Transistor Technologies How to use a MOSFET as a switch?

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MOSFET is one of the best transistors to perform switching functions in a circuit. The voltage-controlled transistor is thermally stable and provides fast switching. MOSFET behaves like a switch in cut-off and saturation regions of operation. Here is an article to understand the operation of MOSFET as a switch and its advantages.

When is it useful to use MOSFET as a switch?

Can we use a Depletion mode MOSFET?

Depletion Mode MOSFET: Depletion mode MOSFET is a “normally ON” device. The transfer characteristics of Depletion type MOSFET is a graph of drain current I_D on Y-axis against gate-to-source V_GS on the X-axis.

MOSFET operates when gate-to-source voltage V_GS is negative and positive both. Such a device cannot be used for switching purposes because it is “normally ON”.

Can we use Enhancement mode MOSFET?

Enhancement Mode MOSFET: Enhancement Mode MOSFET is a great choice for switching purposes because it is a “normally OFF” device that can be turned on. The transfer characteristics of Enhancement mode MOSFET is a graph of drain current I_D on the Y-axis against gate-to-source V_GS on the X-axis. V_T is the threshold voltage beyond which the device turns on.

How does a MOSFET work as a switch?

The transistor MOSFET works as a switch in two operating modes- cut-off and saturation region. MOSFET acts as a short circuit or closed switch in the saturation region and an open switch in the cut-off region.

A MOSFET switching diagram includes load resistance, and gate control circuitry that can be a microcontroller.

Enhancement mode N-channel MOSFET operating modes for switching:

MOSFET as an open switch (V_GS < V_T)

When V_GS is less than V_T but not equal to zero, MOSFET is in the cut-off region. Since V_GS < V_Threshold, no channel formation takes place, and no current flows through the MOSFET.

The transistor behaves like an open circuit and is logically “OFF”.

MOSFET as a short circuit (V_GS > V_T)

Drain characteristics of an enhancement mode MOSFET are obtained by plotting a graph between drain current I_D on the Y-axis and drain-to-source voltage V_DS on the X-axis for different values of V_GS.

For a voltage V_GS > V_T, voltage V_DS is increased. The drain current I_D increases linearly with increasing V_DS until the saturation point-ohmic region. Beyond the saturation point, the drain current I_D remains constant and does not change with increasing V_DS .

The region of operation is termed as the saturation region where MOSFET acts like a short circuit or closed switch and is logically “ON”.

TRANSISTOR TECHNOLOGY

How to use a transistor as a switch?

What are the advantages of using MOSFET as a switch?

There are many advantages of using a MOSFET transistor as a switch.

• There is less noise in semiconductor switches compared to mechanical switches.

• MOSFETs are more thermally stable than BJT because of the positive temperature coefficient.

• Power MOSFETs offer nanosecond switching.

• Low maintenance because there are no moving parts.

• MOSFETs are smaller in size and weight.

• MOSFET switches have lower switching losses.

References

• https://www.electronics-tutorials.ws/transistor/tran_7.html

• https://youtu.be/UJkHL-6mn8s

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