S.C. Binari

Trapping effects and microwave power performance in AlGaN/GaN HEMTs

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 /spl mu/m gate length, an f/sub T/ of 30 GHz and an f/sub max/ of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

Trapping effects in GaN and SiC microwave FETs

It is well known that trapping effects can limit the output power performance of microwave field-effect transistors (FETs). This is particularly true for the wide bandgap devices. In this paper we review the various trapping phenomena observed in SiC- and GaN-based FETs that contribute to compromised power performance. For both of these material systems, trapping effects associated with both the surface and with the layers underlying the active channel have been identified. The measurement techniques utilized to identify these traps and some of the steps taken to minimize their effects, such as modified buffer layer designs and surface passivation, are described. Since similar defect-related phenomena were addressed during the development of the GaAs technology, relevant GaAs work is briefly summarized.

Current collapse and the role of carbon in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy

The two deep traps responsible for current collapse in AlGaN/GaN high electron mobility transistors grown by metalorganic vapor-phase epitaxy have been studied by photoionization spectroscopy. Varying the growth pressure of the high resistivity GaN buffer layer results in a change in the deep trap incorporation that is reflected in the observed current collapse. Variations in the measured trap concentrations with growth pressure and carbon incorporation indicate that the deepest trap is a carbon-related defect, while the mid-gap trap may be associated with grain boundaries or dislocations.

Observation of Deep Traps Responsible for Current Collapse in GaN Metal-Semiconductor Field-Effect Transistors

Deep traps responsible for current collapse phenomena in GaN metal–semiconductor field-effect transistors have been detected using a spectroscopic technique that employs the optical reversibility of current collapse to determine the photoionization spectra of the traps involved. In the n-channel device investigated, the two electron traps observed were found to be very deep and strongly coupled to the lattice. Photoionization thresholds for these traps were determined at 1.8 and at 2.85 eV. Both also appear to be the same traps recently associated with persistent photoconductivity effects in GaN.

GaN FETs for microwave and high-temperature applications

Abstract The d.c., microwave, and high-temperature characteristics of Si-doped MESFETs with and without n+ ohmic contact layers, Si 3 N 4 GaN MISFETs, and AlN GaN HFETs are presented. The highest transconductance and microwave performance were observed for 1 μm gate-length HFETs. These HFETs have a transconductance of 45 mS/mm, an ƒ τ of 8 GHz, and an ƒ max of 22 GHz. The Si-doped MESFETs have good pinch-off characteristics at 400°C and are operational at 500°C. Published by Elsevier Science Ltd.

Fabrication and characterization of GaN FETs

Abstract The current status of GaN-based FET technology and performance is reviewed. The fabrication details and the dc and microwave characteristics of GaN MESFETs that utilize Si-doped channels on semi-insulating buffer layers are presented. MESFETs with a 0.8 μm gate have exhibited an f T and f max of 6 and 14 GHz, respectively. These devices have excellent pinchoff characteristics and a source-drain breakdown voltage of over 85 V. A high-field current-collapse phenomenon is observed in these MESFETs in the absence of light. The characteristics of this current collapse as a function of temperature, illuminating wavelength, and time are described. A model describing the current collapse in terms of hot electron injection into the buffer layer is presented.

H, He, and N implant isolation of n‐type GaN

The effect of ion‐implantation‐induced damage on the resistivity of n‐type GaN has been investigated. H, He, and N ions were studied. The resistivity as a function of temperature, implant concentration, and post‐implant annealing temperature has been examined. Helium implantation produced material with an as‐implanted resistivity of 1010 Ω‐cm. He‐implanted material remained highly resistive after an 800 °C furnace anneal. The damage associated with H implantation had a significant anneal stage at 250 °C and the details of the as‐implanted resistivity were sample dependent. N implants had to be annealed at 400 °C to optimize the resulting resistivity but were then thermally stable to over 800 °C. The 300 °C resistivity of thermally stabilized He‐ and N‐ implanted layers was 104 Ω‐cm, whereas for H‐implanted layers the 300 °C resistivity was less than 10 Ω‐cm.

Improved reliability of AlGaN-GaN HEMTs using an NH/sub 3/ plasma treatment prior to SiN passivation

A passivation method has been developed which reduces the degradation of AlGaN-GaN high electron mobility transistor (HEMT) electrical properties caused by extended dc bias or microwave power operation. The key aspect of this passivation technique is exposure to a low-power NH/sub 3/ plasma prior to SiN deposition. Devices fabricated with the NH/sub 3/ treatment prior to SiN passivation show minimal gate lag and current collapse after extended dc bias operation. In addition, the rate of degradation of the microwave power output while under continuous microwave operation is improved by at least 100 times as compared to SiN passivated HEMTs that were not treated with the NH/sub 3/ plasma.

Persistent photoconductivity in n-type GaN

Persistent photoconductivity has been observed in n-type GaN:Si. The effect is seen at room temperature in both nonoptimally grown films as well as in device quality channel layers. The relaxation dynamics are found to agree with a stretched exponential model of recovery. A comparison between different samples, based upon stretched exponential parameters, Hall measurements, and photoluminescence data is made. The data suggest that the cause of persistent photoconductivity is the same among the different samples and that there is a transition in the relaxation dynamics between room temperature and 130 °C.

Photoionization spectroscopy of traps in GaN metal-semiconductor field-effect transistors

Measurements of the spectral and intensity dependences of the optically-induced reversal of current collapse in a GaN metal-semiconductor field-effect transistor (MESFET) have been compared to calculated results. The model assumes a net transfer of charge from the conducting channel to trapping states in the high-resistivity region of the device. The reversal, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built-in electric field associated with the trapped charge distribution. For a MESFET in which two distinct trapping centers have been spectrally resolved, the experimentally measured dependence upon light intensity was fitted using this model. The two traps were found to have very different photoionization cross-sections but comparable concentrations (4×1011 cm−2 and 6×1011 cm−2), suggesting that both traps contribute comparably to the observed current collapse.