High-voltage SiC and GaN power devices

The recent progress in the development of high-voltage SiC and GaN power switching devices is reviewed. The experimental performance of various rectifiers and transistors, which have been demonstrated, is discussed. Material and processing challenges and reliability concerns on SiC and GaN power devices are also described. The future trends in device development and commercialization are pointed out.     

High-voltage operation of field-effect transistors in siliconcarbide

Buried-gate field-effect transistors with blocking voltages up to 600-700 V have been fabricated in 6H polytype silicon carbide using a trench technology. The devices achieve drain currents of up to 60 mA for a channel width of 0.72 mm and have a turn off gate voltage of about 40 V. We report on the device characteristics and analyze the performance under high-voltage device operation     

Epitaxially-grown GaN junction field effect transistors

Junction field effect transistors (JFETs) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The dc and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (g_m ) of 48 mS/mm and a maximum source-drain current of 270 mA/mm are achieved on a 0.8 μm gate JFET device at V_GS=1 V and V_DS=15 V. The intrinsic transconductance, calculated from the measured g_m and the source series resistance, is 81 mS/mm. The f_T and f_max for these devices are 6 GHz and 12 GHz, respectively. These JFET's exhibit a significant current reduction after a high drain bias is applied, which is attributed to a partially depleted channel caused by trapped hot-electrons in the semi-insulating GaN buffer layer. A theoretical model describing the current collapse is presented, and an estimate for the length of the trapped electron region is given     

High-voltage normally off GaN MOSFETs on sapphire substrates

Gallium nitride self-aligned MOSFETs were fabricated using low-pressure chemical vapor-deposited silicon dioxide as the gate dielectric and polysilicon as the gate material. Silicon was implanted into an unintentionally doped GaN layer using the polysilicon gate to define the source and drain regions, with implant activation at 1100/spl deg/C for 5 min in nitrogen. The GaN MOSFETs have a low gate leakage current of less than 50 pA for circular devices with W/L=800/128 /spl mu/m. Devices are normally off with a threshold voltage of +2.7 V and a field-effect mobility of 45 cm/sup 2//Vs at room temperature. The minimum on-resistance measured is 1.9 m/spl Omega//spl middot/cm/sup 2/ with a gate voltage of 34 V (W/L=800/2 /spl mu/m). High-voltage lateral devices had a breakdown voltage of 700 V with gate-drain spacing of 9 /spl mu/m (80 V//spl mu/m), showing the feasibility of self-aligned GaN MOSFETs for high-voltage integrated circuits.     

Terahertz detection by GaN/AlGaN transistors

Detection of subterahertz and terahertz radiation by high electron mobility GaN/AlGaN transistors in the 0.2-2.5 THz frequency range (much higher than the cutoff frequency of the transistors) is reported. Experiments were performed in the temperature range 4-300 K. For the lowest temperatures, a resonant response was observed. The resonances were interpreted as plasma wave excitations in gated two-dimensional electron gas. Non-resonant detection was observed at temperatures above 100 K. Estimates for noise equivalent power show that these transistors can be used as efficient detectors of terahertz radiation at cryogenic and room temperatures     

The toughest transistor yet [GaN transistors]

GaN transistors withstand extreme heat and are capable of handling frequencies and power levels well beyond those possible with silicon, gallium arsenide, silicon carbide, or essentially any other semiconductor yet fabricated. Frequency and power-handling capabilities of this caliber could make all the difference in amplifiers, modulators, and other key components of the advanced communications networks. GaN transistors could double or triple the efficiency of base-station amplifiers, so that a given area could be covered by fewer base stations or, more likely, be flooded with more data at much higher rates. These same characteristics of speed, high-power handling, and heat resistance would also suit the transistors for many other uses     

Physics-based modeling of submicron GaN permeable base transistors

We present the first physics-based nonstationary modeling of a submicron GaN permeable base transistor. Three different transport models are compared: drift-diffusion, energy balance, and ensemble Monte Carlo. Transport parameters and relaxation times used by the carrier transport equations are consistently derived from particle simulation. The current-voltage (I-V) characteristics predicted with the energy balance model are in good agreement with those obtained from direct Monte Carlo device simulation. On the other hand, the drift-diffusion approach appears to be inadequate for the device under study, even if improved high-field mobility models are adopted     

Methodology for predicting off-state reliability in GaN power transistors

Results of a lifetest across temperature and drain voltage on off-state high power GaN FET test structures are presented. The times to failure (t_f) are fitted to a combination of the Arrhenius model (ln(t_f) ∼ inverse temperature) and the linear field model (ln(t_f) ∼ drain voltage). The estimated activation energy (E_a) is 2.1 eV and the estimated linear field parameter (γ) is 0.03 V⁻¹. Reliability parameters estimated from the test structure data are used to predict the FIT rate for a product level FET using linear scaling of the gate width. Further, the effect of a burn-in and a transient voltage under a duty cycle on the FIT rate are modeled. The FIT rate of the product level FET is larger than that of the test structure. The burn-in and transient voltage similarly reduce the reliability. Contour plots are given that allow trade-offs between these factors in order to meet reliability requirements.     

Deep-level trapping centers in heterostructures for GaN field-effect transistors

Structural and electrophysical properties of heteroepitaxial gallium nitride layers on a sapphire substrate that are grown via the molecular beam epitaxy (MBE) method are studied. The parameters of deep-level trapping centers are determined by the method of the thermostimulated capacitor discharge; the degree of perfection of the film and substrate are determined by the two-crystal X-ray spectrometry method. The following structures are studied: i-GaN (1–2 μm)/GaN 〈Si〉(0.1−0.4 μm) and multilayer structures (Al_0.3Ga_0.7N-GaN-Al_0.3-Ga_0.7N-GaN-Al₂O₃) grown via the MBE method on a sapphire substrate. The effect of reactive ion etching on the energy spectrum of deep-level trapping centers in gallium nitride is studied. The obtained results are used to calculate the energy spectrum of defects in gallium nitride structures. Original Russian Text ? M.S. Andreev, L.E. Velikovskii, T.S. Kitichenko, T.G. Kolesnikova, A.P. Korovin, V.G. Mokerov, S.N. Yakunin, 2007, published in Radiotekhnika i Elektronika, 2007, Vol. 52, No. 7, pp. 880–887.     

Generation-recombination noise in GaN/AlGaN heterostructure fieldeffect transistors

Local levels with a large activation energy E_a~0.8-1.0 eV have been observed in low-frequency noise measurements of GaN/AlGaN heterostructure field effect transistors (HFETs and MOS-HFETs) grown on 4N-SiC substrates. The noise might come from the thin (30 nm) AlGaN barrier layer. The estimates of the level parameters based on this assumption resulted in reasonable values of capture cross section σ_n≈(10^-12-10^-13) cm² and trap concentration N_t≈5-10¹⁶ cm^-3     

DC characteristics of AZO/GaN heterojunction bipolar transistors

A new npn Al-doped zinc oxide (AZO)/GaN heterojunction bipolar transistor (HBT) is presented. The fabricated HBT with an emitter size of 120 x 120 μm² exhibits a current gain of 5.8 at V_BE = 3.0 V. The anomalous high current gain is observed at low current level due to the leakage current. The common-emitter output characteristics demonstrate DC current gain of 1.2. In addition, improved device characteristics are observed after the P₂S₅/(NH₄)₂S treatment.     

Effect of ohmic contacts on buffer leakage of GaN transistors

The effect of ohmic contacts on the buffer leakage of GaN transistors is presented. The buffer leakage for AlGaN/GaN high-electron mobility transistors and GaN MESFETs grown on the same underlying buffer was observed to be different. Controlled experiments show that the increased buffer leakage is due to the nature of the alloyed ohmic contacts and can be minimized if they are screened by the Si doping or by the two-dimensional electron gas.     

Microwave operation of GaN/AlGaN-doped channel heterostructurefield effect transistors

We report on the microwave operation of 1 μm gate AlGaN/GaN doped channel heterostructure field effect transistors (DC-HFET's) with the cutoff frequency f_T of 18.3 GHz. These devices exhibit the cutoff frequency-gate length product in excess of 18 GHz·μm, comparable to that of the state-of-the-art GaAs MESFET's. We explain these improvements in the device performance by the increased sheet carrier density in the device channel and by a reduction in the parasitic series resistances, caused by doping the device channel     

AlGaN/GaN high electron mobility transistors with InGaN back-barriers

A GaN/ultrathin InGaN/GaN heterojunction has been used to provide a back-barrier to the electrons in an AlGaN/GaN high-electron mobility transistor (HEMT). The polarization-induced electric fields in the InGaN layer raise the conduction band in the GaN buffer with respect to the GaN channel, increasing the confinement of the two-dimensional electron gas under high electric field conditions. The enhanced confinement is especially useful in deep-submicrometer devices where an important improvement in the pinchoff and 50% increase in the output resistance have been observed. These devices also showed excellent high-frequency performance, with a current gain cut-off frequency (f/sub T/) of 153 GHz and power gain cut-off frequency (f/sub max/) of 198 GHz for a gate length of 100 nm. At a different bias, a record f/sub max/ of 230 GHz was obtained.     

11.2 W/mm power density AlGaN/GaN high electron-mobility transistors on a GaN substrate

In this letter, high power density AlGaN/GaN high electron-mobility transistors (HEMTs) on a freestanding GaN substrate are reported. An asymmetric Γ-shaped 500-nm gate with a field plate of 650 nm is introduced to improve microwave power performance. The breakdown voltage (BV) is increased to more than 200 V for the fabricated device with gate-to-source and gate-to-drain distances of 1.08 and 2.92 μm. A record continuous-wave power density of 11.2 W/mm@10 GHz is realized with a drain bias of 70 V. The maximum oscillation frequency (f_max) and unity current gain cut-off frequency (f_t) of the AlGaN/GaN HEMTs exceed 30 and 20 GHz, respectively. The results demonstrate the potential of AlGaN/GaN HEMTs on free-standing GaN substrates for microwave power applications.     

High-energy proton irradiation effects on AlGaN/GaN high-electron mobility transistors

AlGaN/GaN high-electron mobility transistors (HEMTs) show decreases in extrinsic transconductance, drain-source current threshold voltage, and gate current as a result of irradiation with 40 MeV protons at doses equivalent to decades in low-earth orbit. The data are consistent with the protons creating deep electron traps that increase the HEMT channel resistance. Postirradiation annealing at 300°C was able to restore ∼70% of the initial g_m and I_DS values in HEMTs receiving proton doses of 5×10¹⁰ cm⁻².     

Investigation of flicker noise and deep-levels in GaN/AlGaN transistors

We report flicker noise measurements combined with deep-level transient spectroscopy of the doped and undoped channel GaN/AlGaN heterostructure field-effect transistors. The low-temperature noise spectra for the doped devices show clear generation-recombination peaks. The value of the activation energy extracted from these noise peaks is consistent with the activation energies measured using deep-level spectroscopy. Our results indicate that the input-referred noise spectral density of the undoped channel devices is much smaller (up to two orders of magnitude) than that of the doped channel devices with comparable electric characteristics. The additional defects due to doping add up to the generation-recombination and flicker noise.     

Photo-ionization spectroscopy of traps in AlGaN/GaN high-electron mobility transistors

Four different layer structures are used to study deep-level traps in AlGaN/GaN high-electron mobility transistors (HEMTs) by photo-ionization spectroscopy. The structures grown on sapphire substrates by metal-organic chemical vapor deposition show nearly identical Hall data. However, the direct current (DC) performance of HEMTs with identical geometry is found to differ strongly. In all structures investigated, two distinct defect levels, namely, at 2.84–2.94 eV and 3.24–3.28 eV, were found from the fits of the photo-ionization cross-sectional data. Additionally, different trap concentrations can be deduced. These are in good correlation with the different transconductance and drain current measured. It is assumed that the defect levels observed are related to the AlGaN surface.     

Role of stress voltage on structural degradation of GaN high-electron-mobility transistors

In GaN high-electron-mobility transistors, electrical degradation due to high-voltage stress is characterized by a critical voltage at which irreversible degradation starts to take place. Separately, cross-sectional TEM analysis has revealed significant crystallographic damage for severely degraded devices. Furthermore, a close correlation between the degree of drain current degradation and material degradation has been reported. However, the role of the critical voltage in physical degradation has not been explored. In this work, we investigate the connection between electrical degradation that occurs around and beyond the critical voltage and the formation of crystallographic defects through detailed electrical and TEM analysis, respectively. We find that a groove in the GaN cap starts to be generated around the critical voltage. At higher voltages, a pit develops that penetrates into the AlGaN barrier. The size of the pit increases with stress voltage. We also observe a good correlation between electrical and material degradation.     

Unpassivated GaN/AlGaN/GaN power high electron mobility transistors with dispersion controlled by epitaxial layer design

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si₃N₄ passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (V_DS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.