Short Answer
Enhancement and depletion modes are two ways in which a MOSFET operates. In enhancement mode, the MOSFET is normally OFF and starts conducting only when a proper gate voltage is applied.
In depletion mode, the MOSFET is normally ON and conducts even without gate voltage. It can be turned OFF by applying a suitable voltage. These modes help control current flow in different applications.
Detailed Explanation
Enhancement and depletion modes in MOSFET
Enhancement mode
Enhancement mode MOSFET is the most commonly used type of MOSFET. In this mode, the transistor is normally in the OFF state when no voltage is applied to the gate terminal.
In an N-channel enhancement MOSFET, when the gate-to-source voltage is zero, there is no conducting channel between the source and drain. Therefore, no current flows through the device.
When a positive voltage is applied to the gate, it creates an electric field that attracts electrons towards the gate region. As the voltage increases, these electrons form a conducting path, called a channel, between the source and drain.
Once this channel is formed, current starts flowing. The strength of the channel increases with increase in gate voltage, which allows more current to flow. This process is called enhancement because the conductivity of the device is increased by applying voltage.
In a P-channel enhancement MOSFET, the working is similar, but a negative gate voltage is used to create the channel using holes.
Enhancement mode MOSFETs are widely used in digital electronics and switching circuits because they remain OFF when no input is given, which helps in saving power.
Depletion mode
Depletion mode MOSFET works in the opposite way compared to enhancement mode. In this type, the MOSFET is normally ON even when no gate voltage is applied.
This happens because a conducting channel already exists between the source and drain. Therefore, current can flow without any external voltage at the gate.
In an N-channel depletion MOSFET, when a negative voltage is applied to the gate, it repels electrons from the channel. This reduces the width of the channel and decreases the current flow.
If the negative voltage is increased further, the channel can be completely closed, and current flow stops. This is called depletion because the charge carriers are reduced or depleted from the channel.
Similarly, in a P-channel depletion MOSFET, a positive gate voltage is used to reduce the flow of holes.
Depletion mode MOSFETs are less commonly used but are useful in certain applications like analog circuits and signal control.
Key differences
The main difference between enhancement and depletion modes is their initial state. Enhancement mode MOSFET is normally OFF, while depletion mode MOSFET is normally ON.
Another difference is how the channel is formed. In enhancement mode, the channel is created by applying gate voltage. In depletion mode, the channel already exists and is controlled by reducing it.
Enhancement mode is widely used in digital circuits due to low power consumption, while depletion mode is used in special applications where a normally ON device is required.
Conclusion
Enhancement and depletion modes define how a MOSFET operates. Enhancement mode requires gate voltage to start conduction, while depletion mode conducts by default and can be turned OFF by voltage. Both modes are important for controlling current in different electronic applications.