased the junction termination area, is proposed and fabricated without any complicated process such as Si-deep etching. Shallow trenches between the floating field limiting rings successfully redistributed the single electric field peak into two peaks so that the breakdown voltage could be increased with the same junction termination area. The experimental results show that the proposed method decreased the junction termination area by more than 25% compared to a conventional field limiting ring structure when breakdown voltages are equal.

We propose a novel junction termination method employing shallow silicon trenches filled with silicon dioxide, which decreases the junction termination radius by 23% while maintaining the identical breakdown voltage. The shallow silicon trenches filled with silicon dioxide and P+ FLRs (Field Limiting Rings) in the proposed structure reduced the electric field concentration in the P+ cathode due to the low dielectric constant of the silicon dioxide. We fabricated the proposed structure in silicon wafers and measured its breakdown voltage. The proposed junction termination method does not require any complex fabrication process such as a deep silicon trench or silicon bevel edge etching. The junction termination radius of the proposed structure employing three trenches was decreased by 23% compared with that of the conventional FLR employing three P+ FLRs. The breakdown voltage of the proposed structure with four trenches was 664V, which is 86% of the ideal parallel-plane breakdown voltage.

We demonstrate the fabrication of 7-octenytrichlorosilane (7-OTS)/trimethylaluminum (TMA) organic–inorganic hybrid films using molecular-atomic layer deposition (MALD). The properties of 7-OTS/TMA hybrid films are extensively investigated using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), and electrical measurements. Our results suggest that uniform and smooth amorphous hybrid thin films with excellent insulating properties are obtained using the MALD process. Films have a relatively high dielectric constant of approximately 5.0 and low leakage current density. We fabricate zinc oxide (ZnO) based thin film transistors (TFTs) using 7-OTS/TMA hybrid material as a back gate dielectric with the top ZnO channel layer deposited in-situ via MALD. The ZnO TFTs exhibit a field effect mobility of approximately 0.43 cm2 V−1 s−1, a threshold voltage of approximately 1 V, and an on/off ratio of approximately 103 under low voltage operation (from −3 to 9 V). This work demonstrates an organic–inorganic hybrid gate dielectric material potentially useful in flexible electronics application.

We proposed a new AlGaN/GaN high-electron-mobility transistor using a trench structure for high-voltage switching applications. The proposed trench structure was designed for the use at the gate edge, which improved the gate leakage current and breakdown voltage. We considered that the thickness of the AlGaN barrier was related to the polarization, surface-state density and leakage current. The surface states at the gate edge were controlled by etching the AlGaN barrier by 22 nm. The gate leakage current of the proposed device was 40 ?A/mm while that of a conventional device was 201 ?A/mm with a reverse gate-drain voltage of 100 V. The suppressed gate leakage current may have been caused by the decrease in the surface states at the gate edge. The breakdown voltage of the proposed device was 762 V while that of the conventional device without a trench structure was 120 V. The forward drain current and transconductance of the proposed device were decreased slightly because the channel resistance was increased in the trench region. The results of this study suggest that the trench structure improves the off-state characteristics of GaN power switches.

A high-voltage GaN Schottky barrier diode (SBD) on a Si substrate was fabricated by surface-O 2 treatment. The surface of the active area was annealed in O 2 ambient at 800 oC after Ohmic-contact formation. The isolation-leakage current, which flowing from a mesa to another one, was considerably suppressed from 2.4×10−3 to 2.7×10−10 A/mm at 100 V by O 2 treatment. The device leakage current was also decreased from 2.4×10−2 to 1.7×10−4 mA/mm at −100 V. The O 2 treatment seems to form group-III oxides on the surface which may passivate surface states. The breakdown voltage of O 2 -treated GaN SBDs with a drift length of 5, 10, and 20 μm were 700, 1400, and 1530 V, respectively.