Fermi energy band diagrams in NMOS

Fermi energy band diagrams in NMOS

• Depletion Mode 

     

    

When a positive gate voltage is applied to a n type MOSFET , holes near the gate

oxide and substrate interface are pushed away due to the same charges. Hence

there exist no excess holes in the region and net charge there becomes zero.

Thus forming a depletion region with all charged particles in pair (electron hole pairs).

Due to the formation of  depletion region the intrinsic energy level becomes equal

to the fermi energy . The shift in the bands is nearly equal to the gate voltage

applied.

• Inversion Mode

When further positive voltage is applied at the gate , free electrons start to get attracted towards the gateocide - substrate interface. Thus near the interface the concentration of n type impurity appears in a p-type substrate. It suggest inversion of the substrate type and so termed as inversion mode.

The intrinsic energy band (Ei) crosses the fermi energy level as Ei lies below Ef for negative doped impurity.

• Accumulation Mode

When negative voltage is applied at the gate terminal of n type MOSFET , a layer of negative charge is formed on the metal gate. This layer attracts the holes in the p-type semiconductor substrate towards the gate oxide - semiconductor interface. Inturn the majority charge carriers of the p-type semiconductor get concentrated near the oxide. 

Due to increase in concentration of holes near the gate the E intrinsic shifts upwards towards conduction band

Author : Dhanesh Manwani