6H silicon carbide MOSFET modelling for high temperature analogue integrated circuits (25-500 degrees C)

The authors report on the effects of elevated ambient and substrate temperatures (25-500 degrees C) on the electrical characteristics of 6H polytype silicon carbide (SIC) MOSFETs. The work focuses primarily on modelling the temperature variations of the large- and small-signal parameters of the devices with a view to assessing their suitability for high-temperature integrated electronics. These parameters include threshold voltages, leakage currents, body-bias effects, and small-signal transconductances and output conductances. Where relevant, the authors' results are compared to silicon MOSFETs and GaAs MESFETs. Above 225 degrees C, the parameter variations of their SIC MOSFETs, including the observation of zero temperature coefficient (ZTC) drain currents, are qualitatively similar to those of Si MOSFETs and of GaAs MESFETs. In contrast with silicon MOSFETs, however, the gate transconductance (g(m)) and the channel mobility (mu) increase with increasing temperature up to 225 degrees C approximately because of a high interface state density. The ON/OFF current ratio of their SIC MOSFETs is at least two orders of magnitude higher than for Si and GaAs FETs above 200 degrees C. Junction leakage current densities measured up to 500 degrees C are several orders of magnitude lower than in high quality Si and GaAs devices, as expected from the higher bandgap energy for their SIC material (approximate to 3 eV). While the junctions retained their electrical integrity, their MOSFETs displayed large gate-to-drain leakage currents above 300 degrees C, apparently due to an oxide reliability problem. Pin-to-pin package leakage is also observed above the same temperature. Despite this partial form of damage, the authors believe that their results confirm the expected potential of SIC MOSFETs for integrated circuit applications above 250 degrees C. The data reported in the paper, for this novel SIC process, are used to derive MOSFET models and SPICE parameters for analogue IC design, and an NMOS operational amplifier (OPAMP) is presented which is expected to operate in the range 25 degrees C to 500 degrees C, where silicon and GaAs technologies are unsuitable.