The
depletion occurs when V
GS
> V
T
and V
DS
> 0 as the electron concentration at channel
increases thereby increasing the hole concentration at metal due to the
presence of external
supply. The conduction band of oxide layer near the semiconductor is tilted upwards
indicating the increase of electrons at the semiconductor.
Thus maintaining the work function and energy gap equally
the conduction band and
valence band are tilted downwards.
Fig: Depletion condition of MOSFET
4) Inversion:
The inversion occurs when V
GS
>> V
T
and V
DS
>> 0.
The band bending of the valence band is so high because intrinsic energy
crosses E
F
in N type
E
F
is above E
I
and in P type E
i
is below E
F
.
Since voltage applied is positive to the gate, electrons travel
towards the gate and
accumulate near the semiconductor-oxide junction resulting in the development of surface
potential. Due to surface potential energy band bending takes place.
Fig: Inversion condition of MOSFET
⮚
VT equation:
● V
T
= V
to
+ ϒ (√(2∅𝐹 + 𝑉𝑆𝐵) − √2∅𝐹 )
At V
SB
= 0 v
T
= V
to
● Body bias: V
SB
if there exists a potential difference between source and
substrate it is known as
body bias.
✔
Check on VSB potential for NMOS for VB = -1v
1.
V
S
connected to +1v
V
SB
= V
S
– V
B
= 1-(-1)
= 2v
When V
SB
is connected with forward bias there are chances that the electrons flow might be
distorted and current direction will change leading to improper function of the device.
