Low temperature threshold behavior of depletion mode devices—Characterization and simulation

INTRODUCTION The changes in threshold voltage characteristics observed between room and liquid nitrogen temperature for depletion and enhancement mode MOS devices are fundamentally different. As described elsewhere (I), the enhancement mode threshold voltage is not affected by carrier freezeout; it shows only an increase in magnitude due to temperature dependent parameter changes combined with an essentially unaltered substrate sensitivity. Actual measurements of depletion mode devices, however, show a significantly different temperature dependency. With decreasing temperature, the negative threshold voltage shift, usually achieved by a shallow donor implantation within an n-channel technology, is reduced as is the substrate sensitivity. Both changes can be accounted for by freezeout within the shallow, compensated, n-type surface layer.