Briefly explain Valence band Conduction band Intrinsic Fermi

Briefly explain: Valence band, Conduction band. Intrinsic Fermi level, Fermi level of PMOS and Fermi level of NMOS Briefly explain in the context of a MOS transistor: Weak Inversion and Strong Inversion. Explain why a scaling down of supply voltage has to be accompanied with a scaling down of threshold voltage as well Explain the operation of a PN junction diode under the following conditions: No bias, Forward-bias and Reverse-bias. Also, draw the V-I characteristic curve for the diode Sketch the cross section of an NMOS transistor. Label its parts and explain its operation. Also, draw the input-output characteristic curves for the transistor for various values of the drain-source voltage (V_DS) What is threshold voltage? How is the nominal threshold voltage for a transistor estimated from the transistor characteristic curve? What is body-biasing? Explain its effect on the effective threshold voltage of an NMOS transistor?

Solution

Valence band is formed due to the atomic valence shell that is completely filled at the zero temperature. Due to thermal excitation, some electrons get enough energy to jump the energy band gap at a time leaving the hole valency.

Conduction band is formed by the atomic valency shell, that is completley empty at the zero temperature. When the electron jumpd to the higher enery level by acquring enough energy, they conduct electricity once they reach the conduction band.

The semiconductors which do not have impurities are known as the intrensic semiconductors. The halfway between the valency band and the conduction band is known as fermi energy level in intrensic semiconductors. It is the function of effective masses of the electrons and holes in the semiconductors. It has equal number of holes in valence band and equal number of electrons in the conduction band. Therefore, it lies in the forbidden gap of the  intrensic semiconductors.

In case of P- channel MOSFET or PMOS positive voltage is applied to the gate, therefore intrensic fermi level movesbelow the fermi level. N- channel MOSFET is known as the NMOS. The conductive channel is present between the source and the drain. Threshold voltage of the NMOS is negative. Here, larger negative voltage is applied to the gate. So, the conduction band and valence band are bent and intrensic fermi level moves above the fermi level.

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