HP HP-Bench-Briefs-1991-01-03 free download

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I r SERVICE INFORMATION FROM HEWLETT-PACKARD 1st Quarter 1991 Field-Effect Transistors Part I1 CMOS Jim BechtoldlEditor means that the channel must be im- - proved for current to flow (i.e., zero Introduction bias will cause minimum current flow). Enhancement-type MOSFETs In the last issue of Bench Briefs, we are diagrammed with a broken line discussed the characteristics of two representing the channel. Depletion- ; fundamental types of field-effect type MOSFETs are "normally-on" de- transistors, the JFET and MOSFET. vices where the channel must be de- We learned that the name field-effect pleted to reduce the current flow (i.e., transistor arises from the fact that cur- current flow occurs with zero bias). rent flow between two of the device They are diagrammed with an unbro- Figure l a . scnematic structure of lunctior f terminals (source and drain) is con- ken line representing the channel. A trolled by an electric field, which in significant difference between JFETs turn is established by a voltage and MOSFETs is that all JFETs are of applied to a third terminal (gate). the depletion type while MOSFETs can be either depletion or enhancement. (`- JFETs MOSFETs can be either p-channel or The schematic structure of a n-chan- n-channel devices. In an n-channel ne1 junction FET is shown in Figure MOSFET, the charge carriers are elec- 1. If a voltage is applied across the trons. In a p-channel MOSFET, the channel so that the drain is positive charge carriers are holes. The charac- with respect to the source, as shown teristics of the p-channel MOSFET are in Figure lb, electrons flow through similar to those of the n-channel ex- the channel from the source to the cept that the polarities of the gate-to- drain producing drain current. The source and drain-to-source voltages magnitude of the drain current is de- are negative instead of positive. riuure IO. mas 10 uroouce wain currenr. termined by the conductivity of the channel and the drain-to-source vol- The n-channel and p-channel MOS- tage. When a negative volta