Conductance of ion-implanted buried-channel MOS transistors

The transfer characteristics of ion-implanted p-channel MOS transistors for small drain-source voltages are measured and compared with a theory based on the in-depth carrier, space charge, and potential profiles, as well as the mobility distribution. MOS transistors with p-n-junctions under the gate area due to ion implantation are characterized by a channel buried in the semiconductor at a depth of 0... 0,2 µm. One result is the classification of implanted transistors into three distinct groups characterized by their buried potential maximum
|\phi_{\max}| \geq 2 \phi_{F}
and surface band bending
|\phi_{max}-\phi_{s}| \geq\leq 2\phi_{F}
at threshold. An extended definition of threshold voltage V
TO
is presented. In the conductance characteristic, V
TO
results from an extrapolation to the subthreshold current. This current is nonzero in heavily implanted transistors and is described well by the theory. In the active region, a buried-channel thickness of 0.1 µm is obtained, which extends over the transition from bulk to surface mobility. Models for the mobility distribution including impurity and surface scattering were used to calculate the active-region conductance. The mobility in buried channels and the related transconductance is governed by scattering at the implanted impurity profile. Beyond a characteristic gate voltage, surface channels are obtained, where the mobility is dominated by surface scattering. A good match of the measured conductance characteristics is obtained both in their characteristic nonlinearities and their dependence on an applied substrate bias.