| Fully depleted SOI MOSFETs have a number of important advantages over their bulk counterparts. Unfortunately, they also suffer from a number of disadvantages, the most important of which is the problem of low drain to source breakdown voltage. Low breakdown voltages are caused by the presence of an inherent parasitic, floating base, bipolar structure, with the source, body, and drain regions of the MOSFET acting as the emitter, base, and collector, respectively, of the parasitic bipolar transistor. Since there is no contact to the body region of a fully depleted SOI MOSFET, the parasitic bipolar device has a floating base. It is this floating body/base that causes the parasitic bipolar device to be so problematic.;In this work, the behavior of the parasitic bipolar device is investigated and explained, both qualitatively and quantitatively. The key result is that the gain of the parasitic bipolar device is inversely proportional to the hole concentration in the body/base. The body region of a fully depleted n-channel SOI MOSFET is completely depleted of holes, by definition, in the zero bias state. But, as the drain voltage is increased, the impact ionization process creates a flow of holes through the body. It is the concentration of these holes that the parasitic gain is inversely proportional to. Since the impact ionization mechanism is exponentially related to drain voltage, the parasitic bipolar gain is found to fall exponentially with increasing drain voltage.;The other significant contributions of this work relate to SOI MOSFET structures designed to ameliorate the parasitic bipolar breakdown problem. The Dual Source SOI MOSFET structure does this by providing a path for the removal of holes from the body/base region. The Heterojunction Source SOI MOSFET decreases the effect of the parasitic bipolar device by reducing the injection efficiency of the source-body/emitter-base junction through the use of a heterojunction. |
