Threshold Voltage in NMOS Depletion

Threshold voltage is an essential parameter that determines whether a MOSFET will conduct or not. It is the voltage applied to the gate of a MOSFET that triggers the conduction of current between its source and drain terminals. The threshold voltage is negative in NMOS depletion, which means that the gate voltage must be less than the threshold voltage to create a conductive channel between the source and drain terminals.

Depletion MOSFET

A depletion MOSFET is a type of MOSFET that is constructed by doping the substrate with impurities that create a conductive channel between the source and drain terminals, even without the application of a gate voltage. The channel is formed by depleting the majority carriers in the substrate region under the gate oxide. The depletion MOSFET has two types: n-type and p-type.

NMOS Depletion

An NMOS depletion MOSFET is constructed by doping an n-type substrate with p-type impurities, which create a conductive channel between the source and drain terminals. When a negative voltage is applied to the gate, it attracts positive charges and creates an additional depletion region in the substrate. This depletion region reduces the width of the conductive channel, which increases the resistance of the MOSFET. As the gate voltage becomes more negative, the depletion region widens, and the conductive channel eventually disappears, leading to no current flow between the source and drain terminals.

Threshold Voltage in NMOS Depletion

The threshold voltage in NMOS depletion MOSFET is the voltage required to create a conductive channel between the source and drain terminals. It is negative because the channel is already present in the substrate due to the doping process. The gate voltage must be less than the threshold voltage to create a conductive channel, which means that the MOSFET is already conducting at zero gate voltage.

Conclusion

In conclusion, the threshold voltage is negative in NMOS depletion MOSFET because the conductive channel is already present in the substrate due to the doping process. The gate voltage must be less than the threshold voltage to create a conductive channel, which means that the MOSFET is already conducting at zero gate voltage.