Qualitative Discussion of
MOS Transistors
Big Picture
• ES230
– Diodes
– BJT
– Op-Amps
• ES330
– Applications of Op-Amps
– CMOS
• Analog applications
• Digital applications
A Crude Metal Oxide
Semiconductor (MOS) Device
Positive charge attract
P-Type Silicon is
negative charges
slightly conductive.
to interface between
insulator and silicon.
The gate
draws no current!
V2 causes movement
of negative charges,
thus current.
V1 can control the
resistivity of the channel.
A conductive path is created
If the density of electrons is
sufficiently high.
Q=CV.
An Improved MOS Transistor
(provide electrons)
(drain electrons)
n+ diffusion allows
electrons move
through silicon.
Typical Dimensions of MOSFETs
tox is made really thin
to increase C, therefore,
create a strong control of Q by V.
These diode must
be reversed biased.
A Closer Look at the Channel
Formulation
Need to tie substrate to GND
to avoid current through PN
diode.
(OFF)
(ON)
VTH=300mV to 500 mV
Free electrons appear at VG=VTH.
Positive charges repel the holes
creating a depletion region, a region free of holes.
MOSFET as a Variable Resistor
As VG increases, the density of electrons increases, the value of
channel resistance changes with gate voltage.
You can build an
attenuator circuit.
(i.e. a voltage divider)
Change Drain Voltage
Resistance determined by VG.
Change Gate Voltage
Higher VG leads to a lower channel resistance, therefore larger slope.
Length Dependence
The resistance of a conductor is proportional to the length.
Dependence on Oxide
Thickness
Q=CV
C is inversely proportional to 1/tox.
Lower Q implies higher channel resitsance.
Width Dependence
The resistance of a conductor is inversely proportional to the cross
section area.
A larger device also has a larger capacitance!
Channel Pinch Off
• Q=CV
– V=VG-VOXIDE-Silicon
• VOXIDE-Silicon can change along the
channel! Low VOXIDE-Silicon implies less Q.
VG-VD is sufficiently large
to produce a channel
No channel
VG-VD is NOT sufficiently large
to produce a channel
Electrons
are swept
by E to drain.
Drain can no longer affect the drain current!
Regions
(No Dependence on VDS)
No channel
Determination of Region
• How do you know whether a transistor
is in the linear region or saturation
region?
– If VDS>(VGS-VTH) and VGS>VTH, then the
device is in the saturation region.
– If VDS<(VGS-VTH) and VGS>VTH, then the
device is in the linear region.
Graphical Illustration
Limited VDS Dependence
During Saturation
As VDS increase,
effective L decreases,
therefore, ID increases.
Pronounced Channel Length
Modulation in small L
Transconductance
• As a voltage-controlled current source, a MOS
transistor can be characterized by its
transconductance:
• It is important to know that
What Happens to gm/ID when
W and ID are doubled?
Body Effect
The threshold voltage will change when VSB=0!
Experimental Data of Body
Effect
The threshold voltage will increase when VSB increases.
What if we drive the base with a
small signal?
Input and Output
Vin, m=1 mV
Vout, m=46 mV
Replace the transistor by its
small signal equivalent circuit
(EQ 5.157)
Comparision:
ADS Simulation: 46
EQ 5.157: 49.33
Small Signal Model for NMOS
Transistor
PMOS Transistor
IV Characteristics of a PMOS
Small Signal Model of PMOS
Small Signal Model of NMOS