Modeling of a nonpinchoff depletion mode MOSFET

A depletion-mode MOSFET is typically formed with a thin channel of opposite conductivity type to the substrate. When a large gate voltage is applied to deplete this channel, an inversion layer is induced. As strong inversion occurs, the depletion layer depth reaches a maximum and cannot be further increased. If this depth is less than the thickness of the channel, the channel cannot be pinched off unless a reverse bias is applied to the substrate. Such a depletion-mode MOSFET is modeled as a MOSFET connected in parallel with a JFET which shares a portion of the channel. The MOSFET has zero background bias so long as the JFET is not pinched off. When the substrate bias V/SUB SS/ is larger than V/SUB p/ to pinch off the JFET, the MOSFET channel depth is reduced, equivalent to applying a background bias to the MOSFET. This background bias V/SUB B/ is shown to be a square root function of V/SUB SS/ and V/SUB p/.