Yi-Che Lee

Threshold voltage control of InAlN/GaN heterostructure field-effect transistors for depletion- and enhancement-mode operation

We describe a method to change the threshold voltage of heterostructure field-effect transistors (HFETs) using InxAl1−xN/GaN heterostructures by using polarization and strain modification in the InAlN barrier layer to realize enhancement-mode operation. The threshold voltage and electronic band structure of the heterostructures were calculated for different indium compositions in the InAlN layer. Band structure calculations predict the enhancement-mode operation of compressively strained InAlN/GaN HFETs with an indium composition higher than 0.25. Studies of InAlN/GaN HFETs with different In alloy compositions show that the sheet resistance increases and the carrier concentration decreases for the heterostructures with increasing indium composition due to changes in the compressive strain and polarization in the InAlN barrier layer. Fabricated HFETs show threshold voltages of −2.5, −0.75, and +0.2 V for In∼0.18Al∼0.82N/GaN, In∼0.22Al∼0.78N/GaN, and In∼0.25Al∼0.75N/GaN HFETs, respectively, corresponding to...

High-Current-Gain Direct-Growth GaN/InGaN Double Heterojunction Bipolar Transistors

We report high-current-gain n-p-n GaN/InGaN double-heterojunction bipolar transistors (DHBTs) using a direct-growth fabrication processing approach. The impact of the indium composition in the base layer was studied, and a burn-in effect using a constant-base-current stressing method was observed. We found that DHBTs with higher indium composition in the InGaN base layer may help reduce the base resistance and lower the surface recombination current but may result in higher bulk recombination current. A device burn-in effect was also studied. The postprocessing current stressing step helps increase free-hole concentration in the base, reduce the bulk recombination current, and enhance the current gain. As a result, a direct-growth GaN/In0.03Ga0.97N DHBT with a peak current gain of 105 and a collector current density > 6.5 kA/cm2 was demonstrated on a sapphire substrate.

NpN-GaN/InxGa1−xN/GaN heterojunction bipolar transistor on free-standing GaN substrate

Data and analysis are presented for NpN-GaN/InGaN/GaN double-heterojunction bipolar transistors (HBTs) grown and fabricated on a free-standing GaN (FS-GaN) substrate in comparison to that on a sapphire substrate to investigate the effect of dislocations in III-nitride HBT epitaxial structures. The performance characteristics of HBTs on FS-GaN exhibit a maximum collector current density of ∼12.3 kA/cm2, dc current gain of ∼90, and maximum differential gain of ∼120 without surface passivation, representing a substantial improvement over similar devices grown on sapphire. This is attributed to the reduction in threading dislocation density afforded by using a homoepitaxial growth on a high-crystalline-quality substrate. The minority carrier diffusion length increases significantly owing to not only a mitigated carrier trap effect via fewer dislocations, but also possibly reduced microscopic localized states.

A Remote-Oxygen-Plasma Surface Treatment Technique for III-Nitride Heterojunction Field-Effect Transistors

We present a paper on the influence of an oxygen-plasma treatment technique for III-nitride (III-N) heterojunction field-effect transistors (HFETs) using a plasma-enhanced atomic layer deposition (PE-ALD) system. The oxygen plasma is excited in a remote location to avoid the plasma damage. After the plasma treatment, the threshold voltage was shifted from -0.25 to 0 V and a two-orders-of-magnitude reduction of the drain leakage current was observed in recessed-gate AlGaN/AlN/GaN HFETs. The capacitance-voltage characteristics of gate diodes showed indistinguishable hysteresis. Improved gate-lag and lowered dynamic ON-resistance were also observed in the fabricated AlGaN/AlN/GaN HFETs after the oxygen-plasma treatment. The results indicate that the oxygen-plasma treatment using a PE-ALD system could be a promising approach to improve the device switching performance in III-N HFETs.

GaN/InGaN avalanche phototransistors

We report on III–nitride (III–N) avalanche phototransistor (APT) action by illuminating ultraviolet (UV) photons onto a GaN/InGaN npn heterojunction bipolar transistor in an open-base configuration. A high responsivity of >1 A/W was measured for the device operating at a collector-to-emitter voltage (VCE) of 68 A/W at VCE = 95 V. The InGaN APT demonstrates the feasibility of using III–N bipolar transistor structures for high-sensitivity UV photodetection applications.

Surface Leakage in GaN/InGaN Double Heterojunction Bipolar Transistors

We report a study on the surface-leakage current in GaN/InGaN double heterojunction bipolar transistors (DHBTs) that are grown on a sapphire substrate. Surface-leakage-current densities on an unpassivated DHBT are 9.6 times 10^-5 - 5.8 times 10^-4 A/cm for JC = 0.5-50 A/cm². A fabricated n-p-n GaN/InGaN DHBT shows the common-emitter dc current gain of 42, the collector-current density of 5.2 kA/cm², and the common-emitter breakdown voltage (BV_CEO) of 75 V.

Temperature-Dependent Characteristics of GaN Homojunction Rectifiers

We report a homojunction gallium nitride (GaN) p-i-n rectifier fabricated on free-standing GaN substrates with the breakdown voltage 800 V and low specific ON-resistance (R_ONA). At 298 K, RONA is 0.28 mΩ-cm² at the current density (J) of 2.5 kA/cm² and the corresponding Baligas figure of merit is 2.5 GW/cm². At a given temperature, R_ONA values decrease with J due to conductivity modulation in the drift region. The ambipolar lifetime (τ_a) is also determined by open-circuit voltage decay measurement. The value for τ_a is 9.6 ns at 298 K and it monotonically increases to 22 ns at 448 K. The reverse I-V measurement reveals the reverse leakage current mechanism is mainly attributed to a field-assisted ionization process from deep-level centers in the space-charge region. The analysis of T-I-V curve yields the Poole-Frenkel coefficient (~3.1 × 10^-4 eV · V^-1/2 · cm^-1/2) and a deep-level trap (~0.7 eV) at zero bias.

Working toward high-power GaN/InGaN heterojunction bipolar transistors

III-nitride (III-N) heterojunction bipolar transistors (HBTs) are a less-explored electronic device technology due to the myriad research issues in material growth, device design and fabrication associated with these devices. For III-N HBTs, npn-GaN/InGaN heterostructures provide the benefits of mitigating the poor base electrical conductivity of p-type GaN and the problematic magnesium incorporation issues. Consequently, InGaN-base III-N HBTs are promising for next-generation high-power RF III-N systems. This paper will describe the current development status of npn GaN/InGaN HBTs grown either on sapphire or free-standing (FS) GaN substrates using optimized metalorganic chemical vapor deposition (MOCVD) and refined HBT processing techniques. Recombination current paths in GaN/InGaN HBTs are studied and small-signal equivalent circuits are developed. The extracted device model indicates that, with further device fabrication technique development, Johnsons figure of merit (JFOM) of GaN/InGaN HBTs can be as high as 5 THz V.

GaN/InGaN Heterojunction Bipolar Transistors With

We report GaN/InGaN n-p-n double-heterojunction bipolar transistors with the collector current density (J_C) >; 16 kA/cm² and the current gain (β) >; 24 grown on a sapphire substrate. The cutoff frequency (f_T) of greater than 5 GHz and the maximum oscillation frequency (f_max) of 1.3 GHz are also measured for a GaN/InGaN heterojunction bipolar transistor at J_C = 4.7 kA/cm².

f_{T} > \hbox{5}\ \hbox{GHz}

We report an electroluminescence (EL) study on npn GaN/InGaN heterojunction bipolar transistors (HBTs). Three radiative recombination paths are resolved in the HBTs, corresponding to the band-to-band transition (3.3 eV), conduction-band-to-acceptor-level transition (3.15 eV), and yellow luminescence (YL) with the emission peak at 2.2 eV. We further study possible light emission paths by operating the HBTs under different biasing conditions. The band-to-band and the conduction-band-to-acceptor-level transitions mostly arise from the intrinsic base region, while a defect-related YL band could likely originate from the quasi-neutral base region of a GaN/InGaN HBT. The IB-dependent EL intensities for these three recombination paths are discussed. The results also show the radiative emission under the forward-active transistor mode operation is more effective than that using a diode-based emitter due to the enhanced excess electron concentration in the base region as increasing the collector current increases.