An area-variable MOS varicap and its application in programmable TAP weighting of CCD transversal filters

A new three-terminal MOS varicap is proposed where the terminal capacitors are made voltage variable not by the modulation of depletion width but by changing the area of inversion under the gate. An MOS capacitor realized on silicon with an impurity gradient along the surface provides the control on the area of inversion because the gate threshold voltage is determined by the doping concentration at the surface. The inhomogeneous doping along the surface is implemented making use of the lateral diffusion from a doped oxide surface. Fabrication details of the capacitor compatible with n-channel silicon gate technology are presented. The C-V relationship for the terminal capacitors is simulated by a piecewise model and agreement with measured results is shown. The Area-Variable MOS Varicap (AVMOSV) is used in implementing an electrically programmable CCD filter with variable TAP weighting. Computer simulation shows considerable promise of area-variable capacitors in TAP weight control and transversal filter realization. Preliminary performance characteristics of a programmable CCD filter are presented.