David Michael Fanning

Electrical and Thermal Performance of AlGaN/GaN HEMTs on Diamond Substrate for RF Applications

Thermal conductivity of the substrate affects the performance of high power RF devices. It is a dominant limiting factor in current state-of- the-art GaN HEMTs on SiC substrate. Due to high thermal conductivity, diamond substrate is an attractive alternative for GaN HEMTs. We have developed device quality GaN-on-diamond wafers using CVD diamond and fabricated 0.25 μm gate length HEMTs. We present detailed electrical and thermal results of the fabricated devices, which show RF power comparable to standard GaN-on-SiC HEMTs. We demonstrate over 25 % lower channel temperature for these devices compared to GaN-on-SiC devices. Electrical results using DC and RF tests and thermal results using IR thermography and micro-Raman spectroscopy are included.

Achieving the Best Thermal Performance for GaN-on-Diamond

GaN-based RF transistors offer impressive power densities, although to achieve the maximum potential offered by GaN, thermal management must be improved beyond the current GaN-on-SiC devices. By using diamond, rather than SiC substrates, transistor thermal resistance can be significantly reduced. It is important to experimentally verify thermal resistance, rather than relying solely on simulation expectations, using measurement results to aid further optimization. The novel thermal characterization methodology presented here combines Raman thermography and simulation to determine the substrate thermal conductivity and GaN/substrate thermal resistance in GaN-on-diamond devices. Measured GaN-on-diamond interfacial thermal resistance is similar to reported values for GaN-on-SiC, whereas the diamond substrate thermal conductivity is substantially higher, resulting in a significantly improved thermal resistance with respect to GaN-on-SiC, with great potential for further improvement.

High Efficiency Ka-Band Power Amplifier MMIC Utilizing a High Voltage Dual Field Plate GaAs PHEMT Process

The design and performance of a high efficiency Ka-band power amplifier MMIC utilizing a 0.15um high voltage GaAs PHEMT process (HV15) is presented. Experimental continuous wave (CW) in-fixture results for the power amplifier MMIC demonstrate up to 5W of saturated output power and 30% associated power added efficiency at 35GHz.

Development of Ka-Band GaAs pHEMTs with Output Power over 1 W/mm

We present GaAs pHEMTs demonstrating output power over 1 W/mm in Ka-band at an operating voltage of 8 V. DC, RF and reliability results are reported. Continuous wave load pull tests at 35 GHz show peak power added efficiency of 52 % and associated gain of 8 dB. The saturated output power of 1.2 W/mm is achieved. Power performance improvement is attributed to a new dielectrically defined 0.15 µm gate process which allows successful operation of these devices at a drain voltage of 8 V. Devices also show excellent small signal performance with maximum cut-off frequency as high as 107 GHz at a drain voltage of 1 V. Using three-temperature accelerated DC life tests, activation energy of 1.45 eV and median life time over 1 million hours at a channel temperature of 150 °C are estimated.

V-band power amplifier MMICs exhibiting low power slump characteristics utilizing a production released 0.15-um GaAs PHEMT process

The design and performance of 0.15-um PHEMT V-band driver and power amplifier MMICs suitable for satellite communication systems is presented. The amplifiers utilize a proven commercially available production released process featuring demonstrated low power slump characteristics and reliability suitable for space applications. Experimental results for the driver amplifier MMIC demonstrate greater than 17 dB of small signal gain, 25dBm saturated output power and 29% power added efficiency at 60GHz. The power amplifier MMIC provides 17dB of small signal gain, up to 27.5dBm saturated output power and 26% power added efficiency. A 1200-hour test at 5dB gain compression resulted in less than 0.5dB of output power drift.