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Reduced Substrate Sensitivity in Field Effect Transistors

IP.com Disclosure Number: IPCOM000079757D
Original Publication Date: 1973-Aug-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Carlson, WH: AUTHOR [+3]

Abstract

This is a method of reducing the substrate sensitivity of a metal-oxide-silicon field-effect transistor (MOSFET) operated in the source-follower mode. The doping profile is graded to peak at the surface of the substrate and to fall off with increasing distance into the substrate, as shown in Fig. 1. The depletion region of the surface channel thereby spreads into increasingly lighter doped material, with the effect of moderating the substrate sensitivity, dV(T)/dV(SX).

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Reduced Substrate Sensitivity in Field Effect Transistors

This is a method of reducing the substrate sensitivity of a metal-oxide-silicon field-effect transistor (MOSFET) operated in the source-follower mode. The doping profile is graded to peak at the surface of the substrate and to fall off with increasing distance into the substrate, as shown in Fig. 1. The depletion region of the surface channel thereby spreads into increasingly lighter doped material, with the effect of moderating the substrate sensitivity, dV(T)/dV(SX).

The threshold voltage (V(T)) equation for a N-channel MOSFET can be written:

(Image Omitted)

As can be seen in the equation, V(SX) increases V(T). For fixed operating voltages, this results in reduced drain current because I(D) decreases as V(T) increases, the other device parameters remaining fixed. Thus, any device whose source is not set at the substrate potential experiences a reduction in I(D), due to the dependence of V(T) on V(SX). From the equation it can be seen that the magnitude of dV(T)/dV(SX) depends on N(A).

Fig. 2 illustrates the reduction of substrate sensitivity achievable with the present technique for a typical N channel MOSFET, having a V(T) of around 0.75 volt. Curve A shows the substrate sensitivity for a standard device. The slope of the curve, dV(T),/dV(SX), is too large to achieve a circuit with a reasonable speed-power product. By decreasing the dopant level to lower V(T) and implementing a substrate bias to restore...