Temperature dependence of FET properties for Cr-doped and LEC semi-insulating GaAs substrates

The temperature dependence of carrier concentration and mobility profiles is measured on Schottky-barrier gate FET´s formed by Si implantation into intentionally Cr-doped Czochralski-grown semi-insulating GaAs substrates and LEC semi-insulating GaAs substrates. The effective depth of the channel implant is at least 100 nm shallower for Cr-doped substrates than it is for LEC substrates. Carrier mobilities are higher for LEC substrates; at low temperature, mobility is higher at the channel-substrate interface than it is nearer the surface. Devices on both types of substrates show free-carrier trapping effects between 160 and 225 K. For LEC substrates, traps are on the channel side of the active channel-substrate interface. For Cr-doped substrates, observation of traps depends on biasing the FET beyond pinchoff; traps are on the substrate side of the channel-substrate interface.