Research Review: N-polar HEMT catches up conventional cousins
ENGINEERS at the University of California, Santa Barbara, have raised the power density bar for N polar HEMTs operating at 4 GHz from 8.1 W/mm to 12.1 W/mm. These transistors and their more common Ga-polar cousins are promising devices for microwave power amplification from L-band to W-band. Lead-author Seshadri Kolluri claims that switching from a Ga-polar HEMT to an Npolar version offers many benefits, including a very low contact resistance and ‘better scalability’. The latter attribute holds the key to reaching higher device frequencies. Higher frequencies are possible because charge in the N-polar devices is induced by the AlGaN barrier below the twodimensional electron gas. This allows the gate to be placed very close to the channel. “However, we need to develop a good insulator to prevent gate leakage in such highly scaled devices,” says Kolluri. The UCSB team that includes Steven DenBaars and Umesh Mishra increased the power density of their HEMTs by 4 W/mm by changing the deposition technology from MBE to MOCVD. “MOCVD grown devices benefit from a lower dislocation density and nitrogen-rich growth conditions, leading to a lower gate leakage and higher breakdown voltages in the device,” explains Kolluri. HEMTs were grown on sapphire off-cut by 4° towards the a-plane, in order to realise a smooth, high quality N-polar film. Transistors with dimensions of 2 μm by 50 μm, a nominal gate length of 0.7 μm and a gatedrain spacing of 0.8 μm were formed by standard lithography. Device testing revealed that the breakdown voltage – defined as the voltage required for a leakage current of 1 mA/mm through the buffer over a 1 μm spacing – was 250 V. Maximum oscillation frequency and cut-off frequency were 40 GHz and 16 GHz, respectively. Biasing the HEMTs in deep class AB mode and performing continuouswave power measurements at 4 GHz (drain bias of 50 V and a quiescent drain current of 121 mA/mm) yielded a gain of 9.8 dB, an output power of 12.1 W/mm and a poweradded efficiency of 55 percent. The output power density at 4 GHz is equal to that for the best Ga-polar AlGaN/GaN HEMT built on sapphire. The UCSB team is now focusing on development of N-polar HEMTs grown by MOCVD on semi-insulating SiC, which should yield higher power densities, due to the superior thermal conductivity of this platform. “We are also working on scaling the devices to gates lengths of 100- 150 nm to achieve better power amplification in the mm-wave range,” says Kolluri. S. Kolluri et al. IEEE Electron. Dev. Lett.
