MOSFET Characteristics

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). It is a transistor that controls the flow of electric current in the circuit. The MOSFETs have 3 main terminals: the Gate (G), the Drain (D), and the Source (S). They are popular because they work efficiently. MOSFETs are able to handle high currents and voltages, making them useful for both small digital circuits and larger power systems.

MOSFET Construction

MOSFET is a transistor that is used as a switch or amplifier and in many other applications. The basic construction of MOSFET can be explained as below:-

• Substrate: MOSFET is constructed on a silicon wafer that is it acts as a base of the device.
• SiO2: A thin layer of insulating material is formed with SiO2 for the exchange of electrons and holes.
• Gate Terminal: A gate terminal is formed on the insulating layer. This controls the flow of current between the drain and the source with the help of gate voltage.
• Source and drain terminals region: These are created on either side of the gate. These are basically doped regions.
• Channel: The region between the gate, drain, and source is known as the channel which controls the flow of charge among them.

Types of MOSFETs

There are two major types of MOSFETs

N-channel MOSFET: More common and efficient, in this the current will only flows when a positive voltage is applied to the gate.

P-channel MOSFET: Current flows when a negative voltage is applied to the gate.

Working Principle of MOSFETs

When some voltage is given to the gate terminal of a MOSFET , it creates an electric field that allows current to flow between the drain of MOSFET and the source of the MOSFET. Hence the amount of the current flowing in the MOSFETs can be controlled by changing the gate voltage according to the need.

Characteristics of MOSFET

Now the central part of the article, her we will learn about the characteristics of MOSFET. There are basically two types of characteristics of a MOSFET:

1. Drain characteristics

2. Transfer characteristics

1. Drain Characteristics of Enhancement Type MOSFET

Characteristics between output current and output voltage. O/P i -> I_D, O/P v -> V_DS , control variable = V_GS. Here we will plot a graph between I_D and V_DS for various levels of V_GS.

Now let's understand by cases:

Case 1: V_GS1 > V_T

(where V_T is the threshold voltage)

V_eff = V_GS1 – V_T

It will directly affect the width of the n-channel

significant amount of drain current will flow through it.

When V_DS is increased to a certain level, drain current becomes constant. This is called pinch-off condition.

V_DS + V_GD – V_GS = 0

V_GD = V_GS – V_DS

V_GD = V_T , pinch off voltage

Case 2: V_GS1 > V_GS2

conductivity of 1 > conductivity of 2

R2> R1

slope of 2 < slope of 1

Now there are few regions in this graph:-

Saturation region : Region in which drain current is constant.

Triode region : Left area of locus of VDS saturation.

Cut-off region : When it can’t achieve the threshold voltage, MOSFET remains OFF. Current is 0 amp.

• With increasing voltage, current flowing through the terminals increases with voltage. You can see this in the graph. VGS more than the threshold voltage is the condition of flowing current.

2. Transfer Characteristics of Enhancement Type MOSFET

Characteristics between output current and input voltage.

V = V_T (Threshold Voltage)

• When the value of voltage across gate and source is less than the threshold voltage

Advantages and Disadvantages of MOSFETS

Advantages

• They consume less power, perfect for battery-operated devices.
• They are easily controlled by small signals.
• They are small and lightweight, fitting well in compact electronic devices.

Disadvantages

• Designing circuits with them can be more complex than with other transistors.
• High-performance versions can be more expensive.

Applications of MOSFETS

• Used in switching power supplies and voltage regulators.
• Common in audio and radio frequency amplifiers
• Used in digital circuits for switching operations

Conclusion

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a transistor used to control electric current in circuits. It has three main terminals: Gate (G), Drain (D), and Source (S), and is built on a silicon substrate with an insulating layer of SiO₂. The gate terminal controls current flow between the drain and source. MOSFETs come in two types: N-channel (current flows with positive gate voltage) and P-channel (current flows with negative gate voltage).

The working principle involves applying a voltage to the gate, creating an electric field that allows current to flow between the drain and source. MOSFET characteristics are divided into drain characteristics (relationship between output current and voltage) and transfer characteristics (relationship between output current and input voltage). Key regions in the drain characteristics include the saturation region (constant drain current), triode region, and cut-off region (no current flow).