Strained Si HFETs for microwave applications: State-of-the-art and further approaches

This paper reviews the RF and noise performance of strained Si heterostructure field-effect transistors. For SiGe n-HFETs the high RF figures of merit f_T = 90 GHz, f_MAX = 188 GHz and noise performance NF_MIN = 0.3 dB at 2.5 GHz demonstrate the capabilities of these devices for microwave circuit applications. Recently some IC demonstrators have been successfully implemented. Strained Si HFETs fabricated on thick SiGe virtual substrates have been traditionally employed as a vehicle of study. Nevertheless these thick relaxed buffers involve some decisive disadvantages: Reduction of the wafer throughput, non-acceptable surface topology if implemented in mixed technologies like CMOS, and, in particular, self-heating due to low thermal conductivity of SiGe alloys compared to Si. In order to overcome these drawbacks several approaches to reduce the buffer thickness have been very recently developed. In this paper the RF and low frequency noise performance of SiGe HFETs prepared on very thin virtual substrates are reported. The potentialities of these technologies have not yet reached their limits. One can expect a great improvement by combining a proper device shrinking with very thin virtual substrates. These heterosystems could become the backbone of strained silicon devices towards microwave applications.