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

GaN-Based RF Power Devices and Amplifiers

Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.

https://science.sciencemag.org/content/sci/327/5961/60.full.pdf

Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500/spl deg/C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (g/sub m/) was obtained at V/sub gs/=0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At V/sub gs/=0 V, the off-state drain leakage current is 28 /spl mu/A/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 /spl mu/m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak g/sub m/ of 148 mS/mm, a current gain cutoff frequency f/sub T/ of 10.1 GHz and a maximum oscillation frequency f/sub max/ of 34.3 GHz.

High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment

This paper presents a method with an accurate control of threshold voltages (Vth) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from -4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 degC. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results confirm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs

Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode

A self-aligned "slant-field-plate" technology is presented as an improvement over the discrete multiple field plates for high breakdown voltage AlGaN/GaN HEMTs. Devices were tested in Fluorinert to eliminate the breakdown of air, which was identified to limit the breakdown voltage in AlGaN/GaN HEMTs. A single integrated field plate, which is self-aligned with the gate, is shown to support more than a kilovolt breakdown voltage (Vbr up to 1900 V was measured with Fluorinert). Devices made with this technology show a good large signal-frequency behavior. Various issues regarding breakdown measurements and interpretation of measurement results are presented

High Breakdown Voltage Achieved on AlGaN/GaN HEMTs With Integrated Slant Field Plates

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 322 W/mm and power-added efficiency (PAE) of 548% at 4 GHz were obtained using devices with dimensions of 055/spl times/246 /spl mu/m/sup 2/ and a field-plate length of 11 /spl mu/m Devices with a shorter field plate of 09 /spl mu/m also generated 306 W/mm with 496% 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

30-W/mm GaN HEMTs by field plate optimization

Recently, electric field modification with GaN-based high-electron-mobility-transistors (HEMTs) using field plates (FP) has resulted in dramatically enhanced power performance. Power densities up to 32 W/mm at 4 GHz have been demonstrated with power-added-efficiency (PAE) of 55%. When scaled to a large periphery, a total output power of 149 W was obtained at 2 GHz. Modern communication applications also require high linearity for power devices. Here we present the linearity performance of GaN-channel HEMTs with various FP lengths at biases up to 108V.

Linearity performance of GaN HEMTs with field plates

The enabling features and performance of GaN based HEMTs as a high power, high bandwidth semiconductor technology are presented. Progress on materials development includes the development of AlGaN and AlN barrier HEMTs with room temperature electron mobility exceeding 2000 cm/sup 2//V-s. Trap free GaN HEMT devices with > 10 W/mm power density and devices with > 70 % efficiency are presented. Operation at > 200 /spl deg/C is reported. Simultaneous linearity and efficiency under class B is presented followed by discussion of mm-wave power performance. Finally, device scaling resulting in a total power > 100 Watts and GaN HEMT circuit demonstrations are presented including mm-wave amplifier with > 3 Watts at 30 GHz and 35 GHz.

AlGaN-GaN HEMTs: material, device, circuit technology and applications

With the rapid progress and maturation over the last several years, wide bandgap GaN-based HEMTs are now regarded as the next generation technology leader for high frequency and high power device applications as stated in U. K. Mishra et al. (2002). The Al/sub x/Ga/sub 1 - x/N-GaN heterostructure system enables high voltage, high current operation, resulting in the demonstration of > 10/spl times/ power performance than GaAs and Si technologies. The high RF power density (W/mm) translates into high watts per unit capacitance (W/pF), resulting in high impedance and simpler matching, an enabler for wide-bandwidth applications. This paper reviews the recent progress in material, device and circuit performance of GaN HEMTs by the Cree team.

R.P. Smith

Papers

30-W/mm GaN HEMTs by field plate optimization

Linearity performance of GaN HEMTs with field plates

AlGaN-GaN HEMTs: material, device, circuit technology and applications

GaN HEMTs: material, device, circuit technology and applications