5.8 The Insulated-Gate Field Effect Transistor

Chapter 5.8 The Insulated-Gate Field Effect Transistor

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 5. TRANSISTOR CIRCUITS1005.8The Insulated-Gate Field Effect TransistorThe insulated-gate FET, also known as a metal oxide semiconductor field effect transistor(MOSFET), is similar to the JFET but exhibits an even larger resistive input impedance dueto the thin layer of silicon dioxide that is used to insulate the gate from the semiconductorchannel. This insulating layer forms a capacitive coupling between the gate and the bodyof the transistor. The consequent lack of an internal DC connection to the gate makes thedevice more versatile than the JFET, but it also means that the insulating material of thecapacitor can be easily damaged by the internal discharge of static charge developed duringnormal handling.The MOSFET is widely used in large-scale digital integrated circuits where its high inputimpedance can result in very low power consumption per component. Many of these circuitsfeature bipolar transistor connections to the external terminals, thereby making the devicesless susceptible to damage.The MOSFET comes in four basic types, N-channel, P-channel, depletion and enhance-ment. The configuration of an N-channel, depletion MOSFET is shown in figure 5.19a. Itsoperation is similar to the N-channel JFET discussed previously: a negative voltage placedon the gate generates a charge depleted region in the N-type material next to the gate,thereby reducing the area of the conduction channel between the drain and source. How-ever, the mechanism by which the depletion region is formed is different from the JFET. Asthe gate is made negative with respect to the source, more positive carriers from the P-typematerial are drawn into the N-channel, where they combine with and eliminate the freenegative charges. This action enlarges the depletion region towards the gate, reducing thearea of the N-channel and thereby lowering the conductivity between the drain and source.For negative applied gate-source voltages the observed effect is much like a JFET, and gmis also about the same size.However, since the MOSFET gate is insulated from the channel, positive gate-sourcevoltages may also be applied without losing the FET effect. Depending on the constructiondetails, the application of a positive gate-source voltage to a depletion-type MOSFET canrepel the minority positive carriers in the depleted portion of the N-channel back into the P-type material as discussed below, thereby enlarging the channel and reducing the resistance.If the device exhibits this behaviour, it is known as an enhancement-depletion MOSFET.A strictly enhancement MOSFET results from the configuration shown in figure 5.19b.Below some threshold of positive gate-source voltage, the connecting channel of N-type ma-terial between the drain and source is completely blocked by the depletion region generatedby the PN junction. As the gate-source voltage is made more positive, the minority positivecarriers are repelled back into the P-type material, leaving free negative charges behind. Theeffect is to shrink the depletion region and increase the conductivity between the drain andsource.5.9Power MOSFET CircuitsTraditionally, MOSFET devices have had the drain-to-source current confined to a thin pla-nar volume of silicon lying parallel to the gate. The limited cross-sectional area of material