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Intro to Mechatronics
18 February 2005
Student Lecture: Transistors
Andrew Cannon
Shubham Saxena
Outline
• What is a Transistor?
• Transistor Properties
• Characteristics and Applications of
– Bipolar Junction Transistor (BJT)
– Field Effect Transistors (FET)
– Power Transistors
What is a Transistor?
•
Electrically Actuated Switch
–
Two operating positions: on and off
• Binary functionality – Permits processing of information
•
Three-terminal semiconductor device
– Control current or voltage between two of the terminals by applying a
current or voltage to the third terminal
•
Amplifiers or Switches
– Configuration of circuit determines whether the transistor will work as
switch or amplifier
A Brief History
•Invented in 1947 at Bell Labs
-John Bardeen, Walter Brattain, and William Schockly
-Nobel Prize in Physics in 1956
•Initial Application
-Replaced Vacuum Tubes: Big and Inefficient
•Today
- Millions of transistors are built on a single silicon chip
What Are The Building Blocks?
Silicon
•Basic building material of most integrated circuits
•Four valence electrons: Possibility for 4 covalent bonds
•Silicon crystal itself is an insulator: no free electrons
Building Blocks
•
Electric conductivity in the Silicon crystal is increased by doping
•
Doping: Adding small amounts of neighbor elements
Building Blocks
Two Dopant Types
1. N-type (Negative)
•
Group V
- Dominant mobile charge carrier: negative electrons
Phosphorous, Arsenic, and Antimony
2. P-type (Positive)
•
Group III
- Dominant mobile charge carrier: positive holes
Boron, Aluminum, and Gallium
N-type
P-type
P-N Junction (Junction Diode)
•Allows current to flow from P to N only
•Density Gradient
- Electrons diffuse to the p region
- Holes diffuse to the n region
•Recombination
- Region near the junction is depleted of mobile charges
•Two types of behavior: Forward and Reverse Biasing
Forward Biasing
• External Voltage lowers the potential barrier at the
junction
• P-N junction drives holes (from the p-type material) and
electrons (from the n-type material) to the junction
• A current of electrons to the left and a current of holes to
the right: total current is the sum of these two currents
Reverse Biasing
•Reverse voltage increases the potential barrier at the junction
•There will be a transient current as both electrons and holes are
pulled away from the junction
•When the potential formed by the widened depletion region
equals the applied voltage, the current will cease except
for the small thermal current. It’s called reverse saturation
current and is due to hole-electrons pairs generated by
thermal energy
Diode Characteristics
• Forward biased (on)- Current flows
– Conduction begins around 0.7 V (Vd )
• Reversed biased (off)- Diode blocks current
– Ideal: Current flow = 0
– Real : Iflow= 10-6 Amps (reverse saturation current)
V threshold
Types of Transistors
• Bipolar Junction Transistor (BJT)
• Field Effect Transistors (FET)
• Power Transistors
Outline
Types of Transistors
• Bipolar Junction Transistor (BJT)
Fundamentals
Representation
Common emitter mode (active)
Operation region
Applications
• Field Effect Transistors (FET)
Fundamentals MOSFET
Operating regimes MOSFET
Fundamentals JFET
Operating regimes JFET
application areas
•
Power Transistors
Fundamentals
Collector
Base
Emitter
BJT
Fundamentals
npn BJT
Common emitter mode (active)
Vc
Collector
Reverse bias
Base
Vb
Forward Bias
Emitter
Hole
E-
Representation
BJT
N-type emitter: more
heavily doped than
collector
I C  I E
Common emitter mode
• Emitter grounded.
• VBE<0.6V: transistor
inactive
• VBE>=0.6V :BaseEmitter conduct
• IB ↑, VBE ↑ (slow)
0.7V , IC ↑
exponentially.(IB
=βIC)
• As IC ↑,voltage drop
across RC increases
and VCE ↓ 0 V.
(saturation) IB ≠βIC
• Q: Operating point
BJT
Q
VCC VCE
IC 

RC RC
Operation region
Operation
Region
Cutoff
IB or VCE
BC and BE
Junctions
IB = Very
Reverse &
small
Reverse
Saturation VCE = Small Forward &
Forward
Active
VCE =
Reverse &
Linear
Moderate
Forward
BreakVCE =
Beyond
down
Large
Limits
BJT
Mode
Open
Switch
Closed
Switch
Linear
Amplifier
Overload
Switch
Applications BJT
Vin(Low ) <
0.7 V
BE forward
bias
Saturation
• logic circuits
• TTL
• lab
Vin(High)
BE not
biased
Cutoff
Amplifier
Applications BJT
• Assume to be in
active region ->
VBE=0.7V
• Find if it’s in active
region by solving
the equations
Field Effect Transistors (FET)
FET: three types
• Metal oxide semiconductor FET
(MOSFET)
• Enhancement mode
• Depletion mode
• Junction FET (JFET)
Fundamentals
MOSFET
Gate, Vg
++++++
Source, Vs
n
n
Drain, Vd
P-substrate
Reverse bias
Id
N-channel enhancement MOSFET
Operating regimes
Cut-off regime: VGS < VT , VGD < VT with
VDS > 0.
Linear or Triode regime:VGS > VT, VGD >
VT , with VDS> 0.
Saturation regime:VGS > VT, VGD < VT (VDS
> 0).
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In the linear regime:
– VGS ") ID ": more electrons in
the channel
– VDS ") ID ": stronger field pulling
electrons out
of the source
• Channel debiasing: inversion
layer ”thins down” from
source to drain)current saturation
as VDS approaches:
VDSsat = VGS − VT
MOSFET
Operating regimes
MOSFET
NMOS
PMOS
Active
region
Saturation
region
Pinch-off
region
Gate: G
Source: S
Drain: D
Depletion Mode Devices
FET
• Physically implanted channel: An n-channel depletion type
MOSFET has an n-type silicon region connecting the n+ source
and drain regions at the top of the p-type substrate.
• The channel depth and its conductivity can be controlled by Vgs in
exactly the same manner as in the enhancement-type device.
• Negative value of Vgs is the threshold voltage
Field Effect Transistors (FET)
•
•
•
•
•
•
•
•
•
•
•
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FETs are useful because there is essentially no input
current
– Thus the output current can be controlled with nearly no
input power
– In this sense, FETs are more nearly ideal transistors than
bipolar junctions are
• Integrated circuits (“chips”) are made by forming many
FET’s on layers of silicon
• Main limitation of FETs is maximum current they can
handle
– For high-current applications the bipolar junction is a better
choice
Fundamentals
JFET
Depletion region grows as the reverse bias across the PN junction is increased
Operating regimes
JFET
Application areas
MOSFET
• Switches: High-current voltage-controlled and
Analog switches
• Drive Motor: DC and stepper motor
• Current sources
• Chips and Microprocessors
• CMOS: Complementary fabrication
JFET
• differential amplifier
Power Transistors
• Designed to conduct large currents and
dissipate more heat. Usually physically larger
than a regular transistor
• Applications where low current devices are
interfaced with high current devices
– Lower gain than signal transistors
• RF amplifiers, motors, solenoid control, lighting
control.
• MOSFET base (flyback) diode
References
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“Introduction to Mechatronics and Measurement
Systems” by D.G. Alciatore, McGraw-Hill
“Microelectronics” by J. Millman, McGraw-Hill
http://www.phys.ualberta.ca/~gingrich/phys395/notes/ph
ys395.html
http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineeringand-Computer-Science/6-012Microelectronic-Devicesand-CircuitsSpring2003/C1EC60A4-4196-4EE6-AAC32775F2200596/0/lecture9.pdf
http://people.deas.harvard.edu/~jones/es154/lectures/lec
ture_4/jfet/jfet.html
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