Silicon Carbide Microwave Devices

Doctoral thesis, 2002

This work deals with silicon carbide (SiC) microwave devices. It treats two devices; the Schottky diode and the metal semiconductor field effect transistor (MESFET). Different MESFET materials, fabrication processes, theoretical models for physical simulation, and device models are presented. The aim of this work is to improve the understanding of these devices and their performance by careful design. An optimisation design technique for the SiC microwave Schottky diode is developed. Depending of the desired punch-through voltage a certain calculated drift layer thickness and doping concentration for the Schottky diode provides an optimum when designing for cut-off frequency. Schottky diodes that behave according to the models presented are demonstrated. For a 5 µm anode radius diode a cut-off frequency (fcut) of 91 GHz was calculated . To our knowledge this is the highest fcut presented for a SiC Schottky diode. A singly balanced diode mixer was measured. The input third order intermodulation intercept point is 31dBm. The mixer has a conversion loss of 5.0 dB at 850 MHz RF. Eleven MESFET structures have been investigated both by physical drift-diffusion (DD) simulations, and measurements. The models used in the DD simulations are described in detail, and approximations of published data have been presented. In particular different buffer structures, and their influence on device performance have been investigated. Measurements on 11 different MESFET materials are presented. In the research performed at Chalmers, devices with very good DC characteristics have been manufactured. The best RF device has a saturated drain current of 270 mA/mm, a maximum transconductance of 35 mS/mm, a breakdown voltage of >100 V, and a pinch-off voltage of -15 V. The high frequency performance of this device are an fmax of 27 GHz, an fT,ext of 8 GHz, and an output power density of 3.2 W/mm with an associated gain of 14 dB at 3 GHz. A resistive FET mixer using a 3.2 mm gate periphery MESFET is presented, and it had a conversion loss of 10 dB, and a IIP3 of 36 dBm at 3.3 GHz.A small signal model for the MESFET in coldFET operation is given.