Define pinch-off voltage of FET?

The pinch-off voltage of a Field-Effect Transistor (FET) is a critical parameter that determines the transistor's behavior in the subthreshold region. It is the gate-source voltage (VGS) at which the channel between the source and drain terminals of the FET is completely depleted of majority carriers, causing the drain current (ID) to drop sharply.

Mathematically, the pinch-off voltage can be expressed as:

$$ V_{P} = V_{T} + \frac{Q_{dep}}{C_{ox}} $$

where:

• $$ V_{T}$$ is the threshold voltage of the FET
• $$ Q_{dep}$$ is the depletion charge in the channel
• $$ C_{ox}$$ is the gate oxide capacitance per unit area

The depletion charge is the charge stored in the depletion region of the FET channel, which is the region where the majority carriers are depleted. The gate oxide capacitance is the capacitance between the gate and the channel.

The pinch-off voltage is an important parameter for designing and analyzing FET circuits. It is used to determine the operating point of the FET, the gain of the FET, and the input/output characteristics of the FET.

Physical Interpretation of Pinch-Off Voltage

The physical interpretation of the pinch-off voltage can be understood by considering the energy band diagram of a FET. When the gate-source voltage is below the pinch-off voltage, the channel is formed, and there is a continuous path for majority carriers to flow from the source to the drain. However, as the gate-source voltage approaches the pinch-off voltage, the depletion region widens, and the potential barrier at the source-channel junction increases. This makes it more difficult for majority carriers to flow from the source to the channel, resulting in a decrease in drain current.

At the pinch-off voltage, the depletion region extends all the way to the source-channel junction, and the potential barrier becomes so high that no majority carriers can flow from the source to the channel. This causes the drain current to drop sharply, and the FET enters the subthreshold region.

Significance of Pinch-Off Voltage

The pinch-off voltage is a significant parameter in FET design and analysis because it:

• Determines the subthreshold characteristics of the FET
• Affects the gain of the FET
• Influences the input/output characteristics of the FET
• Limits the maximum drain current of the FET

By carefully controlling the pinch-off voltage, designers can optimize the performance of FET circuits for specific applications.