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N-polar InN/In0.61Al0.39N heterostructures are grown directly on sapphire by using metalorganic chemical vapor deposition. The thickness of Mg-doped In0.61Al0.39N is 340 nm, and the root-mean-square surface roughness of 20 nm thick InN is ∼3.2 nm. An optional AlN spike grown at 710 °C for 35 s is used either as an interlayer to separate the InAlN buffer from the InN channel or as a part of InAlN nucleation after sapphire nitridation. High-resolution transmission electron microscopy reveals approximately two monolayers of AlN if used as the interlayer. In this case, the concentration of screw and edge threading dislocations in partially strained InN decreased down to 6.5 × 109 and 38 × 109 cm−2, respectively. More importantly, the interlayer inclusion suppressed remote donor and alloy disorder scatterings, providing, at room temperature, the InN free electron mobility and concentration of 620 cm2/V s and 3 × 1013 cm−2, respectively. On the other hand, omitting the AlN spike by InAlN nucleation led to structural deteriorations while buffer resistivity increased to 1.7 kΩ/□. A current density of ∼12–16 A/mm, breakdown field of ∼75 kV/cm, and electron drift velocity of ∼2 × 107 cm/s were determined in InN by applying 10 ns voltage pulses on fabricated test resistors. [ABSTRACT FROM AUTHOR]

We studied the drain current properties of an AlGaN/GaN multi-nano-channel (MNC) high electron mobility transistor (HEMT) fabricated on a sapphire substrate. We observed that the MNC HEMT exhibits the currents almost equal to those in the conventional planar device grown on the same chip. This result was unexpected since the actual gate width on the AlGaN surface in the case of MNC HEMT was only 20 % of that for the planar device. In order to explain our experimental results, we performed a three dimensional (3D) simulation of the planar and MNC HEMTs using the TCAD Sentaurus software. Especially, we calculated the transfer characteristics of the MNC HEMT with a different nanochannel width and compared them with experimental data. The simulation results exhibited a good agreement with experimental ones. On this basis, we showed that the unusual behavior of the current in the MNC HEMT results from the enhancement of the effective electron velocity (v e) under the gate. In particular, we found that v e for the MNC HEMT was about 2.5 times higher than for the conventional HEMT, i.e., 2.44 × 10 7 cm/s, which is close to the peak saturation velocity in GaN (2.5 × 10 7 cm/s). Finally, we showed that such a strong enhancement of v e in the MNC HEMT case is due to the formation of the high electric field in the nanochannel. The results obtained in this work are not limited only to MNC structures but they should also be useful in understanding the electric field and electron velocity distribution in other AlGaN/GaN HEMTs with 3D nanochannels such as AlGaN/GaN FinFETs. [ABSTRACT FROM AUTHOR]

We performed, for the first time, quantitative characterization of electron capture cross sections σ of the interface states at dielectric/III-N heterojunction interfaces. We developed a new method, which is based on the photo-assisted capacitance-voltage measurements using photon energies below the semiconductor band gap. The analysis was carried out for AlGaN/GaN metal-insulatorsemiconductor heterojunction (MISH) structures with Al2O3, SiO2, or SiN films as insulator deposited on the AlGaN layers with Al content (x) varying over a wide range of values. Additionally, we also investigated an Al2O3/InAlN/GaN MISH structure. Prior to insulator deposition, the AlGaN and InAlN surfaces were subjected to different treatments. We found that σ for all these structures lies in the range between 5 × 10-19 and 10-16 cm2. Furthermore, we revealed that σ for dielectric/ AlxGa1-xN interfaces increases with increasing x. We showed that both the multiphonon-emission and cascade processes can explain the obtained results. [ABSTRACT FROM AUTHOR]

We investigate degradation mechanisms in AlGaN/GaN HEMTs which were repeatedly driven into the current-controlled off-state breakdown or subject to 60 s voltage- or current-controlled off state stresses. The current-controlled sweep in to the breakdown allows the sustainability of breakdown that can not be observed in the voltage controlled sweep. Only temporal changes were observed in the HEMT dc performance after repetitive sweeps, which were explained by charging/discharging of the HEMT surface at the gate-to-drain access region and in the GaN buffer below the gate. Similar changes were observed also if high-voltage stress has been applied on the drain; however, permanent degradation appears after 60 s current-controlled breakdown stress. In this case, the drain leakage current, as well as the breakdown current, increases significantly. On the other hand, the breakdown voltage, as well as the gate characteristics, remains unaltered. We suggest that the avalanche-injection process is governing the off-state breakdown event with a dominant role of the potential barrier at the channel-buffer interface. [ABSTRACT FROM AUTHOR]

We address degradation aspects of lattice-matched unpassivated InAlN/GaN high-electron-mobility transistors (HEMTs). Stress conditions include an off-state stress, a semi-on stress (with a partially opened channel), and a negative gate bias stress (with source and drain contacts grounded). Degradation is analyzed by measuring the drain current, a threshold voltage, a Schottky contact barrier height, a gate leakage and an ideality factor, an access, and an intrinsic channel resistance, respectively. For the drain-gate bias <38 V parameters are only reversibly degraded due to charging of the pre-existing surface states. This is in a clear contrast to reported AlGaN/GaN HEMTs where an irreversible damage and a lattice relaxation have been found for similar conditions. For drain-gate biases over 38 V InAlN/GaN HEMTs show again only temporal changes for the negative gate bias stresses; however, irreversible damage was found for the off-state and for the semi-on stresses. Most severe changes, an increase in the intrinsic channel resistance by one order of magnitude and a decrease in the drain current by ∼70%, are found after the off-state ∼50 V drain-gate bias stresses. We conclude that in the off-state condition hot electrons may create defects or ionize deep states in the GaN buffer or at the InAlN/GaN interface. If an InAlN/GaN HEMT channel is opened during the stress, lack of the strain in the barrier layer is beneficial for enhancing the device stability. [ABSTRACT FROM AUTHOR]

The thermal boundary resistance (TBR) and its effects on temperature distribution for GaN layers on Si, SiC or sapphire substrates are investigated. The findings indicate that the role of TBR at the GaN/sapphire interface in the poor heat transfer from GaN to substrate is less important and the substrate cool efficiency is improved, if fewer defects are present at the interface to the GaN epistructure.

An attempt is made to investigate current condition and saturation mechanisms in ungated AlGaN/GaN/SiC transfer length method test structures with contact-to-contact distance L varying from 2 to 32 (mu)m and the current-voltage (I-V) characteristics are measured in the pulsed and in the direct current (dc). The transient interferometric mapping (TIM) results provide an explanation of the current saturation mechanism and directly support conclusions of the I-V characterization.

Self-heating effects in a molecular beam epitaxy-grown GaN/AlGaN/GaN structure on a single crystalline diamond is investigated. A transient interferometric method, in combination with a three dimensional model, is used to describe a pulsed operation of a transistor-like heater, and a micro-Raman technique is used in a steady state. Good agreement is found between the techniques. The thermal conductivity of the diamond is found to be 2200 W/m K, and the thermal boundary resistance to the III-N epi-structure is < 1 × 10-8 m2 K/W. The excellent cooling efficiency of the diamond is manifested by the fast saturation of the temperature at 1 μs and by a record low normalized thermal resistance of 3.5 K mm/W. [ABSTRACT FROM AUTHOR]