The Effect of Voltage on the Threshold Voltage of a MOSFET

The threshold voltage of a MOSFET is a crucial parameter that determines the operation of the device. It is the voltage level at which the MOSFET starts to conduct current between its source and drain terminals. The threshold voltage is affected by various factors, one of which is the voltage applied to the back contact, also known as the substrate or body terminal.

Threshold Voltage
The threshold voltage (Vth) is the voltage level at which the MOSFET transitions from the off-state to the on-state. When the applied voltage at the gate terminal exceeds the threshold voltage, the MOSFET starts to conduct current. Below the threshold voltage, the MOSFET remains in the off-state, and no significant current flows through it.

Effect of Back Contact Voltage
The back contact of a MOSFET is typically connected to the substrate, which can be biased independently. The voltage applied to the back contact affects the behavior of the MOSFET in several ways, including its threshold voltage. Here's how:

1. Body Effect: The voltage applied to the back contact creates an electric field within the substrate. This electric field interacts with the depletion region between the body and the channel of the MOSFET. The depletion region width is inversely proportional to the voltage applied to the back contact. As the depletion region width changes, it affects the channel width and, consequently, the threshold voltage.

2. Threshold Voltage Shift: When a voltage is applied to the back contact, it can shift the threshold voltage of the MOSFET. This shift can be either positive or negative, depending on the polarity of the applied voltage. If the back contact voltage is positive with respect to the source terminal, it can increase the threshold voltage. Conversely, if the back contact voltage is negative, it can decrease the threshold voltage.

3. Substrate Doping: The doping concentration of the substrate also plays a role in determining the threshold voltage. By applying a voltage to the back contact, the substrate doping concentration can be effectively modified, leading to changes in the threshold voltage.

Conclusion
In summary, the voltage applied to the back contact of a MOSFET affects its threshold voltage. This is due to the body effect, which changes the depletion region width and consequently alters the channel width and threshold voltage. The back contact voltage can shift the threshold voltage and modify the substrate doping concentration, further influencing the MOSFET's behavior. Therefore, option 'A' - Threshold Voltage is the correct answer to the given question.