GaN transistors


GaN heterostructure field-effect transistors (HFETs) are promising devices for high-power and high-voltage applications, because the electron density in the channel region formed at the heterointerface is very large due to the internal polarization effects, and the band gap of GaN is very large. As one of the application targets, GaN power amplifiers for cellphone base stations are being studied at a lot of universities and companies in te world. However, the target frequency range is less than 10 GHz, and there have been little reports on the study of GaN transistors for higher frequency. We are working on the development of the GaN HFETs capable of operating in millimeter-wave and submillimeter-wave ranges, especially above the V-band (50-75 GHz). For this research goal, we are studying all material growth, device processing, and device characterization in our group.
	RF-MBE

	X-ray diffraction
GaN transistor features EHigh power DHigh voltage endurance DHig temperature endurance DHigh radiation endurance DLarge amount of resources GaN transistors application in mm-wave range D Car rader (60, 76 GHz) D Car wireless comm. (60, 76 GHz) D Intelligent transportation system, ITS (60, 76 GHz) D Space satellite - Ground comm. (26-40 GHz) D Satellite - Satellite comm.i>30 GHz) D Last-mile comm. (70-100 GHz)
	EB lithographer

	Network analyzer

Latest Research Developments
AlGaN/GaN HFETs with thin and high-Al-content barrier layers The development of faster GaN HFETs are indispensable to the realization of high-speed and high-power ones operating in mm-wave ranges. For this sake, decreasing a gate length to less than 100 nm is one of the most effective ways. However, for a short gate with a length 100 nm, decreasing the thickness of the barrier layer with shortening gate length is also important in maintaining the aspect ratio defined by the gate-channel distance and fabricated gate length high to prevent the effective gate length at the channel interface from increasing. In the case of AlGaN/GaN HFETs, however, there is a large problem that the channel sheet resistance increases since the mobility and electron density decrease when the AlGaN barrier thickness is decreased from 25-30 nm, which is usually used for AlGaN/GaN HFETs, to less than 15 nm. To overcome these problems, we are focusing on AlGaN/GaN HFET structures with a thin and high-Al-content barrier layers, because the enhanced polarization due to the high-Al-content barrier can compensate for the decrease in electron density with decreasing barrier thickness. We succeeded in growing high-quality and crack-free Al0.4Ga0.6N/GaN HFET structures by RF-MBE, which have higher-Al content barriers than the usual ones. We consider that RF-MBE is a suitable technique for growing a high-Al-content AlGaN layer, because its growth temperature is generally 200-300 degree C lower than that in MOCVD. We have fabricated AlGaN(8nm)/GaN HFETs, which demonstrated good DC device characteristics; a high transconductance(gm>400 mS/mm), a high maximum current density (Imax=0.83 A/mm). Those devices also showed excellent RF characteristics for the one with the gate length of 1 micron; a short-circuit current-gain cutoff frequency of ft=13.9 GHz and a maximum oscillation frequency of 26.3 GHz. We are now working on fabricating short-gate devices with a length of less than 100 nm to enhance high-frequency characteristics further.
	I-V curves

	RF small-signal characteristics

	AlGaN barrier thickness dependences of RF and DC characteristics

InAlN/GaN HFET structure For the improvements in the operation power and speed of GaN-based HFETs, decreasing a barrier thickness with increasing an Al composition in barrier is effective to have the large polarization effect. However, it is very difficult to grow an AlGaN layer with an Al composition of larger than 0.5 on GaN coherently, even by RF-MBE, which can grow nitrides at low temperature.
InAlN with Al=0.93 is lattice matched to GaN, thus it is possible to realize InAlN/GaN HFETs with a very large A composition in barrier layers, which can be 0.8-0.9. As a result, larger electron density can be expected for very thin barriers.
We have succeeded in growing high-quality InAlN/GaN HFET structures by RF-MBE and the transistor device operation. This is the first demonstration in the world, to our knowledge. We are now studying to improve the device characteristics by optimizing crystal growth and device processing.

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