Lecture12 1 .pdf - EECS 105 Fall 2003 Lecture 12 Lecture...

Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12
Lecture 12: MOS Capacitors,
transistors
Prof. J. S. Smith
Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
Context
In the last lecture, we discussed
–
PN diodes, and the
–
depletion layer into semiconductor
surfaces.
–
Small signal models
In this lecture, we will apply those
results to:
z
MOS Capacitors (3.7-3.9):
–
Accumulation, Depletion, Inversion
–
Threshold Voltage
–
CV Curve
z
And start the discussion of transistors

Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
MOS structures
z
MOS stands for Metal*-Oxide-semiconductor
z
MOS structures are the heart of field effect
transistors.
z
The MOS structure gives a control mechanism for
the carriers in a layer called the channel by
changing the voltage on a nearby layer called the
gate
*The “metal” is usually a doped polysilicon layer
these days
Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
MOS
z
The MOS structure can be viewed as a PN diode,
with one heavily doped side (the “metal”), and an
extra insulating layer.
z
The extra insulating layer stops currents from
flowing, but the electric fields still penetrate

Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
Charge distribution in a MOS structure
z
At equilibrium, there is a depletion region into the
semiconductor, but since the “metal” has a
LOT
of
carriers per unit volume, it has little or no depletion depth
into it, and any charge on it is at its surface
“Metal”
Oxide
Semiconductor
(n type)
Depletion region
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+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
Reverse Bias
z
Under a reverse bias, there isn’t much difference
between a PN diode and a MOS stack, the insulator
just blocks the minority carriers, so there is zero
current instead of very little current
“Metal”
Oxide
Semiconductor
(n type)
Depletion region
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
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Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
Forward Bias
z
Under a forward bias, the insulating layer blocks
diffusion, so the depletion layer narrows, but there
is still no vertical current.
“Metal”
Oxide
Semiconductor
(n type)
Depletion region + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
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Department of EECS University of California, Berkeley
EECS 105 Fall 2003, Lecture 12 Prof. J. S. Smith
Accumulation
z
Under a higher forward bias, the mobile carriers get pushed
up against the barrier, and start to pile up in a thin layer
there, the accumulation layer
z
The bias where accumulation starts is called flat band
“Metal”
Oxide
Semiconductor
(n type)
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