AlGaN/GaN HFETs for High Power/High Temperature Operation

The University of Sheffield, in collaboration with QinetiQ Ltd., have developed high performance GaN based HFETs on sapphire and Si substrates. The devices were stable at operation temperatures of 500ºC and the I-V characteristics reversible upon cooling to room temperature. Fig. 1 shows the typical behaviour of a 350nm L_g device at 25ºC and 500ºC (on sapphire).

Using FATFET extraction, the mobility of 2DEG structures were found to severely degrade at elevated temperatures due to phonon scattering. Therefore, to minimise high temperature performance degradation, it is important that the devices are operated in the regime where the electron transport under the gate is controlled by the saturation velocity, v_sat, since the latter has a weaker temperature dependence compared to µ. In addition, technologies to define transistor gate lengths down to 70nm and field plates have been established.

Performance indicators:

• Record drain current density of 1.1 A/mm achieved for devices on Si substrates (L_g = 250nm). F_t values of 36 GHz and 22 GHz for those on sapphire and Si substrates respectively (L_g = 250nm).
• Pinch-off drain current density of 5µA/mm.
• Two terminal breakdown voltages of 180V for a 2µm gate-drain spacing HEMT (L_g = 1µm), done using an optimised SiN_x passivation process.
• Minimal Class A and Class B current collapse (gate lag ratio = 0.9) with optimised SiN passivation