Hanxing Wang

Correction to “Thermally Stable Enhancement-Mode GaN Metal-Isolator-Semiconductor High-Electron-Mobility Transistor With Partially Recessed Fluorine-Implanted Barrier”

Al2O3/AlGaN/GaN enhancement-mode metalisolator-semiconductor high-electron-mobility transistor (MIS-HEMT) featuring a partially recessed (Al) GaN barrier was realized by a fluorine plasma implantation/etch technique. By properly adjusting the RF power driving the fluorine plasma, the fluorine plasma is able to produce two desirable results: 1) a well-controlled slow dry etching for gate recess and 2) implanting fluorine ions into the AlGaN barrier. The fluorine ions become negatively charged in the barrier layer and induce a positive shift in the threshold voltage. The proposed MIS-HEMT exhibits a threshold voltage (VTH) of +0.6 V at a drain current of 10 μA/mm, a maximum drive current of 730 mA/mm, an ON-resistance of 7.07 Ω · mm, and an OFF-state breakdown voltage of 703 V at an OFF-state drain leakage current of 1 μA/mm. From room temperature to 200 °C, the device exhibits a small negative shift of VTH (~0.5 V) that is attributed to the high-quality dielectric/F-implanted-(Al) GaN interface and the partially recessed barrier.

High-temperature low-damage gate recess technique and ozone-assisted ALD-grown Al 2 O 3 gate dielectric for high-performance normally-off GaN MIS-HEMTs

A high-temperature (180 °C) gate recess technique featuring low damage and in-situ self-clean capability, in combination with O₃-assisted atomic-layer-deposition (ALD) of Al₂O₃ gate dielectric, is developed for fabrication of high performance normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs), which exhibit a threshold voltage of +1.6 V, a pulsed drive current of 1.1 A/mm, and low dynamic ON-resistance under hard-switching operation. Chlorine-based dry-etching residues (e.g. AlCl₃ and GaCl₃) are significantly reduced by increasing the wafer temperature during the gate recess to their characteristic desorption temperature, while defective bonds like Al-O-H and positive fixed charges in ALD-Al₂O₃ are significantly suppressed by substitution of H₂O with O₃ precursor.

Dynamic Gate Stress-induced VTH Shift and its Impact on Dynamic RON in GaN MIS-HEMTs

Very fast transients of VTH shift and their impact on RON under dynamic AC (1 k-1 MHz) positive gate stress in depletion-mode (D-mode) metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) are revealed. We achieve data acquisition within 120 ns right after each stress pulse throughout the entire stress time range from 10-7 up to 103 s, by virtue of a short stress-to-sense delay of ~100 ns and high sampling rate up to 50 MSa/s. Despite the considerable VTH shift, its impact on RON in D-mode MIS-HEMT is modest, if the device is under sufficient gate overdrive. Furthermore, VTH shift and the consequent RON increase under dynamic stress, which are more relevant to high-frequency switching operation, exhibits frequency dependence within 1 k-1 MHz and is always smaller than that under conventionally used static (constant) stress.

A GaN Pulse Width Modulation Integrated Circuit for GaN Power Converters

We report the first gallium nitride (GaN)-based pulse width modulation (PWM) integrated circuit (IC) featuring monolithically integrated enhancement- and depletion-mode high electron mobility transistors and lateral field-effect rectifiers on the GaN smart power technology platform. The PWM IC is able to generate 1-MHz PWM signal with its duty cycle modulated effectively by a reference voltage (Vc) over a wide range with good linearity. It features a 5 V supply voltage and is composed of a sawtooth generator and a comparator, both of which can be operated at 1 MHz and exhibit proper functionality over a wide temperature range (from 25 °C to 250 °C). This circuit demonstration further proves the feasibility of an all-GaN solution that features monolithically integrated peripheral gate control circuits and power switches for GaN power converters. An all-GaN solution would lead to a compact system with improved efficiency and enhanced reliability.

Schottky-on-heterojunction optoelectronic functional devices realized on AlGaN/GaN-on-Si platform

We demonstrated that the metal-AlGaN/GaN Schottky diode is capable of producing GaN band-edge ultraviolet (UV) emission at 3.4 eV/364 nm under forward bias larger than ~2 V at room temperature. The underlying mechanism of the hole generation/injection and electroluminescence (EL) processes in this Schottky-on-heterojunction light-emitting diode (SoH-LED) was discussed based on the impact ionization of surface states presented in the (Al)GaN barrier layer. By replacing the conventional ohmic drain with a semitransparent Schottky drain, we demonstrated an AlGaN/GaN high-electron-mobility light-emitting transistor (HEM-LET) in which the drain current and EL emission are controlled simultaneously by gate voltage. Switching operation up to 120 MHz was obtained in SoH-LED to demonstrate its potential in providing high-speed on-chip light sources on the GaN electronic device platform.

III-Nitride transistors with photonic-ohmic drain for enhanced dynamic performances

Development of III-nitride high-voltage power devices is still challenged by deep traps that are inevitable in state-of-the-art AlGaN/GaN-on-Si epitaxial heterostructures. In this work, we report a heterojunction field-effect transistor featuring a photonic-ohmic drain, i.e. PODFET, on conventional AlGaN/GaN-on-Si power electronics platform. Photons are synchronously generated with the switching channel current, and they are capable of effectively pumping electrons from the deep surface/bulk traps during each switching cycle. Consequently, the dynamic ON-resistance of the PODFET is significantly reduced compared to that of a conventional ohmic-drain FET.

A GaN pulse width modulation integrated circuit

We report the first GaN-based pulse width modulation (PWM) circuit for integrated GaN gate driver. This circuit is composed of a sawtooth generator and a comparator, both of which exhibit stable operation at temperatures up to 250 °C and operate properly at 1 MHz. The PWM circuit is able to generate PWM signals whose duty cycle is effectively modulated over a wide range by a reference voltage. This successful demonstration suggests the possibility of an all-GaN solution for power converters by monolithically integrating GaN power switches with the peripheral gate drive circuits, leading to a compact solution with reduced parasitics and improved reliability.

An Analytical Model for False Turn-On Evaluation of High-Voltage Enhancement-Mode GaN Transistor in Bridge-Leg Configuration

Compared with the state-of-the-art Si-based power devices, enhancement-mode Gallium Nitride (E-mode GaN) transistors have better figures of merit and exhibit great potential in enabling higher switching frequency, higher efficiency, and higher power density for power converters. The bridge-leg configuration circuit, consisting of a controlling switch and a synchronous switch, is a critical component in many power converters. However, owing to the low threshold voltage and fast switching speed, E-mode GaN devices are more prone to false turn-on phenomenon in bridge-leg configuration, leading to undesirable results, such as higher switching loss, circuit oscillation, and shoot through. In order to expand gate terminals safe operating margin without increasing reverse conduction loss during deadtime, negative gate voltage bias for turn-off and antiparallel diode could be applied to E-mode GaN device. In this paper, with consideration of strong nonlinearities in C–V and I–V characteristics of high-voltage (650 V) E-mode GaN transistors, analytical device models are first developed. Then, we develop an analytical circuit model that combines the circuit parameters with intrinsic characteristics of the high-voltage GaN transistor and antiparallel diode. Thus, key transient waveforms with regard to the false turn-on problem can be acquired, including displacement current and false triggering voltage pulse on gate terminal. The simulated waveforms are then verified on a testing board with GaN-based bridge-leg circuit. In contrast to piecewise switching process models and PSpice simulation, the proposed model exhibits outstanding performances. To provide design guidelines for mitigating false turn-on of GaN transistor, the impacts of different circuit parameters, along with the optimum negative gate voltage bias, are investigated based on the proposed model.

Normally-off GaN MIS-HEMT with improved thermal stability in DC and dynamic performance

We report normally-off Al2O3/AlGaN/GaN metal-isolator-semiconductor high-electron-mobility transistor (MIS-HEMT) with improved thermal stability in DC and dynamic performance. The MIS-HEMTs featuring a partially recessed (Al)GaN barrier were realized by a fluorine-plasma implantation/etch technique. Both the well-controlled slow dry etching for gate recess and implanting fluorine ions into the AlGaN barrier are carried out with CF4 plasma at a relative high RF driving power. The partially recessed barrier leads to improved thermal stability, while the fluorine implantation can convert the device from depletion-mode to enhancement-mode without completely removing the barrier and sacrificing the high mobility heterojunction channel. From room temperature to 200 °C, the device exhibits improved thermal stability with a small negative shift of VTH (~0.5 V) that is attributed to the high-quality dielectric/F-implanted-(Al)GaN interface and the partially recessed barrier.

Impact of Vth shift on Ron in E/D-mode GaN-on-Si power transistors: Role of dynamic stress and gate overdrive

In this work, we revealed the impacts of V_TH shift on R_ON in enhancement-/depletion-mode (E/D-mode) GaN transistors under dynamic AC (1 k-1 MHz) stress. With newly developed fast dynamic characterization techniques, we achieved data acquisition within 120 ns after each stress pulse throughout the entire stress time ranging from 10^-7 s up to 10³ s. V_TH shift and the consequent R_ON increase under dynamic stress, which are more relevant to high-frequency switching operation, exhibits frequency dependence and is smaller than that under conventionally used static stress. Furthermore, distinctive R_ONs susceptibilities to V_TH shift between E-mode and D-mode GaN transistors are revealed and analyzed. The correlation between V_TH shift and R_ON facilitates the assessment on how much V_TH shift a GaN power switch can tolerate.