Jeffrey B. Shealy

AlGaN/GaN HFET reliability

High-voltage AlGaN/GaN HFETs can produce high RF output power with nearly ideal power-added efficiency. But widespread adoption of these HFETs has been limited by a lack of acceptable reliability data for practical communications and radar applications. Device problems that have been observed include dc current and RF output power degradation as a function of time when the device is operating. Sudden and permanent degradation shifts in device performance have also been observed under certain operating conditions. Identified causes of the reliability problems include the quantum mechanical tunneling of electrons on the gate electrode to the surface of the semiconductor adjacent to the gate on the drain side, and a defect generation mechanism that occurs at a high, critical electric field. The gate leakage phenomenon described in this article produces electrons on the surface of the AlGaN layer adjacent to the gate electrode, and this creates a negative charge layer that partially depletes the conducting channel, thereby producing a degradation in dc current and RF output power. The gate leakage current is present when the device is biased and driven with an RF signal, and therefore the charge accumulation increases as a function of operation time. The gate tunnel current is a very sensitive function of surface state density, particularly near the gate edge, and of the magnitude of the electric field at this location. In addition, at a critical magnitude of the electric field defects in the AlGaN layer are created due to mechanical stress on the crystal structure, and these defects act as charge trapping centers. This mechanism is not well understood at this time and is currently the subject of research and investigation. Parameters that affect reliability are a function of device design and surface processing. Improvements in device reliability have been achieved through design modifications to produce improved surface passivation layers that reduce the gate and surface leakage currents and further modifications to reduce the magnitude of the electric field internal to the device. Continuing reliability study is required to fully elucidate the link between observed degradation behavior and physical failure mechanisms in a statistically significant manner.

Low-phase noise AlGaN/GaN FET-based voltage controlled oscillators (VCOs)

The first report of AlGaN/GaN HEMT-based voltage controlled oscillators (VCOs) is presented. Varactor-tuned oscillators implemented using distributed networks oscillate at 6 GHz with high output power (0.5 W), low-phase noise (-92 dBc/Hz SSB noise at 100 kHz offset), and high-tuning bandwidth (10%). The measured phase noise of AlGaN/GaN FETs is compared to the phase noise of GaAs FET and GaAs HBTs at 6 GHz, indicating the AlGaN/GaN FET exhibits equivalent SSB noise to GaAs FETs. These results indicate high power AlGaN/GaN-based VCOs may be used to simplify the line up in a communication radio, while improving the overall efficiency of the radio.

History of GaN: High-Power RF Gallium Nitride (GaN) from Infancy to Manufacturable Process and Beyond

In the early 1990s, gallium nitride (GaN) was deemed an excellent, next generation, semiconductor material for high power/high frequency transistors based on the material parameters of bandgap, electron mobility, and saturated electron velocity (Figure 1). The lack of bulk GaN source material led to the need for GaN growth on mismatched substrates such as Si, SiC and sapphire, but fundamental material development controlled the pace of maturation of GaN technology for both electronic and optoelectronic applications [1]. The development of GaN for RF electronics was significantly aided by the intense development that occurred in the race to first production of blue and, eventually, white light-emitting diodes (LEDs). Ultimately, advancements in the growth of device-grade aluminum gallium nitride (AlGaN)/GaN

Performance and RF Reliability of GaN-on-SiC HEMT's using Dual-Gate Architectures

AlGaN/GaN HEMTs on SiC have been fabricated with dual and single gate device geometries. Subthreshold characteristics and drain bias dependence of large signal parameters were compared to identify differences in electric field. Degradation under RF stress reveals the relative impact of temperature and electric field. The results illustrate the beneficial effects of the dual gate geometry for performance and reliability

Reliability testing of AlGaN/GaN HEMTs under multiple stressors

We performed an experiment on AlGaN/GaN HEMTs with high voltage and high power as stressors. We found that devices tested under high power generally degraded more than those tested under high voltage. In particular, the high-voltage-tested devices did not degrade significantly as suggested by some papers in the literature. The same papers in the literature also suggest that high voltages cause cracks and pits. However, the high-voltage-tested devices in this study do not exhibit cracks or pits in TEM images, while the high-power-tested devices exhibit pits.

Fundamental failure mechanisms limiting maximum voltage operation in AlGaN/GaN HEMTs

The authors report on the fundamental failure mechanisms limiting maximum applied voltage in AlGaN/GaN HEMTs. Device failure in high voltage off state conditions was studied by controlling drain leakage current and maximum applied drain voltage simultaneously. It was found that failure was associated with loss in gate control of channel current and a permanent degradation of gate diode leakage current. No permanent significant change until device failure was observed in ON-state parameters such as Ron, Idss and Idmax, thus distinguishing this failure mode from the inverse pieozo-electric effect as reported in literature.

RF Breakdown and Large-Signal Modeling of AlGaN/GaN HFET's

HFETs fabricated from nitride-based wide bandgap semiconductors can produce RF output power greater than an order of magnitude compared to devices fabricated from traditional semiconductors such as GaAs and InP. Nitride-based HFETs can support drain bias voltages in the range of 40-50 V, and have been biased as high as 120 V using device designs that make use of field-plate technology. However, the RF power produced by these devices is still limited by breakdown phenomena. Breakdown can occur at the gate electrode on the drain side due to a tunnel leakage mechanism, and by RF breakdown in the conducting channel. In this work it is demonstrated that RF breakdown in the conducting channel is the primary mechanism limiting the RF power performance of these devices. Gate leakage current under RF drive produces reliability degradation. A new large signal model that includes RF breakdown is proposed, and it is demonstrated that the new model produces excellent agreement with experimental data

Optimization of Gallium nitride high power technology for commercial and military applications

Next generation commercial and military systems require high power amplifiers (HPAs) with superior performance such as higher efficiency, improved thermal performance, wider bandwidth and higher output power. Using an optimized 0.5um, 48V GaN-on-SiC process, a family of GaN power amplifiers are developed for applications in the frequency range of 30MHz to 4GHz and output power ranging from 8W to 500W. Such devices clearly demonstrate superior power-bandwidth product of GaN for military applications such as radar, military communications and electronic warfare. For commercial applications, a family of linear amplifiers, applicable to 3GPP, LTE and WiMax cellular base stations, offer high efficiency operation. Finally, an optimized GaN process is utilized to develop new cable TV power doubler modules offering 6dB improvement in CIN performance over incumbent GaAs based amplifiers.

Voltage Dependent Characteristics of 48V AlGaN/GaN High Electron Mobility Transistor Technology on Silicon Carbide

Gallium nitride based HEMTs are a promising technology for high voltage, high power, high frequency applications. In addition to the potential for high operating voltage, this technology may also be suited for applications that utilize modulation of the drain voltage to improve overall amplifier efficiency. RFMD has developed a GaN HEMT technology platform on semi-insulating SiC substrates. This technology includes a 0.5 mum gate process and advanced field plate designs to maximize device performance. We report on devices from this technology, operated over a range of drain bias conditions. Performance and reliability results illustrate the compatibility of this device technology for high voltage and variable voltage applications.

GaN Wide Band Power Integrated Circuits

Gallium nitride (GaN) amplifiers have demonstrated very high power density as well as wide bandwidth in previous research. This paper examines their use in supplying flat gain, power, and linearity across a large bandwidth. It demonstrates two types of power amplifiers: a Ft Doubler (FT2) amplifier and a Cascode amplifier, both of which require a simple PCB tune. Both amplifiers show 0.2 to 4GHz bandwidth with 30 dBm PldB output power. The 3GPP WCDMA output power is 20 dBm at -45 dBc ACLR. Also, a WiMAX design is presented for the 3.2-3.8 GHz band to show the feasibility of a GaN HEMT amplifier in a relatively broad band, high linearity commercial application, with 27 dBm output at 2% EVM