Transistors
Camille Cruz
Chase Thompson
Tyler Nelson
September 26, 2013
Outline
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Introduction
Transistors Types
Bipolar Junction Transistors
Field Effect Transistors
Power Transistors
Example
Transistors
BJT (PNP) Electrical
Diagram
Different types and sizes
FET and BJT Transistor
Modern Electronics
First Transistor
• Purpose
▫ To amplify and switch electronic signals on or off
(high or low)
• Modern Electronics
Microprocessor
Motor Controllers
Cell Phones
Vacuum tubes
• Purpose
▫ Used as signal amplifiers and switches
▫ Advantages
 High power and frequency operation
 Operation at higher voltages
 Less vulnerable to electromagnetic pulses
▫ Disadvantages
 Very large and fragile
 Energy inefficient
 Expensive
Invention
• Evolution of electronics
▫ In need of a device that was small, robust, reliable,
energy efficient and cheap to manufacture
• 1947
▫ John Bardeen, Walter Brattain and William Schockly
invented transistor
• Transistor Effect
▫ “when electrical contacts
were applied to a crystal
of germanium, the output
power was larger than
the input.”
General Applications
Doping
• Process of introducing impure elements
(dopants) into semiconductor wafers to form
regions of differing electrical conductivity
Negatively charged Semiconductor
Positively charged semiconductor
Doping Effects
• P-type semiconductors
▫ Created positive charges, where electrons have
been removed, in lattice structure
• N-type semiconductors
▫ Added unbound electrons create negative charge
in lattice structure
• Resulting material
▫ P-N junction
P-N junction
Forward Biasing
Reverse Biasing
• P-N junction
▫ Controls current flow via external voltage
• Two P-N junctions (bipolar junction transistor,
BJT)
▫ Controls current flow and amplifies the current
flow
Transistor Categories
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Semiconductor material
Structure
Polarity
Maximum power rating
Maximum operating frequency
Application
Physical packaging
Amplification factor
Types of Transistors
• Bipolar Junction Transistor (BJT)
• Field Effect Transistors (FET)
• Power Transistors
BJT Introduction
• Bipolar Junction Transistors (BJT)
consists of three “sandwiched”
semiconductor layers
• The three layers are connected to collector
(C), emitter (E), and base (B) pins
• Current supplied to the base controls the
amount of current that flows through the
collector and emitter
BJT Schematic
• NPN
NPN
▫ BE forward bias
▫ BC reverse bias
• PNP
▫ BE reverse bias
▫ BC forward bias
PNP
BJT Characteristic Curves
Transfer Characteristic
• Characteristic curves can be drawn to show other useful parameters
of the transistor
• The slope of ICE / IBE is called the Transfer Characteristic (β)
BJT Characteristic Curves
Input Characteristic
• The Input Characteristic is the base emitter current IBE against
base emitter voltage VBE
• IBE/VBE shows the input Conductance of the transistor.
• The increase in slope of when the VBE is above 1 volt shows that the
input conductance is rising
• There is a large increase in current for a very small increase in VBE.
BJT Characteristic Curves
Output Characteristic
• collector current (IC) is nearly independent of the collector-emitter
voltage (VCE), and instead depends on the base current (IB)
IB4
IB3
IB2
IB1
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 Applications
BJT Switch
• Offer lower cost and substantial reliability over conventional
mechanical relays.
• Transistor operates purely in a saturated or cutoff state (on/off)
• This can prove very useful for digital applications (small current
controls a larger current)
BJT Applications
BJT Amplifier
BJT Applications
BJT Amplifier
Field Effect Transistors (FET)
Chase Thompson
FET Basics
• Electric Field
• Voltage Controlled
• FET includes three distinct pieces
▫ Drain
▫ Source
▫ Gate
FET versus BJT?
Same:
• Applications: amplifier,
switch, etc.
• Relies on PNP or NPN
junctions to allow current
flow
Difference:
• Voltage vs Current Input
• Unipolar vs Bipolar
• Noise
• Higher input impedance
• Fragile and low gain bandwidth
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)
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
JFET
• Reverse Biased PNjunction
• Depletion mode devices
▫ Creates a potential
gradient to restrict
current flow. (Increases
overall resistance)
http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/jfet.html
JFET
• N-channel JFET
• P-channel JFET uses same principles but
▫ Channel current is positive due to holes instead of
electron donors
▫ Polarity of biasing voltage must be reversed
N-Type Characteristics
Characteristics and Applications of FETs
JFETs
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Simplest type of FET – easy to make
High input impedance and resistance
Low Capacitance
Slower speed in switching
Uses?
– Displacement sensor
– High input impedance amplifier
– Low-noise amplifier
– Analog switch
– Voltage controlled resistor
MOSFET
p-channel
• Similar to 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
▫ 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
How does a MOSFET work?
No Voltage to Gate
Source
Voltage to Gate
Drain
Source
Drain
n
n
Simplified Notation
No current flow
“Short” allows current flow
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
MOSFETs
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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
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.
Comparison
Property BJT
MOSFET JFET
Gm
Best
Worst
Medium
Speed
High
Medium
Low
Noise
Moderate
Worst
Best
Good
No
Switch
High-Z Gate No
Yes
Yes
Yes
Yes
ESD
Sensitivity
More
Less
Less
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Questions?