Download MOS Transistor Theory

MOS Transistor Theory
• The MOS transistor is a
majority carrier device having
the current in the conducting
channel being controlled by the
voltage applied at the gate
terminal.
• A pMOS has holes as majority
carriers while an nMOS has
electrons as majority carriers.
• A MOS structure with two
terminals (no drain or source)
can operate in three regions.
• These are:
– Accumulation
– Depletion and
– Inversion
• Of significance is interface at
the parallel plate capacitor.
The MOS Structure
Energy Band Diagrams
Modes of Operation
Mass Law Action
n  p  ni
2
n p0
ni

NA
p p0  N A
2
The Fermi Potential
•
•
The function of temperature and doping concentration that denotes the difference
between the intrinsic Fermi Level Ei and the Fermi Level EF is the Fermi
Potential.
It is given by:
E F  Ei
F 
q
For a p - type semiconduc tor the Fermi Potential is approximat ed by :
n
kT


ln i
Fp
q
N
A
For an n - type semiconduc tor doped with donar concentrat ion N
D
kT N D


ln
Fn
q
ni
The Fermi Potential
•
•
•
The electron affinity of Silicon
which is the potential difference
between the conduction band and
the vacuum (free space) is
expressed as qχ.
The energy required to move an
electron from the Fermi Level into
free space is called the work
function qΦS and is given by: qΦS=
qχ+EC-EF
If we bring the gate material (poly),
the silicon dioxide (SiO2) and the
Semiconductor (Si) together, the
Fermi Levels have to line up to
form a MOS capacitor.
•
•
•
The Fermi potential at the surface also
called surface potential ΦS is smaller in
magnitude than the bulk Fermi potential
ΦF.
If we apply an external voltage to the
gate of the MOS capacitor when the
bulk voltage is 0Vand depending on the
polarity and magnitude of the applied
voltage we end up with three regions of
operation, namely accumulation,
Depletion and inversion.
IF a fairly small gate voltage (positive)
is applied at the gate the region at the
semiconductor/Oxide interface becomes
devoid of mobile carriers.
The Depletion Region
•
•
•
•
The thickness of the depletion region xd can be computed as a function of
the surface potential ΦS.
The amount of the depletion region charge plays an important role in the
analysis of the threshold voltage.
The surface at the Semiconductor/Oxide interface is said to be inverted
when the density of mobile electrons on the surface becomes equal to the
density of holes in the bulk.
This condition dictates that
 F  S