Transistor Technologies How to use a MOSFET as a switch?
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 ID on Y-axis against gate-to-source VGS on the X-axis.
MOSFET operates when gate-to-source voltage VGS 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 ID on the Y-axis against gate-to-source VGS on the X-axis. VT 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:
VGS < VThreshold
Open switch (OFF)
VGS > VThreshold
Short circuit (ON)
MOSFET as an open switch (VGS < VT)
When VGS is less than VT but not equal to zero, MOSFET is in the cut-off region. Since VGS < VThreshold, 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 (VGS > VT)
Drain characteristics of an enhancement mode MOSFET are obtained by plotting a graph between drain current ID on the Y-axis and drain-to-source voltage VDS on the X-axis for different values of VGS.
For a voltage VGS > VT, voltage VDS is increased. The drain current ID increases linearly with increasing VDS until the saturation point-ohmic region. Beyond the saturation point, the drain current ID remains constant and does not change with increasing VDS .
The region of operation is termed as the saturation region where MOSFET acts like a short circuit or closed switch and is logically “ON”.
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.