ME 6405 Student Lecture:
Transistors
Chester Ong
Ajeya Karajgikar
Emanuel Jones
Thursday September 30, 2010
Georgia Institute of Technology
Presentation Outline
1
Transistor Fundamentals
2
Bipolar Junction Transistors
3
Power Transistors
4
Field Effect Transistors
5
Applications of Transistor
Chester Ong
Ajeya Karajgikar
Ajeya Karajgikar
Emanuel Jones
(covered by each speaker in respective topic)
Transistors
Transistors of various type & size
First Transistor
Model, 1947
Used in all modern
electronics
BJT (PNP) Electrical Diagram
Representation
FET Transistor
BJT Transistor
Understanding Transistors (conceptually)
1. What is a Transistor?
•
•
•
•
Basic Purpose of a Transistor
Recognize Transistor Role in Modern Electronics
Understand Reason(s) for its Invention
Comparison to its “predecessor,” the Vacuum Tube
2. How are transistors made?
•
•
•
“Doping” Manufacturing Process
Effect of Doping on Semiconductors
Creation of a P-N Junction via Doping
3. How do transistors work?
•
•
•
•
Depletion Region of a P-N Junction
How to Control Current through a Depletion Region
How a P-N Junction can act as an Electrical Switch
Combination of P-N Junctions -> Transistors
What is a Transistor?
 Basic Purpose
[1] To amplify signals
[2] To electronically switch (no moving parts) a signal on
or off (high/low)
 Role in Modern Electronics
• Basic building blocks for all modern electronics
• Microprocessors, Microcontrollers, Computers,
Digital watches, Digital Logic Circuits, Cell Phones….
Microprocessor
PC & Cell Phones
Motor
Controllers
Headphones
Reason for Transistor’s Invention:
 Early 20th century, vacuum tube was used for signal amplifier &
switch.
Vacuum Tube Radios
ENIAC : 17, 468 vacuum tubes
 Use of vacuum tube* resulted in extremely large, fragile, energy
inefficient, and expensive electronics.
 Evolution of electronics required device that was small, light
weight, robust, reliable, cheap to manufacture, energy efficient:
*Vacuum tube advantages: operation at higher voltages (10K region vs. 1K region of transistors); high power, high frequency
operation (over-the-air TV broadcasting) better suited for vacuum tubes; and silicon transistors more vulnerable to
electromagnetic pulses than vacuum tubes
…and the TRANSISTOR was born!
 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
 Awarded the Nobel Prize in physics (1956)
 Transistor
 is a semiconductor device commonly used
to amplify or switch electronic signals.
First model of Transistor, 1947
Historical Development
1941, Vacuum Tube
1948, the first (Germanium) TR
John Bardeen, Walter Brattain, and William Schockly
1954, Silicon TR
At TI Lab, Ease of processing, lower cost, greater power handling, more
stable temperature characteristics
1958, Integrated Circuit
Individual electronic
components were soldered on
to printed circuit boards.
Sep 2009, 22nm silicon wafer
Intel CEO Paul Otellini, Sep 23 2009
more than 2.9 billion transistors is
packed into an area of fingernail
IC placed all components
in one chip.
Transistor Categories and Types
 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
Various Types of Transistor:
http://en.wikipedia.org/wiki/Category:Transistor_types
 Various Types of Transistors
• Bipolar Junction Transistor (BJT)
• Field Effect Transistors (FET)
• Power Transistors
Understanding Transistors (conceptually)
1. What is a Transistor?
•
•
•
•
Basic Purpose of a Transistor
Recognize Transistor Role in Modern Electronics
Understand Reason(s) for its Invention
Comparison to its “predecessor,” the Vacuum Tube
2. How are transistors made?
•
•
•
“Doping” Manufacturing Process
Effect of Doping on Semiconductors
Creation of a P-N Junction via Doping
3. How do transistors work?
•
•
•
•
Depletion Region of a P-N Junction
How to Control Current through a Depletion Region
How a P-N Junction can act as an Electrical Switch
Combination of P-N Junctions -> Transistors
Doping Manufacturing Process
Doping: “Process of introducing impure elements (dopants) into
semiconductor wafers to form regions of differing electrical conductivity.”
Two Main Manufacturing Processes:
[1] High-temperature furnace diffuse a solid layer of “dopant” onto wafer surface.
[2] Ion implanter: gaseous dopants are ionized (stripped of electrons); accelerated
using an electric field; and deposited in a silicon wafer.
High-Temp Furnace
“Pure” Wafers
“Doped” Wafers
Ion Implanter
Wafer
Refinement
Effect of Doping on Semi-Conductors(1/3)
General Characteristics of Semiconductors:
• Possesses an electrical conductivity somewhere between
insulators & conductors
• Typical material composition is either silicon or germanium
• Semiconductors are more “insulators” than “conductors,” since
semiconductors possess few free electrons (as opposed to conductors,
which have many free electrons)
Doping impurities into a “pure”semiconductor
will increase conductivity.
Doping results in an “N-Type” or “P-Type” semiconductor.
Effect of Doping on Semi-Conductors(2/3)
P-Type Semiconductors : Positively charged Semiconductor
Dopant Material: Boron, Aluminum, Gallium
Effect of Dopant:
•“takes away” weakly-bound outer orbit electrons from semiconductor atom.
•Semiconductor now has “missing” electron or “hole” in its lattice structure.
•Overall material is now positively charged , because material has fewer electrons
but still wants to accept electrons to fill holes in its lattice structure
Effect of Doping on Semi-Conductors(3/3)
N-Type Semiconductors : Negatively charged Semiconductor
Dopant Material: Phosphorous, Arsenic, Antimony (Sb)
Effect of Dopant:
•“adds” electrons to semiconductor atom
•Semiconductor is now negatively charged, because of electron abundance
•Overall material (semiconductor + dopant) wants to donate “extra” electrons to
make lattice structure at its lowest energy state
Creation of P-N Junction via Doping
Remember: Doping introduces impurities into semiconductor material
Remember: Dopant is added to same piece of semiconductor material
Resulting Material: Single, solid material called “P-N Junction”
Example: Boron (P-Type) to side A and Antimony (N-Type) to side B
Positively-charged
P-type Side
Negatively-charged
N-type Side
Lattice structure wants
electrons to fill “holes”
Lattice structure has too
many electrons
What happens at the
point of contact or
“junction?
Understanding Transistors (conceptually)
1. What is a Transistor?
•
•
•
•
Basic Purpose of a Transistor
Recognize Transistor Role in Modern Electronics
Understand Reason(s) for its Invention
Comparison to its “predecessor,” the Vacuum Tube
2. How are transistors made?
•
•
•
“Doping” Manufacturing Process
Effect of Doping on Semiconductors
Creation of a P-N Junction via Doping
3. How do transistors work?
•
•
•
•
Depletion Region of a P-N Junction
How to Control Current through a Depletion Region
How a P-N Junction can act as an Electrical Switch
Combination of P-N Junctions -> Transistors
Depletion Region of P-N Junction
At equilibrium with no external voltage, a thin and constant-thickness
“depletion region” forms between P-type and N-type semiconductors.
In depletion region, free electrons from N-type will “fill” the electron
holes in the P-type until equilibrium.
Negative and positive ions are subsequently created in depletion region.
Ions exhibit a (Coulomb) force which inhibits further electron flow
(i.e. current) across the P-N Junction unless a forward bias
external voltage is applied.
Current through a Depletion Region
 Remember:
•Depletion region is created at equilibrium between P & N-type junction.
•Positive & negative ions are created in depletion region.
•Ions have a Coulomb force which impedes motion of electrons –
essentially insulator property.
Applying External Voltage…
•…of Forward Biasing polarity facilitates motion of free electrons
-> Coulomb force is overcome, electrons flow from N to P
•…of Reverse Biasing polarity impedes motion of free electrons
-> No current flow because of Coulomb force in depletion region
Electrical Switching on P-N Junction
Applying External Voltage…
•…of Forward Biasing polarity facilitates motion of free electrons
•…of Reverse Biasing polarity impedes motion of free electrons
Forward Biasing
•Circuit is “On”
•Current is Flowing
Reverse Biasing
•Circuit is “Off”
•Current not Flowing
Finally – combining all concepts
Semiconductor -> Doping -> P-N Junction -> Depletion Region
-> Ions & Coulomb Force -> External Voltage -> Current on/off
One P-N Junction can control current flow via an external voltage
Two P-N junctions (bipolar junction transistor, BJT) can control current
flow and amplify the current flow.
Also, if a resistor is attached to the output, the resulting voltage output
is much greater than the applied voltage, due to amplified current and
I*R=V.
Presentation Outline
1
Transistor Fundamentals
2
Bipolar Junction Transistors
3
Power Transistors
4
Field Effect Transistors
5
Applications of Transistor
Chester Ong
Ajeya Karajgikar
Ajeya Karajgikar
Emanuel Jones
(covered by each speaker in respective topic)
BJT introduction

BJT = Bipolar Junction
Transistor

3 Terminals



Base (B)
Collector (C)
Emitter (E)
BJT schematic

NPN:
 BE forward
biased
 BC reverse
biased
NPN
PNP

PNP:
 BE reverse biased
 BC forward
biased
BJT formulae
NPN
Current
control
iE  iC  iB
iC    iB
VBE  VB  VE
VCE  VC  VE
β
is the amplification factor and ranges from 20 to 200
It is dependent on temperature and voltage
BJT formulae
NPN
Emitter is more heavily
doped than the collector.
Therefore,
VC > VB > VE
for NPN transistor
BJT formulae
NPN
iC    iE
iB  (1   )iE
iC

 
iB 1  
α is the fraction of electrons that diffuse across the narrow base region
1 – α is the fraction of electrons that recombine with holes in the base region to
create base current
Common Emitter Transistor Circuit
 Emitter is grounded and input voltage is applied to Base
 Base-Emitter starts to conduct when VBE is about 0.6V, iC flows with
iC= β.iB
 As iB further increases, VBE slowly increases to 0.7V, iC rises exponentially
 As iC rises, voltage drop across RC increases and VCE drops toward ground
(transistor in saturation, no more linear relation between iC and iB)
27
Common Emitter Characteristics
Collector current
controlled by the
collector circuit
(Switch behavior)
Collector current IC proportional to
Base current IB
In full saturation
VCE=0.2V
No current flows
28
BJT operating regions
Operating
Region
Parameters
Mode
Cut Off
VBE < Vcut-in
VCE > Vsupply
IB = IC = 0
Linear
VBE = Vcut-in
Vsat < VCE < Vsupply
IC = β*IB
Amplification
Saturated
VBE = Vcut-in,
VCE < Vsat
IB > IC,max, IC,max
>0
Switch ON
Switch OFF
BJT as an amplifier
 Question: What is the minimum Vin that makes the transistor act as an amplifier?
Given:
• RB = 10 kΩ
• RC = 1 kΩ
• β = 100
• VSupply = 10 V
• Vcut-in = 0.7 V
• Vsat = 0.2 V
I
iC = (Vsupply – VCE) / RC
Set VCE = Vsat = 0.2V
iC = (10 – 0.2) / 1000 = 9.8mA
iC = β . iB
iB = iC / β = 0.0098/100 = 0.098mA
RC
II
VSupply
Vin
Vsupply – iC . RC – VCE = 0
I
Vin – iB . RB – VBE = 0
Vin = iB . RB + VBE
Set VBE = Vcut-in = 0.7V
RB
Vin = (0.098) .(10-3).(10000 )+ 0.7V
II
Vin = 1.68V or greater.
BJT as a switch
• From
exercise 3
• Turns on/off coils
digitally
Power Transistors
 Concerned with delivering high power
 Used in high voltage and high current application
In general
Fabrication process different in order to:
 Dissipate more heat
 Avoid breakdown
Different types: Power BJTs, power MOSFETS, etc.
Presentation Outline
1
Transistor Fundamentals
2
Bipolar Junction Transistors
3
Power Transistors
4
Field Effect Transistors
5
Applications of Transistor
Chester Ong
Ajeya Karajgikar
Ajeya Karajgikar
Emanuel Jones
(covered by each speaker in respective topic)
Presented by: Emanuel Jones
• Semiconductor device that depends on
electric field to control the current
• Performs same functions as a BJT;
amplifier, switch, etc.
• Relies on PNP or NPN junctions to
allow current flow
• However, mechanism that controls
current is different from the BJT
• Remember the BJT is bipolar. The FET
is sometimes called a unipolar transistor
• One type of charge carrier
• FETs have three main parts
• Drain
• Source
• Gate
•The body has contacts at the
ends: the drain and source
•Gate surrounds the body and can
induce a channel to because of an
electric field
FET
BJT
Input voltage controls Input current controls
output current
output current
Gate
Drain
Source
Base
Collector
Emitter
Controls flow of current
Current goes out here
Current comes in here
No Voltage to Gate
Source
Voltage to Gate
Drain
Source
Drain
n
n
MOSFET shown here
No current flow
Simplified Notation
“Short” allows current flow
Types of Field-Effect Transistors
Type
Function
Junction Field-Effect Transistor
(JFET)
Metal-Oxide-Semiconductor FET
(MOSFET)
Insulated Gate Bipolar Transistor
(IGBT)
Similar to MOSFET, but different main channel
Organic Field-Effect Transistor
(OFET)
Uses organic semiconductor in its channel
Nanoparticle Organic Memory FET (NOMFET)
“DNAFET”
Uses reversed biased p-n junction to separate gate from body
Uses insulator (usu. SiO2) between gate and body
Combines the organic transistor and gold nanoparticles
Uses a gate made of single-strand DNA molecules
MOSFET
IGBT
JFET
 A single channel of single doped SC material
with terminals at end
 Gate surrounds channel with doping that is
opposite of the channel, making the PNP or
NPN type
n-channel
 Uses reversed biased p-n junction to separate
JFET
gate from body
 Flow of current is similar to water flow through
a garden hose
 Pinch the hose (decrease current channel
width) to decrease flow
 Open the hose (increase channel width) to
increase flow
 Also, the pressure differential from the front
and back of the hose (synonymous with the
voltage from drain to source) effects the flow
p-channel
JFET
JFET analysis
I–V characteristics and output plot of a JFET n-channel transistor.
JFET analysis
IDS : Drain current in saturation region
VGS : Voltage at the gate
Vth : Threshold voltage
VDS : Voltage from drain to source
VP : Pinch-off voltage [1]
[1] - This "pinch-off voltage" varies considerably, even among devices of the same type. For
example, VGS(off) for the Temic J201 device varies from -0.8V to -4V. Typical values vary from
-0.3V to -10V.
MOSFET
 Similar to JFET – remember…
p-channel
 A single channel of single doped SC
material with terminals at end
 Gate surrounds channel with
doping that is opposite of the
channel, making the PNP or NPN
type
 BUT, the MOSFET uses an
insulator to separate gate from
body, while JFET uses a reverse-bias
p-n junction
n-channel
MOSFET
enhanced mode
MOSFET
depleted mode
MOSFET
FETs vary voltage to control current. This illustrates how that works
MOSFET drain current vs. drain-to-source voltage for several values
of VGS − Vth; the boundary between linear (Ohmic) and saturation
(active) modes is indicated by the upward curving parabola.
MOSFET
Triode Mode/Linear Region
Saturation/Active Mode
VGS > Vth and VDS < ( VGS - Vth )
VGS > Vth and VDS > ( VGS - Vth )
VGS : Voltage at the gate
Vth : Threshold voltage
VDS : Voltage from drain to source
μn: charge-carrier effective mobility
W: gate width
L: gate length
Cox : gate oxide capacitance per unit area
λ : channel-length modulation parameter
Characteristics and Applications of FETs
JFETs
•
•
•
•
•
•
Simplest type of FET – easy to make
High input resistance
Low Capacitance
High input impedance
Slower speed in switching
Uses?
– Displacement sensor
– High input impedance amplifier
– Low-noise amplifier
– Analog switch
– Voltage controlled resistor
Characteristics and Applications of FETs
MOSFETs
•
•
•
•
Oxide layer prevents DC current from
flowing through gate
• Reduces power consumption
• High input impedance
Rapid switching
More noise than JFET
Uses?
• Again, switches and amplifiers in
general
• The MOSFET is used in digital
CMOS logic, which uses p- and nchannel MOSFETs as building blocks
• To aid in negating effects that cause
discharge of batteries
Use of MOSFET in battery
protection circuit
Presentation Summary
1
Transistor Fundamentals Chester Ong
•Qualitative explanation of the what & how behind transistors
•General application and history of transistors
•“Physics” behind transistors :
Doping Process, Effect on Semiconductors, & Formation of P-N Junction
Electrical Properties of P-N Junction & using P-N to control / amplify current
2
Bipolar Junction Transistors Ajeya Karajgikar
3
Power Transistors Ajeya Karajgikar
•Introduction & Formulae
•Explain function and characteristics of common emitter transistor
•Describe BJT operating regions
•Applications of BJTs
•Definition and Applications
Field Effect Transistor Emanuel Jones
4
• Use of electric field to change the output current
• JFETs and MOSFETs are most common, and accomplish similar goals as BJTs
• Used for switches, amplification, applications for protecting electronics
5
Applications of Transistor
(covered by each speaker in respective topic)
References (32)
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http://www.utdallas.edu/research/cleanroom/TystarFurnace.htm
http://www.osha.gov/SLTC/semiconductors/definitions.html
http://www.products.cvdequipment.com/applications/diffusion/1/
http://amath.colorado.edu/index.php?page=an-immersed-interface-method-for-modeling-semiconductor-devices
http://www.extremetech.com/article2/0,2845,1938467,00.asp
http://macao.communications.museum/eng/Exhibition/secondfloor/moreinfo/2_10_3_HowTransistorWorks.html
http://fourier.eng.hmc.edu/e84/lectures/ch4/node3.html
http://www.appliedmaterials.com/htmat/animated.html really good video!
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html#c3
http://www.tpub.com/neets/book7/25.htm
http://esminfo.prenhall.com/engineering/wakerlyinfo/samples/BJT.pdf
http://web.engr.oregonstate.edu/~traylor/ece112/lectures/bjt_reg_of_op.pdf
http://www.me.gatech.edu/mechatronics_course/transistors_F09.ppt
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
http://en.wikipedia.org/wiki/Common_emitter
http://en.wikipedia.org/wiki/Diode
http://www.kpsec.freeuk.com/trancirc.htm
http://en.wikipedia.org/wiki/Field-effect_transistor
http://en.wikipedia.org/wiki/JFET
http://en.wikipedia.org/wiki/MOSFET
http://www.slideshare.net/guest3b5d8a/fets
http://www.rhopointcomponents.com/images/jfetapps.pdf
http://cnx.org/content/m1030/latest/
http://www.play-hookey.com/semiconductors/enhancement_mode_mosfet.html
http://www.youtube.com/watch?v=-aHnmHwa_6I&feature=related
http://www.youtube.com/watch?v=v7J_snw0Eng&feature=related
http://info.tuwien.ac.at/theochem/si-srtio3_interface/si-srtio3.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html#c4
http://inventors.about.com/library/inventors/blsolar5.htm
http://thalia.spec.gmu.edu/~pparis/classes/notes_101/node100.html
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/pnjun.html#c3
http://science.jrank.org/pages/6925/Transistor.html - also really good explanation!
Questions?
Thank you!