Yifeng Wu

AlGaN/GaN HEMTs-an overview of device operation and applications

Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

30-W/mm GaN HEMTs by field plate optimization

GaN high-electron-mobility-transistors (HEMTs) on SiC were fabricated with field plates of various dimensions for optimum performance. Great enhancement in radio frequency (RF) current-voltage swings was achieved with acceptable compromise in gain, through both reduction in the trapping effect and increase in breakdown voltages. When biased at 120 V, a continuous wave output power density of 32.2 W/mm and power-added efficiency (PAE) of 54.8% at 4 GHz were obtained using devices with dimensions of 0.55/spl times/246 /spl mu/m/sup 2/ and a field-plate length of 1.1 /spl mu/m. Devices with a shorter field plate of 0.9 /spl mu/m also generated 30.6 W/mm with 49.6% PAE at 8 GHz. Such ultrahigh power densities are a dramatic improvement over the 10-12 W/mm values attained by conventional gate GaN-based HEMTs.

GaN-Based RF Power Devices and Amplifiers

The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance. GaN-based RF power devices have made substantial progresses in the last decade. This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance. The reliability and manufacturing challenges are also discussed.

Very-high power density AlGaN/GaN HEMTs

Research work focusing on the enhancement of large-signal current-voltage (I-V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMTs. 100-150 /spl mu/m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited, 2-mm-wide devices flip-chip mounted on to AlN substrates produced 9.2-9.8 W output power at 8 GHz with 44-47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date.

A 97.8% Efficient GaN HEMT Boost Converter With 300-W Output Power at 1 MHz

A 175-to-350 V hard-switched boost converter was constructed using a high-voltage GaN high-electron-mobility transistor grown on SiC substrate. The high speed and low on-resistance of the wide-band-gap device enabled extremely fast switching transients and low losses, resulting in a high conversion efficiency of 97.8% with 300-W output power at 1 MHz. The maximum efficiency was 98.0% at 214-W output power, well exceeding the state of the art of Si-based converters at similar frequencies.

Gallium nitride based high power heterojunction field effect transistors: process development and present status at UCSB

The development of GaN based devices for microwave power electronics at the University of California, Santa Barbara (UCSB), is reviewed. From 1995 to 2000, the power performance of AlGaN/GaN-on-sapphire heterojunction field effect transistors improved from 1.1 W/mm to 6.6 W/mm, respectively. Compensating the disadvantages of the low thermal conductivity of the sapphire substrate through heat management via flip chip bonding onto AlN substrates, large periphery devices with an output power of 7.6 W were demonstrated. UCSB also fabricated the first GaN based amplifier integrated circuits. Critical issues involved in the growth of high quality AlGaN/GaN heterostructures by metal-organic chemical vapor deposition and the device fabrication are discussed.

GaN microwave electronics

The progress of AlGaN/GaN based HEMTs is reviewed. The mobility achieved in these modulation doped structures is over 1500 cm/sup 2/ V/sup -1/ s/sup -1/ at 300 K with sheet densities of over 1/spl times/1013 cm/sup -2/. Ft of over 50 GHz and fmax of over 90 GHz has been demonstrated. Power density of over 2.6 W/mm at 10 GHz has been achieved.

Cl2 reactive ion etching for gate recessing of AlGaN/GaN field-effect transistors

An effective gate recess etch process has been applied to AlGaN/GaN modulation-doped field-effect transistors (MODFETs), utilizing low power Cl2 reactive ion etching. In comparison to GaAs-based materials, GaN shows a greater robustness to ion damage under ion bombardment at very low ion energies (<70 V). It suggests that a viable gate recess etch process is possible. Recessed gate AlGaN/GaN MODFETs with gate to drain breakdown higher than −80 V have been demonstrated with an optimized lower power Cl2 reactive ion etching.

High Al-content AlGaN/GaN HEMTs on SiC substrates with very high power performance

With the promise of a new level of power performance at microwave frequencies, GaN-based high-mobility-transistors (HEMTs) have attracted sustained research effort and shown steadfast improvements. Previous state-of-the-art included power densities of 5.3-6.9 W/mm at 10 GHz from small devices, as well as a total output power of 9.1 W at 7.4 GHz from a 3-mm-wide device. This performance was obtained using AlGaN/GaN HEMTs grown on SiC substrates for superior heat dissipation. However, these devices used a low Al mole fraction of 14% in the AlGaN layer, which had been shown to be less preferred in an earlier study with devices grown on sapphire substrates. In this work, high Al content AlGaN/GaN HEMTs on SiC substrates are demonstrated with remarkable performance enhancement.

1 - 8-GHz GaN-based power amplifier using flip-chip bonding

We report the first gallium nitride (GaN)-based broad-band power amplifier. The circuit was fabricated on an AlN substrate using AlGaN-GaN power high-electron mobility transistors (HEMTs), grown on sapphire substrates, which were flip-chip bonded for thermal management. The amplifier employed a modified traveling-wave power amplifier (TWPA) topology that eliminated the backward wave of conventional TWPAs. Using four HEMTs each with 0.75-/spl mu/m gate length and 0.75-mm gate periphery, a small-signal gain of /spl sim/7 dB was obtained with a bandwidth of 1-8 GHz. At mid-band, an output power of 3.6 W was obtained when biased at V/sub ds/=18 V and 4.5 W when biased at V/sub ds/=22 V.