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Unformatted text preview: (b) onset of saturation
at pinch-off, VG >
VT and VD = (VG VT);
(c) strong saturation,
VG > VT and VD > Band Diagrams for MOS Capacitors
• Analysis using Energy Band Diagrams
– Modified work functions for the metal and
semiconductor layers are used. The modified work
function describes the energy required to move
electrons from the associated Fermi level to the
conduction band of the insulator
• qΦm is the modified work function for metal
• qΦs is the modified work function for the
• in the semiconductor, qΦF = Ei – EF Band Diagrams for MOS Capacitors
• V < 0 (Accumulation)
– holes accumulate on the semiconductor side of
the SC/Insulator interface. Called hole
EF is raised by an amount qV due to electrons
accumulating at the metal/SC interface.
The charge accumulation results in an electric
field across the insulator
The E field causes a gradient in the oxide
conduction Band Diagrams for MOS Capacitors
– – • Hole accumulation requires that the difference
between EF and Ei to increase in the accumulation
region. This happens by band bending in this
region. The bands bend to reflect the distribution
of charge carriers in the semiconductor.
For hole accumulation in p-type material, the
bands bend upward.
– (Depletion) EF of the metal is lowered by qV as a result of the
Negative charge accumulates at the surface of the
semiconductor in response to the charge.
semiconductor Band Diagrams for MOS Capacitors
– In p-type material, this results in the depletion of
holes at the interface. The negative charge results
from uncompensated ionized acceptors in this
region and electrons accumulating there.
– An E field is created and the energy levels in the
insulator has a gradient
– In the semiconductor, EF remains fixed. The
energy bands bend downward to reflect th...
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