two_terminal_mos.md

Physics of Two Terminal MOS Structure and it’s operation

The MOS Field Effect Transistor (MOSFET) is the fundamental building block of MOS and CMOS digital integrated circuits. Compared to the bipolar junction transistor (BJT), the MOS transistor occupies a relatively smaller silicon area, and its fabrication involves fewer processing steps. These technological advantages, together with the relative simplicity of MOSFET operation, have helped make the MOS transistor the most widely used switching device in LSI and VLSI circuits.

Here, we shall briefly discuss the operation of a two-terminal MOS structure as shown in Figure below.

It may be noted that, due to the work function difference between the metal and semiconductor side, a built-in potential drop takes place inside the MOS system. As a result, there is a small downward bending takes place. The difference in work function is called flat-band voltage. Example: Let the Fermi potential $\phi_f$ =0.2eV. Then semiconductor side work function ($\phi_s$) is calculated as: $\phi_s$ = 4.15+0.55+0.2 =4.9eV. Where 4.15eV is the electron affinity of silicon, 0.55eV is the distance between the conduction band and intrinsic level (half of the band gap, $E_g/2$), and 0.2eV is Fermi potential ($\phi_f$). Similarly, the metal side work function is $\phi_m$ = 4.10eV. Hence, the work function difference between the metal and semiconductor sides is $\phi_{ms}=\phi_{m}-\phi_{s}=4.1-4.9=-0.8eV $

If this amount of voltage can be externally applied then the energy band diagram will be flat. That’s why the name is called flat-band voltage.