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The impact of dynamic threshold voltage ( ${V}_{{\text {th}}}$ ) on the real-time characteristics of GaN direct-coupled FET logic (DCFL) inverter is investigated using an in-situ voltage tracking method. Due to the electrons’ trapping/de-trapping process at the gate dielectric/AlGaN-barrier interface, a two-step positive shift of the threshold input ( ${V}_{{\text {T}}}$ ) is observed in the device-under-test (DUT) inverter circuit, including the start-up periods and the self-adjusted dynamic equilibrium periods. The dependence of the dynamic ${V}_{{\text {T}}}$ upon different temperatures, input voltage magnitude, and operating frequency, is systematically studied and the underlying mechanism is revealed. Compared with the dc ${V}_{\text {T}}$ of the inverter of 3.6 V, the dynamic ${V}_{\text {T}}$ at 1 MHz switching with 8 V input amplitude shifts to ~5 V, which poses a challenge to GaN power integrated circuit (IC).

Plasma enhanced atomic layer deposition (PEALD) SiN gate dielectric is performed on AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor (MIS-HEMT). The fabricated MIS-HEMTs with 5 nm SiN gate dielectric layer exhibits a tiny threshold voltage hysteresis (~ 0.04 V), sufficiently decreased leakage current of ~10−9 A/mm at ${V}_{\text {GS}} =-{60}$ V and good threshold voltage ( ${V}_{\text {TH}}$ ) stability even measured at 200 °C. The increased mobility of 2-D electron gas (2-DEG) and suppressed current collapse are demonstrated, thereby producing a power density of 7.05 W/mm with an associated 34.0 % power-added efficiency (PAE) and a peak PAE of 51.4 % in a continuous-wave mode at 30 GHz.

Integration of an enhancement-mode (E-mode) metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) with a high-performance lateral Schottky barrier controlled Schottky (LSBS) rectifier in a compact layout is fabricated on an ultrathin-barrier AlGaN/GaN heterostructure grown on Si substrate. The state-of-the-art reverse conduction voltage ( ${V}_{{\text {rev}}}$ ) of 0.455 V was achieved in the monolithic integrated E-mode power device with a threshold voltage of 1.55 V. ${V}_{{\text {rev}}}$ exhibited a tiny temperature variation of 0.66% from 0 °C to 150 °C compared with that of 157% for the controlled E-mode HEMT.

In this brief, a high-performance quasi-vertical GaN Schottky barrier diode (SBD) on sapphire substrate with post-mesa nitridation process is reported, featuring a low damaged sidewall with extremely low leakage current. The fabricated SBD with a drift layer of ${1}~\mu \text{m}$ has achieved a very high ON/ OFF current ratio ( ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}{)}$ of $10^{{12}}$ with a low leakage current of $\sim 10^{-{9}}$ A/cm2@-10 V, high forward current density of 5.2 kA/cm2 at 3 V in dc, a low differential specific ON-resistance ( ${R}_{ \mathrm{\scriptscriptstyle ON},\text {sp}}{)}$ of 0.3 $\text{m}\Omega \cdot $ cm2, and ideality factor of 1.04. In addition, a transmission-line-pulse (TLP) ${I}$ – ${V}$ test was carried out and 53 kA/cm2 at 30 V in pulsed measurement was obtained without device failure, exhibiting a great potential for high power applications.

The formation mechanism of the partially crystallized ultrathin layer at the interface between GaN and SiNx grown by low-pressure chemical vapor deposition was analyzed based on the chemical components of reactants and products detected by high-resolution sputter depth profile analysis by X-ray photoelectron spectroscopy. A reasonable mass action equation for the formation of Si2N2O was proposed from the feasibility analysis of the Gibbs free energy changes of the reaction. The high-energy-activated Ga2O on the surface likely assists in the synthesis of the crystallized components. A well-defined 1ML θ-Ga2O3 transition interface was inserted into Si2N2O/GaN pure interface supercell slabs to edit the unsaturated state of the bonds. Low-density states can be achieved when the effective charges of the unsaturated atoms are adjusted to a certain interval.

The combination of ultrathin-barrier (UTB) AlGaN ( x grown by low-pressure chemical vapor deposition (LPCVD) is a promising technique for development of GaN-based millimeter-wave power amplifiers and recess-free enhancement-mode (E-mode) power switches. The LPCVD-SiN x passivation is capable of inducing high density of positive charges at the SiN x /(Al)GaN interface ( $\sim 3.50 \times 10^{13}$ cm−2), ensuring efficient recovery of 2-D electron gas (2-DEG) density that is comparable with conventional AlGaN/GaN heterostructure. Temperature-dependent Hall measurements and scattering mechanism simulations confirm the positive interfacial charges as well as interface states with density below 1013 cm−2 and exert weak remote coulombic scattering of 2-DEG in metal–insulator–semiconductor heterojunction field-effect transistors (MIS-HFETs), which warrants a low ON-resistance of UTB-AlGaN/GaN-based devices. UTB-AlGaN ( x passivation is a compelling technology platform for fabrication of high-frequency power amplifiers and high-efficiency E-mode power switches.

A Ka -band AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) with SiN gate dielectric grown by plasma-enhanced atomic layer deposition (PEALD) is demonstrated. The gate reverse leakage current in a controlled HEMT is reduced by about three orders of magnitude by the PEALD-SiN, contributing to an improved breakdown voltage of 92.5 V (source–drain separation of $2.0~\mu \text{m}$ ) in the fabricated MIS-HEMTs. The MIS-HEMTs also feature small threshold voltage hysteresis in dc transfer and capacitance–voltage characterizations, suggesting a good PEALD-SiN/AlGaN interface. The fabricated MIS-HEMTs deliver a high output power density of 7.16 W/mm and a peak power-added efficiency of 60.3%, respectively, at ${V}_{\text {DS}} =30$ V and ${V}_{\text {DS}} =10$ V in pulse mode at 28 GHz. PEALD-SiN could be a compelling gate dielectric for fabrication of high-power and high-efficiency MIS-HEMTs.

Low damage atomic layer etching (ALE) gate recess is developed for fabrication of millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). Plasma ion induced bombardments to the AlGaN barrier is effectively suppressed by the ALE recess, contributing to a well-controlled recessed surface morphology. The suppressed lattice damage to AlGaN/GaN heterostructure is also reflected by a significantly reduced gate leakage as well as an invisible threshold voltage shift associated with damage induced traps. With a 0.15- $\mu \text{m}$ T-gate fabrication technology, a high power-gain cutoff frequency ${f}_{\text {MAX}}$ of 205 GHz has been achieved. The ALE-recessed AlGaN/GaN HEMTs exhibits a record high power-added-efficiency (PAE) of 43.6% at 40 GHz in a continuous-wave mode. The associated gain and output power density are also remarkably improved compared with controlled HEMTs with conventional gate recess process.

Non-recessed ohmic contact resistance (Rc) on ultrathin-barrier (UTB) AlGaN(

Effects of fluorine plasma etching on Au-free ohmic contacts to SiNx-passivated ultrathin-barrier AlGaN/GaN heterostructure were investigated. It is found that overetching of SiNx passivation layer, which was grown by low-pressure chemical vapor deposition (LPCVD), resulted in strong accumulation of fluorine at the junction between ohmic metal, SiNx passivation, and the (Al)GaN barrier layer. The possible depletion effect of fluorine will cutoff the current conduction path between the ohmic metal and the 2-D electron gas (2DEG) in the access region, leading to degraded contact resistance and 2DEG sheet resistance. The Au-free ohmic contact mechanism can be well fitted by thermionic field emission model, and the extracted Schottky barrier height is increased by ~0.038 eV with F overetching.

The oxygen plasma surface treatment prior to ohmic metal deposition was developed to reduce the ohmic contact resistance (RC) for AlGaN/GaN high electron mobility transistors (HEMTs) on a high-resistive Si substrate. The oxygen plasma, which was produced by an inductively coupled plasma (ICP) etching system, has been optimized by varying the combination of radio frequency (RF) and ICP power. By using the transmission line method (TLM) measurement, an ohmic contact resistance of 0.34 Ω∙mm and a specific contact resistivity (ρC) of 3.29 × 10–6 Ω∙cm2 was obtained with the optimized oxygen plasma conditions (ICP power of 250 W, RF power of 75 W, 0.8 Pa, O2 flow of 30 cm3/min, 5 min), which was about 74% lower than that of the reference sample. Atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDX), and photoluminescence (PL) measurements revealed that a large nitrogen vacancy, which was induced near the surface by the oxygen plasma treatment, was the primary factor in the formation of low ohmic contact. Finally, this plasma treatment has been integrated into the HEMTs process, with a maximum drain saturation current of 0.77 A/mm obtained using gate bias at 2 V on AlGaN/GaN HEMTs. Oxygen plasma treatment is a simple and efficient approach, without the requirement of an additional mask or etch process, and shows promise to improve the Direct Current (DC)and RF performance for AlGaN/GaN HEMTs.

The formation mechanism of the partially crystallized ultrathin layer at the interface between GaN and SiN

In this work, the optimization of reverse leakage current (I R) and turn-on voltage (V T) in recess-free AlGaN/GaN Schottky barrier diodes (SBDs) was achieved by substituting the Ni/Au anode with TiN anode. To explain this phenomenon, the current transport mechanism was investigated by temperature-dependent current–voltage (I–V) characteristics. For forward bias, the current is dominated by the thermionic emission (TE) mechanisms for both devices. Besides, the presence of inhomogeneity of the Schottky barrier height (qφ b) is proved by the linear relationship between qφ b and ideality factor. For reverse bias, the current is dominated by two different mechanisms at high temperature and low temperature, respectively. At high temperatures, the Poole–Frenkel emission (PFE) induced by nitrogen-vacancy (V N) is responsible for the high I R in Ni/Au anode. For TiN anode, the I R is dominated by the PFE from threading dislocation (TD), which can be attributed to the decrease of V N due to the suppression of N diffusion at the interface of Schottky contact. At low temperatures, the I R of both diodes is dominated by Fowler–Nordheim (FN) tunneling. However, the V N donor enhances the electric field in the barrier layer, thus causing a higher I R in Ni/Au anode than TiN anode, as confirmed by the modified FN model.

We report a high-performance GaN Schottky barrier diode (SBD) on a sapphire substrate with a novel post-mesa nitridation technique and its application in a high-power microwave detection circuit. The fabricated SBD achieved a very high forward current density of 9.19 kA cm−2 at 3 V, a low specific on-resistance (R ON,sp) of 0.22 mΩ cm2 and breakdown voltage of 106 V. An extremely high output current of 400 mA was obtained when the detected power reached 38.4 dBm@3 GHz in pulsed-wave mode with a small anode diameter of 70 μm. Meanwhile, broadband detection at frequencies ranging from 1 to 6 GHz was achieved at 33 dBm in continuous-wave mode.

Gallium nitride (GaN) has attracted increased attention because of superior material properties, such as high electron saturation velocity and high electrical field strength, which are promising for high-power microwave applications. We report on a high-performance vertical GaN-based Schottky barrier diode (SBD) and its demonstration in a microwave power limiter for the first time. The fabricated SBD achieved a very low differential specific on-resistance (RON,sp) of 0.21 mΩ·cm2, attributed to the steep-mesa technology, which assists in reducing the spacing between the edge of the anode and cathode to 2 µm. Meanwhile, a low leakage current of ~10−9 A/cm2@−10 V, a high forward current density of 9.4 kA/cm2 at 3 V in DC, and an ideality factor of 1.04 were achieved. Scattering parameter measurements showed that the insertion loss (S21 ) was lower than −3 dB until 3 GHz. In addition, a microwave power limiter circuit with two anti-parallel diodes was built and measured on an alumina substrate. The input power level reached 40 dBm (10 watts) in continuous-wave mode at 2 GHz, with a corresponding leakage power of 27.2 dBm (0.5 watts) at the output port of the limiter, exhibiting the great potential of GaN SBD in microwave power limiters.

Constant-capacitance deep-level transient Fourier spectroscopy is utilized to characterize the interface between a GaN epitaxial layer and a SiNx passivation layer grown by low-pressure chemical vapor deposition (LPCVD). A near-conduction band (NCB) state ELP (EC – ET = 60 meV) featuring a very small capture cross section of 1.5 × 10–20 cm–2 was detected at 70 K at the LPCVD-SiNx/GaN interface. A partially crystallized Si2N2O thin layer was detected at the interface by high-resolution transmission electron microscopy. Based on first-principles calculations of crystallized Si2N2O/GaN slabs, it was confirmed that the NCB state ELP mainly originates from the strong interactions between the dangling bonds of gallium and its vicinal atoms near the interface. The partially crystallized Si2N2O interfacial layer might also give rise to the very small capture cross section of the ELP owing to the smaller lattice mismatch between the Si2N2O and GaN epitaxial layer and a larger mean free path of the electron in the ...

We report on the electrical and microstructural characterization of Au-free Ti/Al/Ti/TiN contacts for AlGaN/GaN heterostructures. Ultra-low Au-free ohmic contact has been obtained with contact resistance and specific contact resistivity as low as $0.21~\Omega \cdot \text{mm}$ and ${1.16}\times {10}^{-6} \Omega \cdot \,\,\text{cm}^{2}$ , respectively. The ohmic alloy temperature is reduced as low as 550 °C by pre-ohmic recess of the AlGaN barrier and optimization of the thickness of bottom Ti layer. We found that interfacial layer formation of AlN between the ohmic metal and AlGaN surface is crucial to realize a low contact resistance with reduced low annealing temperature by a combination of electrical ${I}$ – ${V}$ characterization and high-resolution transmission electron microscopy analysis. Furthermore, we suggest a hypothesis that the bottom Ti layer plays a catalytic role for the Al–N reaction with optimized thickness.

In this letter, a high-temperature (HT) gate recess technique is implemented into the fabrication of high-performance millimeter-wave AlGaN/GaN high-electron-mobility transistors (HEMTs). By virtue of the low damage feature of the HT recess, a high extrinsic transconductance of 422 mS/mm, a three-terminal breakdown voltage of 134 V with a source–drain separation of 2.4 $\mu \text{m}$ , and a remarkable low Schottky gate leakage current of $1.6\times 10^{-6}$ A/mm at ${V}_{\textsf {GS}} = -\textsf {60}$ V are achieved, which is at least two orders of magnitude lower than the HEMTs with gate recessed at room temperature. By employing a 0.2- $\mu \text{m}$ T-shaped gate technology, a current-gain cutoff frequency ${f}_{T}$ of 81 GHz and a unit-power-gain frequency ${f}_{\textsf {MAX}}$ of 194 GHz are realized. The current collapse in the HT-recessed AlGaN/GaN HEMTs is significantly suppressed, contributing to a record high power-added-efficiency of 42.8%, as well as high output power density of 5.1 W/mm at 35 GHz in a continuous-wave mode.

(Al)GaN recess-free normally OFF technology is developed for fabrication of high-yield lateral GaN-based power devices. The recess-free process is achieved by an ultrathin-barrier (UTB) AlGaN/GaN heterostructure that features a natural pinched-off 2-D electron gas channel. The top–down manufacturing technique overcomes the challenges in etching of AlGaN barrier with well-controlled depth and uniformity, which is especially attractive for fabrication of normally OFF GaN-based high-electron-mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs) on large-size Si substrate. With SiNx passivation grown by low-pressure chemical-vapor deposition, on-resistance of the UTB-AlGaN/GaN-based power devices can be significantly reduced. High-uniformity low-hysteresis normally OFF HEMTs and Al2O3/AlGaN/GaN MIS-HEMTs are successfully demonstrated on the UTB AlGaN/GaN-on-Si platform. It is also a compelling technology platform for manufacturing high-performance GaN-based lateral power diodes, and normally OFF p-(Al)GaN heterojunction field-effect transistors.

Effects of oxide/III-nitride interface and oxide states on the threshold voltage instability ( $\boldsymbol{V}_{\mathbf{TH}}$ -Instability) of normally-OFF Al 2 O 3 /AlGaN/GaN MIS-HFETs, i.e., metal-insulator-semiconductor heterojunction field-effect transistors, were revealed by constant-capacitance deep level transient spectroscopy (CC-DLTS). It is confirmed that a technique of in-situ remote plasma pretreatments could effectively suppress the $\boldsymbol{D}_{\mathbf{it}}$ with level depth ( $\boldsymbol{E}_{\mathbf{C}}-\boldsymbol{E}_{\mathbf{T}}$ ) larger than 0.4 eV down to below $1.3\times 10^{12}\ \mathbf{cm}^{-2}\mathbf{eV}^{-1}$ , in spite of the presence of a discrete level with $\boldsymbol{E}_{\mathbf{C}}-\boldsymbol{E}_{\mathbf{T}}$ and capture cross section ( $\boldsymbol{\sigma}_{\mathbf{n}}$ ) being 0.33 eV and $4.0\times 10^{-15}\ \mathbf{cm}^{2}$ respectively. However, electron charging of oxide states occurs when the MIS-HFETs are pulsed by a high positive gate bias (e.g., > 8 V), as confirmed by a reduced tunnel constant $\boldsymbol{d}_{0}$ of 0.79 nm. High electric field induced tunnel filling of gate oxide states could be an assignable cause for the positive bias temperature instability in normally-OFF III-nitride MIS-HFETs.

Silicon nitride (SiNx) grown by low-pressure chemical vapor deposition (LPCVD) at a reduced growth temperature (650 °C), is utilized for fabrication of GaN metal–insulator-semiconductor (MIS) power devices. An atomically sharp interface between the LPCVD-SiNx and GaN was achieved, featuring a disordered region with only a thickness of 2.5 ~ 5 A. A fabricated LPCVD-SiNx/GaN/AlGaN/GaN MIS diode exhibits a sharp two-step capacitance–voltage behavior with small frequency dispersion of 0.4 V in the right step. The improved interface was quantified by a well-elaborated constant-capacitance deep-level transient Fourier spectroscopy (CC-DLTFS), delivering quite low density of 1.5 × 10 13 cm - 2 eV - 1 at the level depth of 30 meV and about 4 × 10 11 ~ 1.2 × 10 12 cm - 2 eV - 1 at the level depth of 1 eV. Distributions of interface states can be described by a proposed physics-based decoupling function featuring an exponential law. A discrete level at the SiNx/GaN border or in the barrier layer with the level depth and the capture cross section being 0.8 eV and 5.5 × 10 - 14 cm 2 respectively, can thus be detached from the interface states.

Ultra-thin-barrier (UTB) AlGaN/GaN heterostructure is utilized for fabrication of normally-OFF GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs). The sheet resistance of 2-D electron gas in the UTB Al0.22Ga0.78N(5-nm)/GaN heterostructure is effectively reduced by SiNx passivation grown by low-pressure chemical vapor deposition, from 2570 to $334~\Omega $ /□. The fabricated Al2O3/AlGaN/GaN MIS-HEMTs exhibit normally-OFF behavior with good $V_{\mathrm {TH}}$ uniformity and low $V_{\mathrm {TH}}$ -hysteresis. 20 mm-gate-width power devices featuring a low $R_{\sc {\mathrm{ on}}}$ of $0.75~\Omega $ ( $\text{I}_{\mathrm {D,MAX}} =6.5$ A) are also demonstrated on the platform.

Constant-capacitance deep-level transient Fourier spectroscopy is utilized to characterize the interface between a GaN epitaxial layer and a SiN

High-performance AlGaN/GaN-on-Si diodes are fabricated with lateral Schottky barrier controlled Schottky rectifier (LSBS) on thin-barrier (5nm) AlGaN/GaN heterostructures, which features a recess-free process, enabling better electrostatic control to pinch off the channel under the anode region. In this way, low leakage currents (3-orders of magnitude lower than conventional recessed Schottky-barrier-diode (SBD)) and a high reverse breakdown voltage of 1700 V (@10μA/mm) are reached, together with a low onset voltage of only 0.37 V and a record low on-state resistance R on, sp of 10 mΩ·cm2 for GaN-based SBDs with an Anode-to-Cathode distance (L AC ) of 10 μm. This is attributed to the combination of an effectively preserved 2DEG by LPCVD SiN x passivation and a hybrid Schottky /Ohmic anode. Thanks to the recess free technology and low damage in the Schottky region, stable preliminary HTRB performance are obtained. The proposed diode fabrication is compatible with GaN depletion/enhancement MIS-HEMTs process flow, enabling integration in the promising smart GaN platform.

The effects of GaN channel layer thickness on dc and RF performance of AlGaN/GaN high-electron mobility transistors (HEMTs) with a state-of-the-art composite AlGaN/GaN (1/1 μm) buffer were systematically investigated. Although HEMTs with a thick GaN channel layer exhibit slight degraded dc and RF small-signal performance associated with short-channel effects, they demonstrate significantly enhanced OFF-state breakdown voltage and RF large-signal performance. The 1-mm HEMTs with a 150-nm-thick GaN channel layer feature a 1.4 dB higher saturated POUT and about 10% higher PAE than that with a 50-nm-thick GaN channel layer, in both Classes AB and B operation conditions. Pulse I-V characterization reveals that the buffer-related current collapse is also suppressed in the thick GaN channel sample as compared with the thin one, suggesting that a thick GaN channel layer will not only reduces the deep traps in the channel, but also reduces the electron capture probability by deep traps in the composite AlGaN/GaN buffer. The selection of a proper GaN channel layer thickness is thus of great importance to the designation of GaN-based power amplifiers for various applications.

A physical insight into the capture and emission behavior of interface/oxide states in a GaN-based metal-oxide-semiconductor (MOS) structure is of great importance to understanding the threshold voltage (VTH) instability in GaN power transistors. A time-dependent VTH shift in Ni/Al2O3/AlGaN/GaN MOS-HFETs (heterojunction field-effect transistors) and a distribution of Al2O3/III-nitride interface states (Dit) were successfully characterized by constant-capacitance deep level transient spectroscopy. It is found that in situ remote plasma pretreatments in plasma-enhanced atomic-layer-deposition could suppress Dit (EC-ET > 0.4 eV) down to below 1.3 × 1012 cm−2 eV−1. Under high applied gate bias (e.g., VG > 8 V), tunnel filling of oxide states in the Al2O3 dielectric comes into play, contributing to remarkable VTH instability in the MOS-HFETs. The tunnel distance between the 2D Electron Gas (2DEG) channel and oxide states ET,ox in the Al2O3 dielectric decreases from 3.75 to 0.82 nm as VG increases from 2 to 8 V. A further increase of VG to 11 V makes the Fermi level approach ET,ox (EC − ET ∼ 1.62 eV), which may enable direct filling. High electric field induced tunnel filling of gate oxide states could be an assignable cause for VTH instability in normally-OFF III-nitride MOS-HFETs.

Gallium nitride (GaN)-based vertical power Schottky barrier diode (SBD) has demonstrated outstanding features in high-frequency and high-power applications. This paper reviews recent progress on GaN-based vertical power SBDs, including the following sections. First, the benchmark for GaN vertical SBDs with different substrates (Si, sapphire, and GaN) are presented. Then, the latest progress in the edge terminal techniques are discussed. Finally, a typical fabrication flow of vertical GaN SBDs is also illustrated briefly.

In this letter, we report high-performance enhancement-mode (E-mode) Al2O3/AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) fabricated with high-temperature low-damage gate recess technique. The high-temperature gate recess is implemented by increasing the substrate temperature to 180 °C to enhance the desorption of chlorine-based etching residues during the dry etching of AlGaN barrier. High-crystal-quality Al2O3 gate dielectric was grown by atomic-layer deposition using O3 as the oxygen source to suppress hydrogen-induced weak bonds. The fabricated E-mode MIS-HEMTs exhibit a threshold voltage of 1.6 V, a pulsed drive current of 1.13 A/mm, and very low OFF-state standby power of $6.8 \times 10^{\mathrm {-8}}$ W/mm at $V_{\mathrm {GS}} =0$ V and $V_{\mathrm {DS}} =30$ V. At 4 GHz and in pulse-mode operation, the output power density and power-added efficiency were measured to be 5.76 W/mm and 57%, both of which are the highest for GaN-based E-mode MIS-HEMTs reported to date.

Low-pressure chemical vapor deposition (LPCVD) technique is utilized for SiN x passivation of AlGaN/GaN high-electron-mobility transistors (HEMTs). A robust SiN x / AlGaN interface featuring high thermal stability and well-ordered crystalline structure is achieved by a processing strategy of “passivation-prior-to-ohmic” in HEMTs fabrication. Effective suppression of surface-trap-induced current collapse and lateral interface leakage current are demonstrated in the LPCVD-SiN x passivated HEMTs, as compared with conventional plasma-enhanced chemical vapor deposition-SiN x passivated ones. Energy dispersive X-ray spectroscopy mapping analysis of SiN x /AlGaN interfaces suggests the interface traps are likely to stem from amorphous oxide/oxynitride interfacial layer.

An obvious increase in the gate leakage current has been commonly observed in GaN HEMTs, after Plasma-enhanced chemical vapor deposition (PECVD) SiN passivation has been observed to obviously increase. This paper presents an Al/SiN stack layer passivation structure. The high gate leakage current in GaN HEMTs caused by the PECVD SiN passivation is distinctly reduced by 2 to 3 orders of magnitude by introducing a thin Al layer. It is mainly attributed to the Al layer blocking and minimizing the damage for the (Al)GaN surface in the build-up of the luminance process of PECVD SiN and then reducing the surface trap density. TEM mapping and SRIM software simulation reveal that neither damage nor inter-diffusion is demonstrated at the Al/AlGaN interface, where a continuous crystalline region is observed. The moderate current collapse suppression and 32.8% improvement in VBR are achieved in GaN HEMTs with Al/SiN passivation.

An X-band AlGaN/GaN high-electron-mobility transistor (HEMT) on SiC substrate with high microwave power performances has been achieved. Its small-signal characteristics with a gate-length of 0.4 μm showed a unity current gain cut-off frequency (fT) of 22 GHz and a maximum oscillation frequency (fmax) of 65 GHz. The GaN HEMT device with a gate width of 1 mm exhibited a continuous-wave saturated output power of 10.2 W and a linear gain of 14.8 dB at 8 GHz, and successfully achieved the power-added efficiency (PAE) as high as 69.2%, which is very suitable for X-band power applications.

This letter reports a 0.2- μm gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on an Si substrate passivated with an AlN/SiNx (4/20 nm) stack layer. The 4-nm-thick AlN was grown by plasma-enhanced atomic-layer-deposition. The AlN/SiNx-passivated HEMTs exhibit a high maximum drain current of 930 mA/mm, an three-terminal OFF-state breakdown voltage (BVDS) of 119 V, and a small threshold voltage shift of 130 mV in a wide drain bias range (VDS=3-24 V). Owing to the additional positive polarization charge in the AlN passivation layer, the access resistance Rs in the GaN-on-Si HEMTs is significantly reduced while maintaining small parasitic gate-drain capacitance Cgd, contributing to a high power-gain cutoff frequency fMAX of 182 GHz and a high Johnson's figure of merit of BVDS × f T of 6.43 × 1012 V/s simultaneously. The accuracy of the RF performance is verified by a small signal modeling based on measured S-parameters.

This paper concentrates on the impact of SiN passivation layer deposited by plasma-enhanced chemical vapor deposition (PECVD) on the Schottky characteristics in GaN high electron mobility transistors (HEMTs). Three types of SiN layers with different deposition conditions were deposited on GaN HEMTs. Atomic force microscope (AFM), capacitance–voltage (C–V), and Fourier transform infrared (FTIR) measurement were used to analyze the surface morphology, the electrical characterization, and the chemical bonding of SiN thin films, respectively. The better surface morphology was achieved from the device with lower gate leakage current. The fixed positive charge Q f was extracted from C–V curves of Al/SiN/Si structures and quite different density of trap states (in the order of magnitude of 1011–1012 cm−2) was observed. It was found that the least trap states were in accordance with the lowest gate leakage current. Furthermore, the chemical bonds and the %H in Si–H and N–H were figured from FTIR measurement, demonstrating an increase in the density of Q f with the increasing %H in N–H. It reveals that the effect of SiN passivation can be improved in GaN-based HEMTs by modulating %H in Si–H and N–H, thus achieving a better Schottky characteristics.

The thermal stability and electrical characteristics of GaN high-electron-mobility transistors (HEMTs) were investigated. Storage tests were carried out at 400°C for 48 h to study the ohmic-contact stability by means of the transmission line model. It was found that Ti/Al/Ni/Au ohmic contacts were stable and had superior thermal performance, but the Schottky contact may be more sensitive to the temperature. After thermal storage for 48 h at 400°C, the Schottky barrier height was increased, and the ideality factor decreased. Two types of isolation structures were also investigated under the same condition. DC tests were implemented to study the phenomenon and provide feedback for potential process improvements.

A C-band linear power amplifier is successfully developed with a one-chip 2 mm AlGaN/GaN high electron mobility transistors (HEMTs). Two kinds of matching circuits for the linear power amplifier are compared. Besides, stabilization methods for the amplifier are also discussed. At 5.4 GHz, the developed GaN HEMTs linear power amplifier delivers a 37.2 dBm (5.2 W) cw P1 dB output power with 9 dB linear gain and 55.7% maximum power-added efficiency (PAE) with a drain voltage of 25 V. To our best knowledge, the achieved PAE is the state-of-the-art result ever reported for 2 mm gate width single die GaN-based hybrid microwave integrated power amplifier at C-band.

Temperature-dependent ON-state breakdown (BV ON ) loci of AlGaN/GaN high-electron-mobility transistors (HEMTs) were experimentally demonstrated for the first time. With gate-current extraction technique, impact ionization was revealed to be responsible for the ON-state breakdown in our device as the HEMTs is marginally turned on. The characteristic electric-field E i of impact ionization was extracted exhibiting an U-shaped temperature dependence from 40 to -30 o C, with minimum E i occurs at -10 o C. Such anomalous temperature dependence of E i together with the kink effect observed in our devices stem from coulomb scattering effect of the acceptor-like traps in AlGaN/GaN heterostructures.

We have performed constant voltage stress (CVS) tests on GaN-on-SiC HEMT to investigate the drain current drift. Two kinds of current drift behavior are observed in CVS. The off-state drain voltage step stress tests are carried out to confirm the electric field dependent current drift. A critical voltage for drain current recovery is observed. We suggest that the recovery of drain current is due to holes generation near the heterojunction interface or the detrapping of acceptors in the barrier layer. The drain current drift is balanced by the current degradation and recovery mechanism.