VLSI Lecture 7 - Threshold Voltage (It's 4 Components)
In enhancement type MOSFET, no conducting channel is present at zero gate bias.
Threshold voltage is defined as the minimum gate-to-source voltage required to induce or create a conducting channel.
Threshold voltage has four physical components:
1. Flat band voltage
2. Gate-to-source voltage component to change surface potential.
3. Gate-to-source voltage component to offset the depletion region charge.
4. Gate-to-source voltage component to offset(nullify) the fixed charges in gate-oxide and in silicon-oxide interface.
Flat band voltage is defined as a workfunction difference between the gate metal and the semiconductor when no charge is present in oxide-semiconductor interface.
External gate bias voltage must be changed to achieve the surface inversion hence to change surface potential by −2ΦF. Surface potential is developed due to accumulation of charge near oxide-semiconductor interface.