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Metal Oxide Semiconductor Field-Effect Transistors

Jun 20, 2019

Metal oxide semiconductor field-effect transistors (MOSFETs) are incredibly popular transistors that in some ways resemble JFETs. For instance, when a small voltage is applied at its gate lead, the current flow through its drain-source channel is altered. However, unlike JFETS, MOSFETs have larger gate lead input impedances (≥1014 Ω, as compared with 109 Ω for JFETs), which means that they draw almost no gate current whatsoever. This increased input impedance is made possible by placing a metal oxide insulator between the gate-drain/source channel. There is a price to pay for this increased amount of input impedance, which amounts to a very low gate-to channel capacitance (a few pF), through the gate and destroy the MOSFET. (Some MOSFETs are designed with safeguards against this breakdown-but not all.) Both enhancement-type and depletion-type MOSFETs come in either n-channel or p-channel forms.

MOSFETs are perhaps the most popular transistors used today; they draw very little input current, are easy to make (require few ingredients), can be made extremely small, and consume very little power. In terms of applications, MOSFETs are used in ultrahigh input impedance amplifier circuits, voltage-controlled “resistor” circuits, switching circuits, and found with large-scale integrated digital ICs. Like JFETs, MOSFETs have small transconductance values when compared with bipolar transistors. In terms of amplifier applications, this can lead to decreased gain values. For this reason, you will rarely see MOSFETs in simple amplifier circuits, unless there is a need for ultrahigh input impedance and low input current features.

OHMIC REGION MOSFET is just beginning to resist. In this region, the MOSFET behaves like a resistor.

ACTIVE REGION MOSFET is most strongly influenced by gate-source voltage (VGS) but hardly at all influenced by drain-source voltage (VDS).

CUTOFF VOLTAGE (VGS, off) Often referred to as the pinch-off voltage (Vp). Represents the particular gate-source voltage that causes the MOSFET to block most all drain-source current flow.

BREAKDOWN VOLTAGE (BVDS) The drain source voltage (VDS) that causes current to “break through” MOSFET’s resistive channel.

DRAIN CURRENT FOR ZERO BIAS (IDSS) Represents the drain current when gate source voltage is zero volts (or when gate is shorted to source).

TRANSCONDUCTANCE (gm) Represents the rate of change in the drain current with change in gate-source voltage when drain-source voltage is fixed for a particular VDS. It is analogous to the transconductance (I/Rtr) for bipolar
transistors.

MOSFET Drivers may come with a fourth lead, called the body terminal. This terminal forms a diode junction with the drain-source channel. It must be held at a non conducting voltage [say, to the source or to a point in a circuit that is more negative than the source (n-channel devices) or more positive than the source (p-channel devices)]. If the base is taken away from the source (for enhancement-type MOSFETs) and set to a different voltage than that of the source, the effect shifts the threshold voltage VGS,th by an amount equal to 1⁄2VBS 1/2 in the direction that tends to decrease drain current for a given VGS. Some instances when shifting the threshold voltage becomes important are when leakage effects, capacitance effects, and signal polarities must be counterbalanced. The body terminal of a MOSFET Drivers is often used to determine the operating point of a MOSFET by applying an incremental ac signal to its gate.

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