High mobility Ge 2DHG based MODFETs for low-temperature applications

Modulation-doped field-effect transistors (MODFET) that are typically based on either a two-dimensional electron gas or a two-dimensional hole gas (2DHG) are highly suitable for low-temperature applications due to the high mobilities attained by the charge carriers in the device channel. The Ge 2DHG is particularly interesting, since Ge exhibits the highest hole mobility among all semiconductors. In this paper, we report on the temperature-dependent direct current-characteristics of normally-on MODFETs based on a high mobility Ge 2DHG. Here, we specifically investigate device characteristics down to cryogenic temperatures and analyze the impact of different MOD-doping on device performance. We find that the largest On-Off ratio of I-On/I-Off = 3.2 x 10(6) and lowest sub-threshold swing (SS) of SS = 64 mV dec(-1) at a temperature of T = 7.5 K are attained for a MOD-doping of N-A = 5 x 10(17) cm(-3), while the highest effective mobility mu(eff) = 864 cm(2) V(-1)s(-1) is obtained from a device with a MOD-doping of N-A = 1.1 x 10(18) cm(-3). Furthermore, while effective mobilities in the devices are strongly reduced compared to Hall mobilities measured in the unstructured Ge 2DHGs, both quantities can be seen to follow a similar trend. Our results motivate further investigations of Ge 2DHG MODFETs for applications in cryogenic low-noise amplifiers, besides their well-established potential for spintronic applications.