Design and Fabrication of Silicon Carbide RF MOSFET for L- and S-band applications

This thesis describes realisation of 4H-SiC radio-frequency metal-oxide-semiconductor field effect transistors (RF MOSFETs). Such transistors are in principle very attractive devices for high power and high frequency electronics. They are intended as a direct replacement for their silicon counterparts, offering higher power. In order to enable high frequency operation together with high voltage handling capability the concept of buried anti-punch-through (APT) is introduced and implemented in RF devices. The buried APT prevents punch-through of the channel region without seriously affecting the inversion channel mobility and the threshold voltage of the transistor. 4H-SiC RF MOSFET with output power density of 1.9W/mm at 3GHz from a 0.8mm device is demonstrated. This is twice the output power density from Si MOSFETs. Previously, insufficient quality of the silicon dioxide/4H-SiC interface has resulted in very low inversion channel mobility (10-40cm2/Vs) in such devices and unacceptable current output and frequency response. Major improvements in channel mobility and current capacity are reported in this thesis. These results make feasible fabrication of high frequency SiC MOSFETs. Transistors with peak field effect mobility above 150cm2/Vs are demonstrated. Furthermore, MOSFETs with aluminium ion implanted gate channels are investigated. In contrast to previous reports, high quality SiO2/SiC interface is obtained both when the gate oxide is grown on p-type epitaxial material and when grown on ion implanted regions. The mobility reduction with increasing acceptor density follows the same functional relation as in n-channel Si MOSFETs