introduction to semiconductor technologyapachepersonal.miun.se/~gorthu/halvledare/l6_eng.pdf ·...
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![Page 1: Introduction to semiconductor technologyapachepersonal.miun.se/~gorthu/halvledare/L6_eng.pdf · Introduction to semiconductor technology. Outline – 6 Junctions • Metal-semiconductor](https://reader030.vdocuments.us/reader030/viewer/2022040112/5e7f127d4bf6ae1a9b799c98/html5/thumbnails/1.jpg)
Introduction to semiconductor
technology
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Outline
– 6 Junctions • Metal-semiconductor junctions
– 6 Field effect transistors • JFET and MOS transistors
• Ideal MOS capacitance
• Actual MOS capacitance
• MOS transistor “current equation" (L7)
• MOS-transitorn "transfer equation" (L7)
• MOS-transitorn channel mobility (L7)
• Substrate bias effect (L7)
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Metal-semiconductor junctions, rectifying
Vacuum energy level
Note the
importance of
clean surfaces
prior deposition
of metal
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Metal-semiconductor junctions, rectifying
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Metal-semiconductor junctions, rectifying,
with bias
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Metal-semiconductor junctions, ohmic, n-
substrate
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Metal-semiconductor junctions, ohmic, p-
substrate
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Metal-semiconductor junctions, ohmic,
tunneling
Although there is a barrier
between the metal and
semiconductor, it is possible to
make a good ohmic contact,
hard doping under the metal so
that the tunneling occurs
If the metal has high barrier
height to n-type, the most likely
low barrier height for p-type.
Exceptions exist, so called
Fermi-level pinning
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Field effect transistors (FET)
Junction-FET Metal-oxid-halvledar-FET
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Junction-FET (pinch of the channel and saturation)
When the drain voltage
increases reverse bias the
gate/drain.
The depletion region spreads
and chokes the channel, Id stop
increasing and becomes
constant.
Compare with constant current
generator
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Junction-FET (Gate control)
With modified gate voltage level
can the saturation of the current
be controlled.
For a p+n diode applies; a
L
When w = a, is precisely depleted, we have reached pinchoff
voltage
Note, the junction should not be biased in
forward direction
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Junction-FET (current-voltage characteristics, long
channel)
Current in a cross-section
caused by a voltage drop
case
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Junction-FET (current-voltage characteristics,
long channel)
Valid up to Vp
VD-VG=VP
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Junction-FET (current-voltage characteristics,
long channel)
At saturation applies
Verifierad
experimentellt
IDSS=Idsat (VG=0)
VD-VG=VP
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Short channel effects
• With a short channel increases the electric field and the
charge carrier reach saturation velocity
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MOS-transistorn
Voltage is applied on the gate
and capacitive attracts
electrons to form a leading
channel
Or
Depleting the channel to block
the transistor
G
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MOS-transistorn
MOS transistor with
conducting channel (inversion
layer)
Incipient pinch off channel with
applied drain voltage
Strong saturation
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Ideal MOS capacitance
metal
SiO2
Semiconductor
Work function is measured
from the oxides conduction
band, “modified"
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Ideal MOS capacitance
Accumulated holes is collected
by the interface oxide
semiconductors
++
++
+
Tilt of the conduction band
caused by the electric field
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Ideal MOS capacitance
Depletion region is formed
nearest the oxide/
semiconductor interface
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Ideal MOS capacitance
Inversion, a layer of
electrons are formed at
oxide/ semiconductor
interface
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Ideal MOS capacitance, strong
inversion
n conc (inversion) = p doping in the substrate
Describes the band bending f(x)
Band bending at the
interface, due to surface
potential
Example 3-5
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Ideal MOS capacitance
Space charge density as
a function of surface
potential
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Ideal MOS capacitance, in inversion
equal number of charges in the
metal as in the semiconductor
NOTE no charges in oxide in
this case.
In true MOS structures, are
always charges in oxide
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Ideal MOS capacitance, in inversion
Inversions charge is not included in
the drawings for the electric field and
potential
Voltage drop
across oxide
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Ideal MOS-kapacitans, in inversion
metal
SiO2
semiconductor w
W is calculated as if
it were a n+p-diode
Maximum depletion
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Ideal MOS capacitance, in inversion
The charge (depending on fixed ionized doping atoms) in the depletion
area in strong inversion can then be written:
Fs inv 2)(
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Ideal MOS capacitance
”oxide "capacitance in series with depletion capacitances
Measurement at low
frequencies (100Hz)
Measurement at high
frequencies (1 MHz)
d w
Cd
C sd
ii
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Actual MOS Capacitances
• Change in workfunction metal
SiO2
Semiconductor
Poly silicon
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Actual MOS Capacitances
Charges in
oxide
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Actual MOS Capacitances
Difference in workfunction between the metal (polysilicon)
semiconductors influences on the threshold voltage VT