Transcript Lecture 17

Lecture #17 MOS transistors
MIDTERM coming up a week from
Monday (October 18th)
Next Week: Review, examples,
circuits
Reading: MOS: chapter 14
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Midterm
• Monday, October 18,
• In class
• One page, one side of notes
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Topics
Today:
• Metal Oxide Semiconductor (MOS) Transistors
–
–
–
–
–
–
Physical structure
Physical operation
Circuit symbol and current/voltage designations
Modes of operation
I-V Relationship
Solution of MOS circuits
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MOS transistor
• The bipolar transistor controls current
trough a reverse biased diode, so it is
intrinsically a current controlled device,
and it produces a current output.
• A MOS transistor uses a voltage to pinch
off a conductive channel, and therefore is
like a voltage controlled switch.
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MOS
• In a MOS device, a voltage is applied to a metal
layer, which pushes away mobile carriers in a
semiconductor layer.
• The metal is separated from the semiconductor
by an insulating layer, usually an oxide.
Metal
Oxide
Semiconductor
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MOS
• If the voltage on the metal is negative, it
attracts holes into the semiconductor
• If the voltage on the metal is positive, it
attracts electrons into the semiconductor
Metal
Oxide
------------------------Semiconductor
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The MOS as a switch
• If there are mobile carriers in the semiconductor,
they can conduct a current. (switch closed)
• A device which uses electrons needs a positive
voltage on the metal to conduct
– called an NMOS device
• A device which uses holes needs a negative
voltage on the metal to conduct.
– called a PMOS device
• If there are no mobile carriers under the oxide,
then no current can flow (switch open)
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NMOS and PMOS
What determines if the mobile carriers that a
device uses are holes or electrons?
• The contacts on either end.
• If the contacts on the ends are N type, they will
allow electrons to flow under the oxide, but holes
would be blocked by a reverse biased diode
• If the contacts on the ends are P type, they will
allow holes to flow under the oxide, but electrons
are blocked from flowing by a reverse biased
diode.
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NMOS (N-Channel Metal Oxide
Semiconductor)
Transistor
gate
source
metal
n-type
metal
oxide insulator
drain
metal
n-type
p-type
metal
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NMOS Transistor in Equilibrium
gate
source
metal
_ + n-type+ _
+ +
_
_
metal
oxide insulator
h
h
drain
metal
_ + n-type
+ _
+
_+
_
p-type
h
h
metal
A PN junction separates the N regions from the P regions with a depletion
region.
No current flows between the source and the drain because of these depletion
regions, even if there is a voltage difference between the drain and the source
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NMOS Transistor in Cutoff
VGS > 0
gate
- +
source
metal
n-type
+ + +
_
_
_
metal
oxide insulator
_
_ _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
metal
When a small, positive VGS is applied, holes “move away” from the
gate.
There is still no current flow between the source and the gate
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NMOS Transistor Channel
VGS > VTH(n)
gate
- +
source
metal
n-type
+ + +
_
_
metal
oxide insulator
ee_ e _e_ e _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
h
h
h
h
h
metal
When VGS is larger than a threshold voltage VTH(n), the attraction to the gate
is so great that free electrons collect there.
The applied VGS creates a channel under the gate (an area with free
electrons).
Now current can flow if there is a voltage from the source to the drain.
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NMOS Transistor Drain Current
- +
VGS > VTH(n)
VDS > 0
gate
- +
source
metal
n-type
+ + +
_
_
metal
oxide insulator
ee_ e _e_ e _
_
drain
metal
n-type
+ + +
_
_
p-type
h
h
h
h
h
h
h
h
h
h
metal
When a positive VDS is applied, the free electrons flow from the source to the
drain. (Positive current flows from drain to source).
The amount of current depends on VDS, as well as the number of electrons in
the channel, channel dimensions, and material.
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NMOS Transistor CircuitVSymbol
DS
- +
VGS
drain
gate
- +
source
ID
IG
metal
oxide insulator
metal
n-type
metal
n-type
p-type
metal
G
IG
ID
S
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-
VDS +
D
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NMOS I-V Characteristic
G
IG
ID
S
-
VDS +
D
• Since the transistor is a 3-terminal device, there
is no single I-V characteristic.
• Note that because of the insulator, IG = 0 A.
• We typically define the MOS I-V characteristic as
ID vs. VDS
for a fixed VGS.
• The I-V characteristic changes as VGS changes.
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NMOS I-V Curves
ID triode mode
saturation mode
VGS = 3 V
VDS = VGS - VTH(n)
VGS = 2 V
VGS = 1 V
cutoff mode (when VGS < VTH(N))
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VDS
16
Saturation in a MOS transistor
• At low Source to drain voltages, a MOS transistor looks
like a resistor which is “turned on” by the gate voltage
• If a more voltage is applied to the drain to pull more
current through, the amount of current which flows stops
increasing→ an effect called pinch-off.
• Think of water being sucked through a flexible wall tube.
Dropping the pressure at the end in order to try to get
more water to come through just collapses the tube.
• The current flow then just depends on the flow at the
input: VGS
• This is often the desired operating range for a MOS
transistor, as it gives a current source at the drain as a
function of the voltage from the gate to the source.
• Note the different use of the word saturation for MOS
and Bipolars
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