2.2.3 High Frequency Device Operation

The preceding discussion focused on high-power switching for power conversion. Many of these arguments can also be applied to devices used to generate and amplify RF signals used in radar and communications applications. In particular, the high breakdown voltage and high thermal conductivity coupled with high carrier saturation velocity allow SiC microwave devices to handle much higher power densities than their silicon or GaAs RF counterparts, despite SiC's disadvantage in low-field carrier mobility. A variety of microwave devices with impressive DC and RF performance including metal-semiconductor field effect transistor (MESFETs), static-induction transistors (SITs) and heterojunction bipolar transistors (HBTs) can be fabricated from SiC-based semiconductors [46].

SiC MESFETs with cutoff frequency $f_\mathrm{t}=25$ GHz have been fabricated [41]. These devices produced 1.75 W/mm RF power with 45.5% power-added efficiency (PAE) at 6 GHz. SiC MESFETs with RF output power on the order of 2.8 W/mm at 1.8 GHz and 2.27 W/mm with 65.7% PAE for a class B amplifier have been reported at 850 MHz [47]. A SiC MESFET with an $f_\mathrm{t}$ of 42 GHz has been reported recently [16], indicating that these devices should be capable of producing excellent RF performance through X band and potentially to K band. SiC SITs show the highest pulsed power density of any solid state transistor and are the most advanced of the SiC microwave power devices. For example, single package SIT output powers of 900 Watt UHF with PAE 0f 78% have been obtained with high drain efficiency [16]. T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation