Hyungtak Kim

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm/sup 2//Ns, respectively, For electron sheet density near 1/spl times/10/sup 13//cm/sup 2/, Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f/sub t/ of 106 GHz for 0.15 /spl mu/m gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of /spl sim/4 W/mm, while large periphery devices have /spl sim/2 W/mm, both thermally limited. Performance, without and with Si/sub 3/N/sub 4/ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates.

Influence of barrier thickness on the high-power performance of AlGaN/GaN HEMTs

The dependence of current slump in AlGaN/GaN HEMTs on the thickness of the AlGaN barrier was observed. Power measurements on a 2/spl times/125/spl times/0.3 /spl mu/m AlGaN/GaN HEMT made on Silicon Carbide (SiC) substrates with an AlGaN thickness of 10 nm gave a saturated output power of 1.23 W/mm at 8 GHz whereas a device with the same dimensions fabricated on samples with an AlGaN barrier of 20 nm gave a saturated output power of 2.65 W/mm at the same frequency. RF load line measurements clearly show the reduction of RF full channel current as compared to dc full channel current and the increase in the RF knee voltage compared to the dc knee voltage, with the effect being more pronounced in thin barrier samples. Passivation improved the large signal performance of these devices. A 1/spl times/150/spl times/0.3 /spl mu/m transistor made on AlGaN(20 nm)/GaN structure gave a saturated output power of 10.7 W/mm (40% power added efficiency) at 10 GHz after passivation. This represents the state of the art microwave power density for AlGaN/GaN HEMTs. Heating of the transistors during high-power operation of these devices becomes the important factor in limiting their performance after passivation.

Effects of SiN passivation and high-electric field on AlGaN-GaN HFET degradation

The authors report on the effects of silicon nitride (SiN) surface passivation and high-electric field stress (hot electron stress) on the degradation of undoped AlGaN-GaN power HFETs. Stressed devices demonstrated a decrease in the drain current and maximum transconductance and an increase in the parasitic drain series resistance, gate leakage, and subthreshold current. The unpassivated devices showed more significant degradation than SiN passivated devices. Gate lag phenomenon was observed from unpassivated devices and removed by SiN passivation. However, SiN passivated devices also showed gate lag phenomena after high-electric field stress, which suggests possible changes in surface trap profiles occurred during high-electric field stress test.

High-power broad-band AlGaN/GaN HEMT MMICs on SiC substrates

Broadband, high power cascode AlGaN/GaN HEMT MMIC amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small signal gain of 19 dB was realized. A broadband amplifier MMIC using these cascode cells in conjunction with a lossy-match input matching network was designed, fabricated, and evaluated, showing a useful operating range of DC-8 GHz with an output power of 5-7.5 W and a PAE of 20-33% respectively. A nonuniform distributed amplifier (NDA) based on this same process yielded an output power of 3-6 W over a DC-8 GHz bandwidth with an associated PAE of 13-31%.

Reliability Evaluation of High Power AlGaN/GaN HEMTs on SiC Substrate

Undoped AlGaN/GaN HEMTs grown on SiC substrates have recently demonstrated record output power density of 10.7 W/mm (cw) and total output power of 10 W at 10 GHz (L. F. EASTMAN, Joint ONR/MURI Review (5/15-16, 2001), CA (USA) [1]). In this paper, we present the results of reliability tests performed on undoped AlGaN/GaN HEMTs on SiC under dc and rf stress conditions. Undoped AlGaN/GaN HEMTs on SiC substrates have been submitted to on-wafer dc and rf stress conditions at a room temperature and the degradation in device performance induced by hot electron and thermal effects have been observed.

High-field effects in silicon nitride passivated GaN MODFETs

This paper presents a detailed study of high-field effects in GaN MODFETs. Degradation of DC characteristics and change of flicker noise due to hot electron and high-reverse current stresses in Si/sub 3/N/sub 4/ passivated GaN MODFETs have been investigated. The authors observe that during hot electron stress, electron trapping in the barrier layer and interface state creation occur. These cause a positive shift of V/sub t/, reduce I/sub D/, skew the transfer characteristics, and degrade g/sub m/. Flicker noise (1/f) measurements show that after hot electron stress, the scaled drain current noise spectrum (S/sub I(D)//I/sub D//sup 2/) decreases in depletion, but increases only slightly in strong accumulation, corroborating the creation of interface states but only a small creation of transition-layer tunnel traps that contribute to 1/f noise. During high-reverse current stress, electron trapping dominates for the first 50-60 s and then hole trapping and trap creation begin to manifest. However, there still is net electron trapping under the gate after one hour of stress. The degradation processes bring about a positive shift of V/sub t/, degrade I/sub D/ and g/sub m/, and increase reverse leakage. After high-reverse current stress, S/sub I(D)//I/sub D//sup 2/ increases substantially in strong accumulation, indicating the creation of transition layer tunnel traps.

High power monolithic AlGaN/GaN HEMT oscillator

A monolithic X-band oscillator, based on AlGaN/GaN HEMT with 1.5 mm total gate periphery, has been designed, fabricated and characterized. The oscillator delivers 1.7 W at 9.556 GHz into 50 ohm load when biased at V/sub ds/=30 V and V/sub gs/=-5V, with a DC-to-RF efficiency of 16%. Phase noise was estimated to be -87dBc/Hz at 100 kHz offset and 30 kHz bandwidth. Experimental results show great promise for AlGaN/GaN HEMT MMIC technology to be used in future high power microwave source applications.

High-power broadband AlGaN/GaN HEMT MMICs on SiC substrates

Broadband, high power cascode AlGaN/GaN HEMT MMIC amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small signal gain of 19 dB was realized. A broadband amplifier MMIC using these cascode cells in conjunction with a lossy-match input matching network was designed, fabricated, and evaluated, showing a useful operating range of DC-8 GHz with an output power of 5-7.5 W and a PAE of 20-33% respectively. A nonuniform distributed amplifier (NDA) based on this same process yielded an output power of 3-6 W over a DC-8 GHz bandwidth with an associated PAE of 13-31%.

High-Quality ICPCVD

We have developed a high-quality SiO2 deposition process using an inductively coupled plasma chemical vapor deposition system for use as a gate oxide of AlGaN/GaN-on-Si metal-oxide-semiconductor heterostructure field-effect transistor (MOSHFET) for power switching applications. A breakdown field of ~12 MV/cm was achieved using the optimized deposition conditions that were successfully applied in fabrication of the normally off AlGaN/GaN-on-Si MOSHFETs. The fabricated device exhibited excellent characteristics: a maximum drain current density of 375 mA/mm, a threshold voltage of 3 V, and a breakdown voltage of 820 V.

\hbox{SiO}_{2}

AlGaN-GaN high electron mobility transistors (HEMTs) have shown great potential for high temperature/high power electronics. However, the study on the reliability of GaN-based devices is still at the initial stages. In this work, we report the degradation characteristics of AlGaN HEMTs under various stress conditions such as DC stress (gate current extraction and hot electron cycles) and RF input drive stress at room temperature. While AlGaN-GaN HEMTs have shown robust reliabilities under gate current extraction stress, degradation due to hot carriers was observed under hot electron cycles and RF input drive stress. Si/sub 3/N/sub 4/ passivation was found to provide better reliability than SiO/sub 2/ passivation under DC stress and RF input drive stress.