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I have heard that mosfets can be converted into resistors if they are "depleted". Can somebody explain how this is done as simply as possible...I don't have an EE background.


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3 Answers 3

MOSFETs are resistors. By how much the channel conducts current is determined by construction of the MOSFET and the voltage on the gate. A depletion type MOSFET will conduct with no voltage applied to the gate, and require a voltage to increase the channel's resistance, while for an enhancement type it's just the other way around.

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Depletion-mode MOSFETs are devices that will provide a voltage drop when current is drawn through them. They can be used in places where something like a pull-up resistor would be required, but they don't act like a typical resistor. A typical resistor will, throughout its useful operating range, drop a voltage proportional to current. A 10Kohm resistor will drop ten volts per milliamp, or ten millivolts per microamp. By contrast, a depletion-mode MOSFET might drop one volt at 100uA, but drop 0.5 volts at 75uA, or 3 volts at 120uA.

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A voltage applied to the gate will reduce the width of source-drain channel. Less width/space to flow means less current. That's same as the function of a resistor. Its simple as that.

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