Field Effect
Transistors (1)
Dr. Wojciech Jadwisienczak
EE314
http://www.plasticlogic.com
Q: How we can do this?
A: A new generation of MOSFETs for plastic electronics
1.Construction of MOS
2.NMOS and PMOS
3.Types of MOS
4.MOSFET Basic Operation
5.Characteristics
6.Small-Signal Equivalent Circuits
7.Examples
Chapter 12: Field
Effect Transistors
pp. 544-577
Figure 2 -120 nm footprint T-gate with
self-aligned source and drain contacts.
Figure 4 – showing the lateral etching control offered by “digital” recess
etching. In both micrographs, the depth of the recess is 30 nm. The
micrograph on the left shows a “wide” gate recess with large lateral extent
whilst the micrograph on the right shows a recess tightly defined around the
gate footprint.
http://www.three-fives.com/latest_features/webzine_features/iainthainestory.html
Building A MOSFET Transistor Using Silicon
http://micro.magnet.fsu.edu/electromag/java/transistor/index.html
It is done. Now, how does it
work?
JFET and MOSFET Transistorsor
JFET – Junction Field Effect Transistor
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
n-channel MOSFET & p-channel MOSFET
Symbol
L=0.1-10mm
W=0.5-500mm
SiO2 Thickness=0.05-0.1mm
Device characteristics depend on L,W, Thickness, doping levels
n-channel MOSFET Basic Operation
Operation in the Cutoff region
pn junction:
forward
bias, reverse
bias
iD=0
for vGS<Vt0
Schematic
When vGS=0 then iD=0 until vGS>Vt0 (Vt0 –threshold voltage)
n-channel MOSFET Basic Operation
Operation in the Triode Region
For viDS<vGS-Vt0 and vGS>Vt0 the NMOS is operating in the
triode region
Resistor like characteristic
(R between S & D,
Used as voltage controlled R)
For small vDS, iD is proportional
to the excess voltage vGS-Vt0
n-channel MOSFET Basic Operation
Operation in the Saturation Region (vDS is increased)
Tapering
of the
channel
iD is smaller
when
vDS is
larger

2
i D  K 2v GS  Vt 0 v DS  v DS
When vGD=Vt0 then the channel
thickness is 0 and

 W  KP
K  
L 2
i D  K vGS  Vt 0 
2
Device parameter KP for
NMOSFET is 50 mA/V2
n-channel MOSFET Basic Operation
Characteristic
Example 12.1
Channel length
modulation
id depends on vDS in
saturation region
(approx: iD =const in
saturation region)
2
i D  Kv DS
p-channel MOSFET Basic Operation
It is constructed by interchanging the n and p regions of
n-channel MOSFET.
Symbol
Characteristic
How does operate
p-channel MOSFET?
-voltage polarities
-iD current
-schematic
Load-Line Analysis of NMOS Amplifier
It is a graphical analysis similar to load-line analysis of pn diode.
Schematic
We look for operating
point
vGS
vDS
Analysis…
Input
loop
vGS (t )  vin (t )  VGG
vGS (t )  sin( 200t )  4
Output
loop
VDD  RD i D (t )  v DS (t )
Load
line
20  RDiD (t )  vDS (t )
Load-Line Analysis of NMOS Amplifier
Load line
20  RDiD (t )  vDS (t )
Taking iD=0 or vDS=0 we find
out the quiescent operating
point Q
The quiescent values
vGS (t )  vin (t )  VGG
vin(t)=0 then iDQ=9 mA
and vDSQ=11V
Points A & B
intersection of
curve and the
load-line
Load-Line Analysis of NMOS Amplifier
Input signal vin (t )  1sin( 200t ) (peak-to-peak amplitude is 2V)
12V
Inverse
operation
The positive peak of the input occurs at the same time as the min.
value of vDS. These are not symmetrical sinusoids! (nonlinear distortion)
Bias Circuits
Analysis of amplifier circuits is often undertaken in two steps:
(1) The dc circuit analysis to determine the Q point. It involves the
nonlinear equation or the load-line method. This is called bias
analysis
(2) Use a linear small-signal equivalent circuit to determine circuit
parameters
The fixed-plus
self-bias circuit
Analysis…
VG  VDD
Equivalent
circuit
R2
R1  R2
vGS
VG  vGS  RS i D
Assume the VRG=0
For saturation region
iD  K vGS  Vt 0 
2
v DS  VDD  RD  RS i D
Input
Output
vDS
Bias Circuits
Plot of
VG  vGS  RS i D
Disregarded root
for vGS<Vt0
Use only larger
root for vGS and
smaller for iD
Example 12.2
and
iD  K vGS  Vt 0 
2