The electrical equivalent component for MOS structure is:
The MOS structure acts as a capacitor with metal gate and semiconductor acting as parallel plate conductors and oxide as dielectric between them.
The Fermi potential is the function of:
The Fermi potential, which is a function of temperature and doping, denotes the difference between the intrinsic Fermi level and the Fermi level.
The direction of electric field when the gate voltage is zero:
Metal being more positive compared to semiconductor.. Electric field exists from metal to semiconductor.
Consider a MOS structure with equilibrium Fermi potential of the doped silicon substrate is given as 0.3eV. Electron affinity of Si is 4.15eV and metal is 4.1eV. Find the built in potential of the MOS system:
Surface potential: qΦs = 4.15eV+0.55eV+0.3eV=5.0eV
qΦm-qΦs = 4.1eV – 5.0eV =-0.9eV.
When gate voltage is negative for enhancement mode n-MOS, the direction of electric field will be:
When gate voltage is negative, holes in substrate are attracted towards surface creating electric field from semiconductor to metal.
At threshold Voltage, the surface potential is:
When surface potential reaches –fermi potential, the surface inversion occurs. The gate voltage which brings these changes is known as threshold voltage.
Surface inversion occurs when gate voltage is:
Surface inversion occurs when gate voltage is equal to threshold voltage.
The energy band diagram of the MOS system when gate voltage is zero is:
The energy band diagram of enhancement mode nMOSFET when gate voltage is zero is :
For enhancement mode n-MOSFET, the threshold voltage is:
For enhancement mode n-MOSFET, the threshold voltage is positive quantity.
The threshold voltage depends on:
The threshold voltage depends on: The workfunction difference between gate and channel, The gate voltage component to change surface potential, The gate voltage component to offset the depletion charge and fixed charges in gate oxide