A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a transistor that amplifies or switches electronic signals. It works by varying the channel width through which charge carriers (electrons or holes) can flow. Here’s a breakdown of how a MOSFET works:
Structure of a MOSFET
- Source: The terminal through which carriers enter the channel.
- Drain: The terminal through which carriers leave the channel.
- Gate: The terminal that controls the flow of carriers in the channel.
- Body (or Substrate): The semiconductor material (usually silicon) on which the MOSFET is built. It can be either p-type or n-type.
There are two main types of MOSFETs:
- n-channel MOSFET: Uses electrons as charge carriers.
- p-channel MOSFET: Uses holes as charge carriers.
Operation Modes
- Cutoff Mode: When the gate-source voltage (V_GS) is less than the threshold voltage (V_th), the MOSFET is off, and no current flows from drain to source (I_DS ≈ 0).
- Linear (or Ohmic) Mode: When V_GS is greater than V_th and the drain-source voltage (V_DS) is small, the MOSFET operates like a variable resistor. The current (I_DS) through the channel is proportional to V_DS.
- Saturation (or Active) Mode: When V_GS is greater than V_th and V_DS is large enough that the channel is pinched off near the drain, the current (I_DS) becomes almost independent of V_DS and is primarily controlled by V_GS.
Working Principle
n-channel MOSFET (Enhancement Mode):
- Off State (Cutoff): When the gate voltage (V_G) is less than the threshold voltage (V_th), there is no conductive channel between the source and drain, and the MOSFET is off.
- On State (Linear/Saturation): When V_G is greater than V_th, it creates an electric field that attracts electrons towards the gate, forming a conductive channel between the source and drain. The number of electrons (hence the channel conductivity) increases with increasing V_G, allowing current to flow from drain to source.
p-channel MOSFET (Enhancement Mode):
- Off State (Cutoff): When the gate voltage (V_G) is more optimistic relative to the source (for p-channel, this means V_GS is less negative), there is no conductive channel between the source and drain.
- On State (Linear/Saturation): When V_G is less positive (more negative), it creates an electric field that repels electrons, forming a conductive channel of holes between the source and drain. The number of holes increases with decreasing V_G, allowing current to flow from source to drain.
Key Parameters
- Threshold Voltage (V_th): The minimum gate-source voltage required to create a conductive channel.
- Gate Capacitance (C_g): The capacitance between the gate and the channel.
- Transconductance (g_m): The rate of change of the drain current with respect to the gate voltage.
- On-Resistance (R_DS(on)): The resistance between the drain and source when the MOSFET is on.
Applications
MOSFETs are widely used in various applications due to their high switching speed and efficiency. Common uses include:
- Switching power supplies.
- Amplifiers.
- Motor controllers.
- Digital circuits (like CMOS logic gates).
Visualization
- Off State (Cutoff):
- No channel: Source| | Drain
- On State (Linear/Saturation):
- n-channel: Source — (electrons) — Drain
- p-channel: Source — (holes) — Drain
FAQs
How does MOSFET work as an amplifier?
A MOSFET acts as an amplifier by controlling a large current flowing between the drain and source terminals with a much smaller voltage applied to the gate terminal. When a small input voltage is applied to the gate, the conductivity of the channel between the drain and source changes by modifying the electric field within the MOSFET. This change in conductivity leads to a proportional change in the drain current. By appropriately biasing the MOSFET in its active region and connecting it to a load resistor, the variations in the input signal voltage at the gate result in corresponding, amplified variations in the output voltage across the load resistor, thus achieving amplification.
How does MOSFET work as a switch?
A MOSFET acts as a switch by using the voltage applied to its gate terminal to control the current flow between the drain and source terminals. When the gate-to-source voltage exceeds a certain threshold, it creates a conductive channel between the drain and source, allowing current to flow. This enables the MOSFET to act as a switch, turning the current on and off as needed in a circuit.
How MOSFET acts as capacitor?
Through its gate-to-channel capacitance, a MOSFET can act as a capacitor. When a voltage is applied to the gate terminal, it creates an electric field across the insulating layer between the gate and the channel, forming a parallel-plate capacitor. This capacitive behaviour is crucial in high-frequency applications and affects the MOSFET’s switching characteristics and overall circuit performance.
Understanding how MOSFETs work is fundamental for designing and analyzing electronic circuits. The ability to control a large current with a small voltage makes them incredibly useful in various electronic applications.