The influence of substrate bias fixed charge in the buried insulator on the gain of the parasitic bipolar inherent in silicon-on-insulator MOSFETs

When Silicon-On-Insulator (SO) MOSFETS are operated at high drain voltages, the drain-source current becomes strongly influenced by the common emitter current gain of the parasitic, lateral bipolar transistor. For small channel lengths, the gain is limited only by the emitter efficiency and bipolar action can then give rise to high off-state leakage. From measurements performed on lateral SOI diodes it is demonstrated that the recombination rate in the source/body depletion region can be enhanced by the application of a substrate bias sufficient to cause depletion at the body/buried oxide interface, This results in a reduction in the common emitter current gain and from Gummel plots a reduction in gain of 20% was measured for an increase in substrate voltage of 5 volts. Also, fixed positive charge in the buried oxide is expected to produce the same effect for an n-channel transistor, as a depletion region will form automatically at the body/buried oxide interface.