Student Lecture by:
•Giangiacomo Groppi
•Joel Cassell
•Pierre Berthelot
September 28th 2004
Lecture outline
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Historical introduction
Semiconductor devices overview
Bipolar Junction Transistor (BJT)
Field Effect Transistors (FET)
Power Transistors
Transistor History
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Invention: 1947,at Bell Laboratories.
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John Bardeen, Walter Brattain, and William
Schockly developed the first model of
transistor (a Three Points transistor, made
with Germanium)
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They received Nobel Prize in Physics in
1956 "for their researches on
semiconductors and their discovery of the
transistor effect"
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First application: replacing vacuum tubes
(big & inefficient).
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Today: millions of Transistors are built on a
single silicon wafer in most common
electronic devices
First model of Transistor
What is a transistor ?
• The Transistor is a three-terminal,
semiconductor device.
• It’s possible to control electric current or
voltage between two of the terminals (by
applying an electric current or voltage to the third
terminal). The transistor is an active component.
• With the Transistor we can make amplification devices or
electric switch. Configuration of circuit determines
whether the transistor will work as switch or amplifier
• As a miniature electronic switch, it has two operating
positions: on and off. This switching capability allows
binary functionality and permits to process information in
a microprocessor.
Semiconductors
Most used semiconductor:
• Silicon
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Basic building material of most integrated circuits
Has four valence electrons, in its lattice there are 4 covalent bonds.
Silicon crystal itself is an insulator: no free electrons
Intrinsic concentration (ni) of charge carriers: function of Temperature
(at room temp. 300K ni = 1010 /cm3)
Semiconductors 2
• Electric conductibility in the Silicon crystal is increased
by rising the temperature (not useful for our scope) and
by doping.
• Doping consists in adding small amounts of neighbor
elements.
Semiconductors 3: Doping
Two Dopant Types
1.
N-type (Negative)
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2.
Donor impurities (from Group V) added to the Si crystal lattice.
Dominant mobile charge carrier: negative electrons.
 Group V elements such as Phosphorous, Arsenic, and
Antimony.
P-type (Positive)
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Acceptor impurities (from Group III) added to the Si crystal lattice.
Dominant mobile charge carrier: positive holes.
 Group III elements such as Boron, Aluminum, and Gallium.
N-type
P-type
The simplest example:
• It’s also called Junction
Diode
• Allows current to flow from P
to N only.
• Because of the density gradient,
electrons diffuse to the p region,
holes to the n region.
• Because of the recombination, the
region near the junction is
depleted of mobile charges.
• Two types of behavior: Forward
and Reverse biased.
p-n junction
Forward bias
Forward biasing:
• The external Voltage lowers the potential barrier at the junction.
• The 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: the
total current is the sum of these two currents.
Reverse bias
• Reverse biasing:
• Reverse voltage increases the potential barrier at the junction.
• There will be a transient current to flow 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
– It needs about 0.7 V to start conduction (Vd )
• Reversed biased (off)- Diode blocks current
– Ideal: Current flow = 0
– Real : Iflow= 10-6 Amps (reverse saturation current)
V threshold
Bipolar Junction Transistor (BJT)
• 3 adjacent regions of doped
Si (each connected to a lead):
– Base. (thin layer,less doped).
– Collector.
– Emitter.
• 2 types of BJT:
npn bipolar junction transistor
– npn.
– pnp.
• Most common: npn (focus
on it).
Developed by
Shockley (1949)
pnp bipolar junction transistor
BJT npn Transistor
• 1 thin layer of p-type, sandwiched between 2 layers of n-type.
• N-type of emitter: more heavily doped than collector.
• With VC>VB>VE:
– Base-Emitter junction forward biased, Base-Collector reverse biased.
– Electrons diffuse from Emitter to Base (from n to p).
– There’s a depletion layer on the Base-Collector junction no flow of eallowed.
– BUT the Base is thin and Emitter region is n+ (heavily doped) 
electrons have enough momentum to cross the Base into the Collector.
– The small base current IB controls a large current IC
BJT characteristics
• Current Gain:
– 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
• The current Gain is expressed
in terms of the β (beta) of the
transistor (often called hfe by
manufacturers).
• β (beta) is Temperature and
Voltage dependent.
• It can vary a lot among
transistors (common values for
signal BJT: 20 - 200).
I C  I E
I B  (1   ) I E
IC

 
IB 1
npn Common Emitter circuit
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Emitter is grounded.
Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=*IB.
Increasing IB, VBE slowly increases to 0.7V but 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)
Common Emitter characteristics
Collector current
controlled by the
collector circuit.
(Switch behavior)
In full saturation
VCE=0.2V.
No current flows
Collector current
proportional to
Base current
The avalanche
multiplication of
current through
collector junction
occurs: to be
avoided
BJT as Switch
•Vin(Low ) < 0.7 V
•BE junction not forward
biased
•Cutoff region
•No current flows
•Vout = VCE = Vcc
•Vout = High
•Vin(High)
•BE junction forward biased (VBE=0.7V)
•Saturation region
•VCE small (~0.2 V for saturated BJT)
•Vout = small
•IB = (Vin-VB)/RB
•Vout = Low
BJT as Switch 2
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Basis of digital logic circuits
Input to transistor gate can be analog or digital
Building blocks for TTL – Transistor Transistor Logic
Guidelines for designing a transistor switch:
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VC>VB>VE
VBE= 0.7 V
IC independent from IB (in saturation).
Min. IB estimated from by (IBminIC/).
Input resistance such that IB > 5-10 times IBmin because 
varies among components, with temperature and voltage and RB
may change when current flows.
– Calculate the max IC and IB not to overcome device
specifications.
Operation point of BJT
• Every IB has a corresponding
I-V curve.
• Selecting IB and VCE, we can
find the operating point, or Q
point.
• Applying Kirchoff laws around
the base-emitter and collector
circuits, we have :
IB = (VBB-VBE)/RB
VCE = Vcc – IC*RC
VCC VCE
IC 

RC RC
Operation point of BJT 2
VCC VCE
IC 

RC RC
Q
Load-line curve
BJT as amplifier
•Common emitter mode
•Linear Active Region
•Significant current Gain
Example:
•Let Gain,  = 100
•Assume to be in active
region -> VBE=0.7V
•Find if it’s in active
region
BJT as amplifier 2
VBE  0.7V
I E  I B  I C  (   1) I B
VBB  VBE
5  0.7
IB 

 0.0107mA
RB  RE *101
402
I C   * I B  100 * 0.0107  1.07mA
VCB  VCC  I C * RC  I E * RE  VBE 
 10  (3)(1.07)  (2)(101* 0.0107)  0.7 
 3.93V
VCB>0 so the BJT is in
active region
Operation region summary
Operation
Region
Cutoff
IB or VCE
Char.
IB = Very
small
Saturation VCE = Small
Active
Linear
VCE =
Moderate
Breakdown
VCE =
Large
BC and BE
Junctions
Reverse &
Reverse
Forward &
Forward
Reverse &
Forward
Beyond
Limits
Mode
Open
Switch
Closed
Switch
Linear
Amplifier
Overload
Field Effect Transistors
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1955 : the first Field effect transistor works
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Increasingly important in mechatronics.
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Similar to the BJT:
– Three terminals,
– Control the output current
BJT
Terminal
FET
Terminal
Base
Gate
Collector Drain
Emitter
Source
Field Effect Transistors
• Three Types of Field Effect Transistors
– MOSFET (metal-oxide-semiconductor field-effect transistors)
• Enhancement mode
• Depletion mode
– JFET (Junction Field-effect transistors)
• The more used one is the n-channel enhancement mode MOSFET,
also called NMOS
MOSFET
(enhancement mode n-channel)
Symbols (base connected
to the source or not)
Depletion mode
Enhancement mode
The arrow head indicates the
direction of the pn substratechannel junction
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N-channel => Source and Drain are n type
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Enhancement mode =>
Increase VGS to make the travel from D to S easier for the
electrons
NMOS Behavior
VGS < Vth
• IDS=0
VGS > Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
VDS > VBreakdown
IDS increases quickly
Should be avoided
NMOS Characteristic
For VDS > VPinchoff , the base
current is a function of VGS
Active
region
Pinchoff
Point
Saturation
region
NMOS Vs PMOS
– Symbols:
NMOS Vs PMOS
VGS > Vth Vth < 0
• IDS=0
VGS < Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
VDS > VBreakdown
IDS increases quickly
Should be avoided
NMOS uses
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High-current voltage-controlled switches
Analog switches
Drive DC and stepper motor
Current sources
Chips and Microprocessors
• CMOS: Complementary fabrication
NMOS Example
For Vpinchoff < VDS < 0
And VGS > VTH
JFET overview
The circuit symbols:
JFET design:
JFET Behavior
Can be used with VG=0
JFET Behavior
Can be used with VG < 0
JFET Behavior
VGS > Vth
• IDS=0
VGS < Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
VDS > VBreakdown
IDS increases quickly
Should be avoided
JFET uses
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Small Signal Amplifier
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Voltage Controlled Resistor
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Switch
FET Summary
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General:
• Signal Amplifiers
• Switches
JFET:
For Small signals
Low noise signals
Behind a high impedence system
Inside a good Op-Ampl.
MOSFET:
Quick
Voltage Controlled Resistors
RDS can be really low : 10 mOhms
Power Transistors
• In General
– Fabrication is different in order to:
• Dissipate more heat
• Avoid breakdown
– So Lower gain than signal transistors
• BJT
– essentially the same as a signal level BJT
– Power BJT cannot be driven directly by HC11
• MOSFET
– base (flyback) diode
– Large current requirements
References
• “Introduction to Mechatronics and Measurement Systems” by D.G.
Alciatore, McGraw-Hill
•“Microelectronics” by J. Millman, McGraw-Hill
•Several Images from Internet: some websites are:
•http://www.engr.colostate.edu/~dga/mechatronics/figures/
•http://www.ecse.rpi.edu/~schubert/Course-ECSE-6290 SDM-2/
•http://hyperphysics.phy-astr.gsu.edu/hbase/solids/diod.html