Thomas R. Prunty

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.

Microwave performance of AlGaN/GaN metal insulator semiconductor field effect transistors on sapphire substrates

Metal-insulator-semiconductor field effect transistors (MISFETs) from surface-passivated undoped AlGaN/GaN heterostructures on sapphire were fabricated. Measured static output characteristics includes full channel currents (I/sub dss/) of roughly 750 mA/mm with gate-source pinchoff voltages of -10 V and peak extrinsic transconductancies (g/sub m/) of 100-110 mS/mm. Increased surface roughness resulting from a gate recess process to reduce the pinchoff voltage introduces gate leakage currents in the micro-amps regime. With evidence for reduced dc-to-rf dispersion from pulsed gate transfer characteristics, these devices at 4 GHz with 28.0 V bias generated maximum output power densities of 4.2 W/mm with 14.5 dB of gain and 36% power added efficiency.

An AlGaN/GaN high-electron-mobility transistor with an AlN sub-buffer layer

The AlGaN/GaN high-electron-mobility transistor requires a thermally conducting, semi-insulating substrate to achieve the best possible microwave performance. The semi-insulating SiC substrate is currently the best choice for this device technology; however, fringing fields which penetrate the GaN buffer layer at pinch-off introduce significant substrate conduction at modest drain bias if channel electrons are not well confined to the nitride structure. The addition of an insulating AlN sub-buffer on the semi-insulating SiC substrate suppresses this parasitic conduction, which results in dramatic improvements in the AlGaN/GaN transistor performance. A pronounced reduction in both the gate-lag and the gate-leakage current are observed for structures with the AlN sub-buffer layer. These structures operate up to 50 V drain bias under drive, corresponding to a peak voltage of 80 V, for a 0.30 µm gate length device. The devices have achieved high-efficiency operation at 10 GHz (>70% power-added efficiency in class AB mode at 15 V drain bias) and the highest output power density observed thus far (11.2 W mm-1). Large-periphery devices (1.5 mm gate width) deliver 10 W (continuous wave) of maximum saturated output power at 10 GHz. The growth, processing, and performance of these devices are briefly reviewed.

1.5-kV and 2.2-m \(\Omega \) -cm \(^{2}\) Vertical GaN Transistors on Bulk-GaN Substrates

In this letter, vertical GaN transistors fabricated on bulk GaN substrates are discussed. A threshold voltage of 0.5 V and saturation current >2.3 A are demonstrated. The measured devices show breakdown voltages of 1.5 kV and specific ON-resistance of 2.2 mΩ-cm 2 , which translates to a figure-of-merit of V BR 2 /R ON ~1 × 10 9 V 2 Ω -1 · cm -2 .

Vertical Power p-n Diodes Based on Bulk GaN

There is a great interest in wide-bandgap semiconductor devices and most recently in monolithic GaN structures for power electronics applications. In this paper, vertical p-n diodes fabricated on pseudobulk low defect density (104-106 cm-2) GaN substrates are discussed. Homoepitaxial low-pressure metal organic chemical vapor deposition growth of GaN on its native substrate and being able to control and balance the n-type Si doping with background C impurity has allowed the realization of vertical device architectures with drift layer thicknesses of 6 to 40 μm and net carrier electron concentrations of 4 × 1015 to 2.5 × 1016 cm-3. This parameter range is suitable for applications requiring breakdown voltages (BVs) of 600 V-4 kV with a proper edge termination strategy. Measured devices demonstrate near power device figure of merit, that is, differential specific on-resistance (Rsp) of 2 mΩcm2 for a BV of 2.6 kV and 2.95 mΩcm2 for a 3.7-kV device, respectively. The improvement in the substrate quality over the last few years has resulted in the fabrication of diodes with areas as large as 16 mm2, with BVs exceeding 700 V and pulsed (100 μs) currents of 400 A. The structures fabricated are utilized to study in detail the temperature dependency of I-V characteristics, impact ionization and avalanche characteristics, and extract (estimate) modeling parameters such as electron mobility in the GaN c-direction (vertical) and hole minority carrier lifetimes. Some insight into device reliability is also provided.

Growth and passivation of AlGaN/GaN heterostructures

Abstract The undoped AlGaN/GaN heterostructure (Ga-face) produces a polarization induced two-dimensional electron gas (2DEG) at the heterointerface. The as-grown structure has a fixed positive charge on its surface to oppose the negative polarization charge on the top AlGaN layers surface. Although sensitive to atmospheric conditions this positive surface charge is not sufficient in density to prevent significant depletion of the 2DEG. The ideal surface passivation is an insulator, acting as an encapsulant, which provides a fixed positive charge to neutralize the AlGaN polarization charge. We demonstrate that certain types of silicon nitride thin films can result in a charge balance and virtually eliminate depletion of the underlying 2DEG. A simple polarization model is used to explain the Hall and capacitance–voltage data on AlGaN/GaN heterostructures grown by organometallic vapor phase epitaxy. By changing the passivation layer thickness, the surface depletion can be systematically controlled, allowing the dependence of electron mobility on two-dimensional charge densities to be determined for samples grown on sapphire and SiC substrates. A metal–insulator–semiconductor structure of this type can be used to deplete the electrons induced at the AlGaN/GaN interface allowing insulated gate transistors to be realized.

3.7 kV Vertical GaN PN Diodes

There is a great interest in wide band-gap semiconductor devices for power electronics application. In this letter, vertical GaN p-n diodes fabricated on bulk GaN substrates are discussed. The device layers are grown by MOCVD on low defect density (104 cm-2) bulk GaN substrates. The measured devices show breakdown voltages of 3.7 kV with an area differential specific on-resistance (Rsp) of 2.95 mΩ-cm2.

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.

Hot electron induced degradation of undoped AlGaN/GaN HFETs

Abstract This work presents the effects of hot electron stress on the degradation of undoped Al 0.3 GaN 0.7 /GaN power HFET’s with SiN passivation. Typical degradation characteristics consist of a decrease in the drain current and maximum transconductance, an increase in the drain series resistance, gate leakage and a subthreshold current. Degradation mechanism has been investigated by means of gate lag measurements (pulsed I – V ) and current-mode deep level transient spectroscopy (DLTS). Stressed devices suffered from aggravated drain current slump (DC to RF dispersion) which indicates possible changes in surface charge profiles occurred during hot electron stress test. The DLTS was used to identify the trap creation by hot electron stress. The DLTS spectra of stressed device revealed the evidence of trap creation due to hot electron stress.