What Is a MOSFET?

The Metal Oxide Semiconductor Field Effect

Transistor (MOSFET) is one type of FET
transistor. In these transistors, the gate
terminal is electrically insulated from the
current carrying channel so that it is also
called as Insulated Gate FET (IG-FET). Due to
the insulation between gate and source
terminals, the input resistance of MOSFET may
be very high.

Like JFET, the MOSFET also acts as a voltage controlled resistor when no current flows into the
gate terminal. The small voltage at the gate terminal controls the current flow through the
channel between the source and drain terminals.

Basically, MOSFETs are electronic devices used to switch or amplify voltages in circuits. It is a
voltage controlled device and is constructed by three terminals, namely Drain (D), Source (S)
and Gate (G) and also one more (optional) terminal called substrate or Body(B).

MOSFET Construction/ Structure

 The p-
type

semiconductor forms the base of the MOSFET.

 The two types of the base are highly doped with an n-type impurity which is marked as
n+ in the diagram.
 From the heavily doped regions of the base, the terminals source and drain originate.
 The layer of the substrate is coated with a layer of silicon dioxide for insulation.
 A thin insulated metallic plate is kept on top of the silicon dioxide and it acts as a
capacitor.
 The gate terminal is brought out from the thin metallic plate.
  A DC circuit is then formed by connecting a voltage source between these two n-type
regions.

MOSFET Types

The classification of MOSFET based on the construction and the material used is given below in
the flowchart.

MOSFETs are of two classes: Enhancement mode and depletion mode. Each class is available as
n-channel or p-channel; hence overall they tally up to four types of MOSFETs.

Depletion Mode

When there is no voltage across the gate terminal, the channel shows maximum conductance.
When the voltage across the gate terminal is either positive or negative, then the channel
conductivity decreases.

The depletion type MOSFET transistor is equivalent to a “normally closed” switch. The
depletion type of transistors requires gate – source voltage (VGS) to switch OFF the device.

The symbols for depletion mode

of MOSFETs in both N-channel and P-
channel types are shown above. In
the above symbols, we can

P-channel
N-channel
observe that the fourth terminal (substrate) is connected to the ground, but in discrete
MOSFETs it is connected to source terminal. The continuous thick line connected between the
drain and source terminal represents the depletion type. The arrow symbol indicates the type
of channel, such as N-channel or P-channel.

In this type of MOSFETs a thin layer of silicon is deposited below the gate terminal. The
depletion mode MOSFET transistors are generally ON at zero gate-source voltage (VGS). The
conductivity of the channel in depletion MOSFETs is less compared to the enhancement type of
MOSFETs.

Enhancement Mode

When there is no voltage across the gate terminal, then the device does not conduct. When
there is the maximum voltage across the gate terminal, then the device shows enhanced
conductivity.

Enhancement mode means, that whenever the voltage toward the gate terminal of this
MOSFET increases, then the current flow will be increased more from drain to source until it
reaches the highest level.

The Enhancement mode MOSFET is equivalent to “Normally Open” switch and these types of
transistors require a gate-source voltage to switch ON the device. The symbols of both N-
channel and P-channel enhancement mode MOSFETs are shown below.

P-channel N-channel

Here, we can observe that a broken line is connected between the source and drain, which
represents the enhancement mode type. In enhancement mode MOSFETs, the conductivity
increases by increasing the oxide layer, which adds the carriers to the channel.

Generally, this oxide layer is called as ‘Inversion layer’. The channel is formed between the
drain and source in the opposite type to the substrate, such as N-channel is made with a P-type
substrate and P-channel is made with an N-type substrate. The conductivity of the channel due
to electrons or holes depends on N-type or P-type channel respectively.

Operating Regions of MOSFET

A MOSFET is seen to exhibit three operating regions. Here, we will discuss those regions.

 Cut-Off Region- The cut-off region is a region in which there will be no conduction and
as a result, the MOSFET will be OFF. In this condition, MOSFET behaves like an open
switch.
 Ohmic Region- The ohmic region is a region where the current (IDS)increases with an
increase in the value of VDS. When MOSFETs are made to operate in this region, they
are used as amplifiers.
 Saturation Region- In the saturation region, the MOSFETs have their IDS constant in
spite of an increase in VDS and occurs once VDS exceeds the value of pinch-off voltage
VP. Under this condition, the device will act like a closed switch through which a
saturated value of IDS flows. As a result, this operating region is chosen whenever
MOSFETs are required to perform switching operations.

The circuit below shows the configuration of MOSFET when it is used as a switch.

In the circuit arrangement, an Enhancement-mode N-channel MOSFET is used to switch a

simple lamp “ON” and “OFF.” The input fate voltage Vgs is adjusted to an appropriate positive
voltage to switch “ON” the device and the voltage level is set to a negative value or zero to turn

it “OFF.”