ME 6405 Student Lecture
Transistor
Cong Zhao
Feifei Qian
Kuan-Wen Tung
October 2, 2012
Georgia Institute of Technology
Contents
1
Introduction to Transistor
2
Bipolar Junction Transistor
3
Field Effect Transistor
4
Power Transistor
(Speaker: Cong Zhao)
(Speaker: Feifei Qian)
(Speaker: Kuan-Wen Tung)
(Speaker: Kuan-Wen Tung)
Introduction
 Question #1: How can we control the TV with
remote-controller?
 Question #2: How can a computer recognize
0(off) and 1(on) for computing?
Amplifier and Electronic Switch are needed.


Amplifier: any device that changes, usually increases, the amplitude of a signal.
andthat
Electronic
areclosing
needed.
ElectronicAmplifier
Switch: switch
the physicalSwitch
opening and
is achieved by
applying appropriate electrical control signals.
Introduction
 Early 20th century, vacuum tube was used for the amplifier and switch.
Vacuum Tube Radio
ENIAC, the first general-purpose electronic
computer, contains 17,468 vacuum tubes.
 However, Vacuum Tube is too big, fragile, and energy-consuming.
Transistor solved this problem.
Introduction – Invention of Transistor
 Invention



In 1947, John Bardeen, Walter Brattain,
and William Schockly, researchers at
Bell Lab, invented Transistor.
They found Transistor Effect: “when
electrical contacts were applied to a
crystal of germanium, the output power
was larger than the input.”
John Bardeen, Walter Brattain,
and William Schockly
In 1956, they were awarded the Nobel
Prize in physics.
First model of Transistor, 1947
Introduction – Definition of Transistor
A transistor is a three-terminal semiconductor
device that amplifies or switches the flow of
current between two terminals by varying the
current or voltage between one of the terminals
and a third.
Introduction – Semiconductor
 Semiconductor



is a material that has an electrical resistivity between that of a
conductor and an insulator.
has a few charge carriers(“holes” or free electrons).
the conductivity increases by adding impurities or dopants(doping).
Silicon is used in most
commercial
semiconductors
Introduction – Doping
 Doping: add neighboring elements to the semiconductor to
make it electropositive or electronegative
 P(positive)-type doping is adding Acceptor impurities (from Group III)
to the semiconductor in order to increase holes.
 Ex: Silicon doped with Boron
 N(negative)-type doping is adding Donor impurities (from Group V)
with more electrons in order to increase free electrons.
 Ex: Silicon doped with Phosphorous
Add Group III(Boron)
Add Group V (Phosphorous)
Introduction – P-N Junction
 PN Junction

is a junction formed by P-type and
N-type semiconductors together in
very close contact.
What happens at the junction?

Thin depletion region forms near
junction at an equilibrium condition
(Zero bias)

Near the p–n interface, electrons
from N diffuses into P, leaving
positively charged ions in N
region; holes from P diffuses into
N, leaving negatively charged ions
in P region

Ions exhibit a force which inhibits
further carriers flow across the P-N
Junction unless a forward bias
external voltage is applied.
Introduction – Forward and Reverse Bias
 Forward bias




-V pumps electrons into the N-region.
+V pumps more holes into the P-region.
Excess of charge in P and N region will
apply pressure on the depletion region
and will make it shrink.
→ current flows
 Reverse bias




-V sucked out electrons from N-region.
+V sucked out holes from P-region.
The depletion layer widens and it
occupies the entire diode(p-n).
→ current doesn’t flow
Introduction – Transistor
 Transistor is comprised by two P-N junctions:
Base-Collector junction and Base-Emitter junction.
Introduction – Types of Transistor
 Transistor are categorized by
Semiconductor material: germanium, silicon, gallium arsenide, etc.
Structure: BJT, FET, IGFET (MOSFET), IGBT
Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
Maximum power rating: low, medium, high
Maximum operating frequency: low, medium, high
Application: switch, audio, high voltage, etc.
Physical packaging: through hole, surface mount, ball grid array, etc.
Amplification factor








 General Purpose Transistors



Bipolar Junction Transistor (BJT)
Field Effect Transistors (FET)
Power Transistors
NPN BJT
n-channel
JFET
BIPOLAR
JUNCTION
TRANSISTOR
by: Feifei Qian
Introduction
• A bipolar junction transistor (BJT) is a three terminal
semiconductor
device
in
which
the
operation
depends on the interaction of both majority and
minority carriers and hence the name Bipolar.
• It is used in amplifier and oscillator circuits, and as a
switch in digital circuit.
• It has wide applications in computers, satellites and
other modern communication system.
Structure and Symbols
Three Regions in BJT
 Emitter is heavily doped so that
it can inject a large number of
charge carriers into the base.
 Base is lightly doped and very
thin, therefore it passes most of
the injected charge carriers from
the emitter into the collector.
 Collector is moderately doped
and larger in size, so that it
collects the major portion of the
majority carriers supplied by the
emitter.
Transistor biasing




NPN:
BE forward
biased
BC reverse
biased
PNP:
BE reverse
biased
BC forward
biased
BJT Transistor Operation
http://www.learnabout-electronics.org/bipolar_junction_transistors_05.php
Regions of Operation
BJT Switch “On”
Linear mode
BJT Switch “Off”
Transistor as a Switch
 When a transistor is used as a switch, it is usually required
to be brought alternatively in the saturation and cut-off
conditions.
 When in saturation condition, it should carry heavy current,
so the voltage drop across the transistor is as near to zero
as possible. It may be considered as “closed switch”.
 When in cut-off condition, it should carry almost no current
so that it may be considered to be an “open switch”.
Transistor as an Amplifier
Transistor Configurations
Parameter
Common Emitter
Common Collector
Common Base
Voltage gain
High (about 100)
Unity (1)
Medium (10-50)
Current Gain
High (50 - 800)
High (50 -800)
Low (<1)
Transistor as an Amplifier
• Use small
digital signal
to turn coils
on/off
Field Effect Transistor, FET
By Kuan-Wen Tung
What makes a FET?
•
3 terminal device with:
•
•
•
•
•
Drain
Source
Gate
The body has contacts at the ends:
the drain and source
Gate surrounds the body and can
induce a channel by use of electric
field
FET
Input voltage controls
output current
BJT
Input current controls
output current
Gate
Drain
Source
Base
Collector
Emitter
Controls flow of current
Current goes out here
Current comes in here
What is a FET?
•
•
•
•
Semiconductor device that
uses electric field to control
current, thus, the channel
conductivity.
Unipolar transistor involving
single carrier type operation,
in contrast to BJT.
Relies on PNP or NPN
junctions to allow current
flow
Performs same functions as a
BJT; amplifier and switch.
FET
How does FET work?
E=id*R+vd
Basic operation:
 Assume a 2-terminal device with nonlinear I-V characteristics
 Add a third terminal to control the I-V curve by changing voltage

Result: change in the current flowing through the first 2 terminals
Application:
 Amplification : increase current id by increasing gate voltage.
 Switching : change gate voltage to switch device on, vice versa.
Types of Field-Effect Transistors
Type
Function
Metal-Oxide-Semiconductor FET
(MOSFET)
Junction Field-Effect Transistor
(JFET)
Uses insulator (usu. SiO2) between gate and body
Uses reversed biased p-n junction to separate gate
from body
Insulated Gate Bipolar Transistor
(IGBT)
Similar to MOSFET, but different main channel
Organic Field-Effect Transistor
(OFET)
Uses organic semiconductor in its channel
Combines the organic transistor and gold
nanoparticles
Nanoparticle Organic Memory FET (NOMFET)
NPN
nMOS
PNP
pMOS
NPN
nMOS
PNP
pMOS
NPN
nMOS
PNP
pMOS
Metal-Oxide-Semiconductor, MOSFET
 Four terminal device:

Source, gate, drain, and body
(substrate) that connects to source
 Enhancement mode of
operation

nMOS

Positive gate voltage required to
conducting channel for nMOS device
to turn on.
Negative gate voltage for pMOS
device to turn on.
How does a MOSFET work?
nMOS device in enhancement mode
 Structure:



Equilibrium
energy
band diagram
Operation:

VGS > Vth
Device formed on lightly
doped p-type substrate.
Source and drain are
heavily doped with n-type.
Oxide layer separates gate
from Si surface.
Result: N-P-N type, nMOS.
Enhancement Mode of Operation
FETs vary voltage to control current.
Triode Mode/Linear Region
VGS > Vth and VDS < ( VGS - Vth )
Saturation/Active Mode
VGS > Vth and VDS > ( VGS - Vth )
VGS : Voltage at the gate
Vth : Threshold voltage
VDS : Voltage from drain to source
Characteristics and Applications of
MOSFET
•
•
•
•
Oxide layer prevents DC current from flowing through gate
• Reduces power consumption
• High input impedance
Rapid switching
Noisy due to undesired capacitive behavior
Applications:
• Used as switches and amplifiers in general
• Used in digital CMOS logic, which uses p- and nchannel MOSFETs as building blocks
• CMOS: Complementary MOS
Symbol: Not Function/ Inverter
Circuit Layout
Power Transistors
•
•
•
•
•
•
Built to carry and dissipate more power as opposed to tiny transistor
has low current gain
Used in high voltage and high current application
Used in high switching frequency of 100 kHz (switch in <1 μs)
Provides current limit protection
Cannot withstand reverse voltage
• Different types: Power BJTs, power MOSFETS, etc.
• Application: RF and microwave power transistors, multiple
chip device
• Limitation: limited to dc voltage for inverters
References

[1] “Solid State Electronic Devices" Ben G. Streetman.

[2] Student Lecture Fall 2010.

[3] Student Lecture Fall 2011.

[4] “Electronic Circuits – Handbook for Design and Applications” U. Tietze and Ch. Schenk.

[5] “Transistors." http://en.wikipedia.org/wiki/Transistor

[6] “FET.” http://en.wikipedia.org/wiki/Field-effect_transistor

[7] “JFET.” http://en.wikipedia.org/wiki/JFET

[8] “MOSFET.” http://en.wikipedia.org/wiki/MOSFET

[9] “MOSFET Animation.” http://www.youtube.com/watch?v=v7J_snw0Eng

[10] “Class Lecture #6 on Digital Arithmetic 2012.” Dr. Charles Ume

[11] “Class Lecture #4 on CMOS Inverter 2010.” Dr. Miltiadis Hatalis
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