Modelling the cryogenic properties of germanium for emerging liquid hydrogen power applications

Abstract

In recent years, there has been an increase in research focused towards the reduction and/or elimination of greenhouse emissions from applications used in everyday life. In addressing this, liquid hydrogen has been highlighted as an attractive alternative fuel source for commercial vehicles due to it’s lower weight, higher power density and zero greenhouse emissions in comparison to petrol and diesel fuels. Incorporating such a fuel source however introduces a cryogenic environment of 20 K affecting the power electronics used to deliver the power from source to load. Herein, the physical properties of semiconductors influencing the overall efficiency of devices within an H-bridge circuit are considered. From this, germanium is hypothesised to be the most suitable semiconductor for power devices at or near temperatures of 20 K. Closed-loop models are developed for the carrier concentration, carrier mobility, carrier velocity, for both electrons and holes as a function of doping concentration and temperature with critical analysis of the range of suitability for each. Multiple models are also developed for both carrier concentration and carrier mobility which offer a trade off depending on whether one requires accuracy or simplicity in calculation. A significant influence on the device characteristics of MOSFETs is that of the oxide/semiconductor interface. For the first time, ZrO2 is fabricated directly on germanium substrates through the thermal oxidation of zirconium on germanium. The interface state density of these capacitors are comparable to literature values offering a much cheaper and simpler fabrication method for high-κ dielectric formation on germanium substrates. The leakage current density of the ZrO2 MOS capacitors are low in comparison to reported values and are shown to decrease with decreasing temperature. With the physical models of both bulk and interfacial germanium, multiple PiN germanium diodes are simulated using technology computer aided design (TCAD) that show the potential for germanium power devices with breakdown voltages in excess of 800 V at room temperature and 400 V at 20 K. Simulations of vertical power MOSFETs incorporating a ZrO2 interlayer show great promise for low temperature power electronics at or near 20 K where other commercial devices experience significant resistive losses. With the work conducted here, vertical power MOSFETs fabricated using germanium and ZrO2 open the gateway for low voltage applications incorporating liquid hydrogen fuel cells.