September 28th 2004

1
Transistors

• Student Lecture by
• Giangiacomo Groppi
• Joel Cassell
• Pierre Berthelot

• September 28th 2004

2
Lecture outline

• Historical introduction
• Semiconductor devices overview
• Bipolar Junction Transistor (BJT)
• Field Effect Transistors (FET)
• Power Transistors

3
Transistor History

• Invention 1947,at Bell Laboratories.
• John Bardeen, Walter Brattain, and William
  Schockly developed the first model of transistor
  (a Three Points transistor, made with Germanium)
• They received Nobel Prize in Physics in 1956 "for
  their researches on semiconductors and their
  discovery of the transistor effect"
• First application replacing vacuum tubes (big
  inefficient).
• Today millions of Transistors are built on a
  single silicon wafer in most common electronic
  devices

First model of Transistor
4
What is a transistor ?

• The Transistor is a three-terminal, semiconductor
  device.
• Its 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.

5
Semiconductors

• Most used semiconductor
• Silicon
• 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)

6
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.

7
Semiconductors 3 Doping

• Two Dopant Types
• N-type (Negative)
• 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)
• 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.

8
The simplest example p-n junction

• Its 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.

9
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.

10
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. Its called reverse saturation current
  and is due to hole-electrons pairs generated by
  thermal energy.

11
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)

12
BJT
13
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.
• pnp.
• Most common npn (focus on it).

Developed by Shockley (1949)
14
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 VCgtVBgtVE
• Base-Emitter junction forward biased,
  Base-Collector reverse biased.
• Electrons diffuse from Emitter to Base (from n to
  p).
• Theres a depletion layer on the Base-Collector
  junction ?no flow of e- allowed.
• 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

15
BJT characteristics

• Current Gain
• a is the fraction of electrons that diffuse
  across the narrow Base region
• 1- a 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 ß
  (beta) of the transistor (often called hfe by
  manufacturers).
• ß (beta) is Temperature and Voltage dependent.
• It can vary a lot among transistors (common
  values for signal BJT 20 - 200).

16
npn Common Emitter circuit

• Emitter is grounded.
• Base-Emitter starts to conduct with VBE0.6V,IC
  flows and its ICbIB.
• 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)

17
Common Emitter characteristics
Collector current controlled by the collector
circuit. (Switch behavior) In full saturation
VCE0.2V.
Collector current proportional to Base current
The avalanche multiplication of current through
collector junction occurs to be avoided
No current flows
18
BJT as Switch

• Vin(Low ) lt 0.7 V
• BE junction not forward biased
• Cutoff region
• No current flows
• Vout VCE Vcc
• Vout High

• Vin(High)
• BE junction forward biased (VBE0.7V)
• Saturation region
• VCE small (0.2 V for saturated BJT)
• Vout small
• IB (Vin-VB)/RB
• Vout Low

19
BJT as Switch 2

• 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
• VCgtVBgtVE
• VBE 0.7 V
• IC independent from IB (in saturation).
• Min. IB estimated from by (IBmin IC/b).
• Input resistance? such that IB gt 5-10 times IBmin
  because b 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.

20
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 ICRC

21
Operation point of BJT 2
Load-line curve
Q
22
BJT as amplifier

• Common emitter mode
• Linear Active Region
• Significant current Gain

• Example
• Let Gain, b 100
• Assume to be in active region -gt VBE0.7V
• Find if its in active region

23
BJT as amplifier 2
VCBgt0 so the BJT is in active region
24
Operation region summary
25
FET
26
Field Effect Transistors

• 1955 the first Field effect transistor works

• Increasingly important in mechatronics.

• Similar to the BJT
• Three terminals,
• Control the output current

27
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)

• Each in p-channel or n-channel

• The more used one is the n-channel enhancement
  mode MOSFET, also called NMOS

28
MOSFET (enhancement mode n-channel)
The arrow head indicates the direction of the pn
substrate-channel junction

• N-channel gt Source and Drain are n type
• Enhancement mode gt
• Increase VGS to make the travel from D to S
  easier for the electrons

29
NMOS Behavior

• VGS lt Vth
• IDS0

VGS gt Vth 0 lt VDS lt VPinch off
Depletion mode (or active region), gate holes are
repelled. ? variable resistor (controled
by VGS)
VDS gt VPinch off Inversion mode (or saturation
region), IDS constant.
VDS gt VBreakdown IDS increases quickly Should be
avoided
30
NMOS Characteristic
31
NMOS Vs PMOS

• Symbols

32
NMOS Vs PMOS

• VGS gt Vth Vth lt 0
• IDS0

VGS lt Vth 0 lt VDS lt VPinch off
Depletion mode (or active region), gate holes are
repelled. ? variable resistor (controled
by VGS)
VDS gt VPinch off Inversion mode (or saturation
region), IDS constant.
Analogous to the pnp BJT
VDS gt VBreakdown IDS increases quickly Should be
avoided
33
NMOS uses

• High-current voltage-controlled switches
• Analog switches
• Drive DC and stepper motor
• Current sources
• Chips and Microprocessors
• CMOS Complementary fabrication

34
NMOS Example
35
JFET overview
36
JFET Behavior
Can be used with VG0
37
JFET Behavior
Can be used with VG lt 0
38
JFET Behavior

• VGS gt Vth
• IDS0

VGS lt Vth 0 lt VDS lt VPinch off
Depletion mode (or active region), gate holes are
repelled. ? variable resistor (controled
by VGS)
VDS gt VPinch off Inversion mode (or saturation
region), IDS constant.
Analogous to the pnp BJT
VDS gt VBreakdown IDS increases quickly Should be
avoided
39
JFET uses

• Small Signal Amplifier
• Voltage Controlled Resistor
• Switch

40
FET Summary

• 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
41
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

42
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/fi
  gures/
• http//www.ecse.rpi.edu/schubert/Course-ECSE-6290
  SDM-2/
• http//hyperphysics.phy-astr.gsu.edu/hbase/solids/
  diod.html