PARASITIC BIPOLAR GAIN IN FULLY DEPLETED N-CHANNEL SOI MOSFETS

Fully depleted SOI MOSFET's include an inherent parasitic lateral bipo lar structure with a floating base. We present hare the first complete physically based explanation of the bipolar gain mechanism, and its d ependence on bias and technological parameters. A simple, one-dimensio nal physical model, with no fitting parameters, is constructed, and is shown to agree well with simulations and measurements performed on a new type of SOI MOSFET structure. It is shown that parameters which af fect the gain, such as SOI layer thickness, body doping concentration, and gate and drain voltages, do so primarily by affecting the concent ration of holes in the body region. Thus, current gain falls dramatica lly with increasing drain voltage due to the associated impact ionizat ion driven increase in the hole concentration. Gummel plots of this pa rasitic bipolar indicate an apparent ideality factor of 0.5 for the ho le current, due to the body hole concentration's dependence on drain v oltage.