Introduction to Transistors

1
Introduction to Transistors

• Presented October 23, 2001
• Chris Green
• Carl Hanna
• Ancil Marshall
• Kwame Ofori

2
Overview

• Introduction History
• Semiconductors
• Operation of Transistors
• Transistor Types
• Applications
• Examples
• Questions
• Conclusion

3
Background

• Invented at Bell Laboratories in 1947.
• John Bardeen, Walter Brattain, and William
  Schockly received Nobel Prize in Physics in 1956
  for Inventing Transistors.
• First application telephone signal amplification
• Replaced cumbersome and inefficient vacuum tubes
• Transistors can now be found on a single silicon
  wafer in most common electronic devices

4
Background

• Model of First Transistor

5
What are Transistors?

• Versatile three lead semiconductor devices whose
  applications include electronic switching and
  modulation (amplification)
• Transistors are miniature electronic switches.
• Configuration of circuit determines whether the
  transistor will serve a switch and amplifier
• Building blocks of the microprocessor, which is
  the brain of the computer.
• Have two operating positions- on and off.
• Binary functionality of transistors enables the
  processing of information in a computer.

6
Semiconductors

• Silicon
• Basic building material of most integrated
  circuits
• Has four valence electrons, which allow it to
  form four covalent bonds.
• Silicon crystal is an insulator-- no free
  electrons.

7
Semiconductors

• Resistance to current flow in the silicon crystal
  is reduced by adding small amounts of foreign
  impurities, which is referred to as doping.
• Doping transforms a silicon crystal from a good
  insulator into a viable conductor hence, the
  name semiconductor.

8
Semiconductors

• Two Dopant Types
• N-type (Negative) Free flowing electrons are
  added to the silicon crystal structure.
• Examples include Group V elements including
  Phosphorous, Arsenic, and Antimony.
• P-type(Positive)- Lack electrons and serve as
  potential slots for migrating electrons.
• Examples include Group III elements such as
  Boron, Aluminum, and Gallium

9
Comparison of Energy Bands

• Semiconductor resembles an insulator, but with a
  smaller energy band.
• Small energy band makes it a marginal conductor

10
Simple Semiconductors Diodes

• Diode is the simplest semiconductor.
• Allows current to flow in one direction only.

11
Diode Sign Conventions

• Power dissipated by a load () quantity
• Current flows from () ? (-)
• Forward Biased
• Supplied Current flows with natural (hole)
  diffusion current
• Reversed Biased
• Supplied Current fights against natural diffusion
  (hole) current and diode orientation

12
Forward-Bias Example

• Charge Diffusion aided by Supply Current
• Current is allowed through easily

P-N Junction (Depletion
Region / Offset voltage 0.7V)
p (positive charges Dominate)
- - - - - - - - -

n (negative charges dominate)
Diode Electric Field Supplied Current Diffusion
(hole) Current
13
Reverse-Bias Example

• Charges cannot diffuse unless supplied current
  flows towards n

(Depletion Region)
p (positive charges Dominate)
- - - - - - - - -

n (negative charges dominate)
Diode Electric Field Supplied Current Diffusion
(hole) Cuurent
14
Diodes States

• Forward biased (on)- Current flows
• Real Need about 0.7 V to initiate electron-hole
  combination process.
• Reversed biased (off)- Diode blocks current
• Ideal- Current flow 0
• Real Iflow 10-6 Amps

15
Bipolar Junction Transistors (BJT)

• Three Layers in a BJT
• Collector
• Base (very thin)
• has fewer doping atoms
• Emitter
• Two Types of BJTs
• PNP (figure on left)
• operates with outgoing base current
• NPN (figure on right)
• operates with incoming base current

i
i
16
BJT Schematic Representation
Corresponds to
Corresponds to
17
BJT Operation Characteristics

• IC vs. VCE graph allows us to determine operating
  region.
• Works for any IB or VCE
• VBE tops out around 0.7V

18
BJT Operation Regions
19
Cutoff NPN BJT
Collector current
C
n
V2
Base current
Reverse Biased
B
p

Reverse biased
n
V1
Emitter current
E
20
Saturated NPN BJT
Collector current
C
n
V2
- - - -
Forward biased
Base current
B
p

- -
Forward biased
n
V1
Emitter current
E
21
Active Linear NPN BJT
Collector current
C
n
V2
- - -
Base current
Reverse Biased
B
p
- - -

- - -
Forward biased
n
V1
Emitter current
E
22
Possible Uses for BJTs

• Can act as Signal Current Switch (Cutoff Mode)
• Can act as Current Amplifier (Active Region)
• Where
• Beta intrinsic amp property (20 - 200)

23
FIELD-EFFECT TRANSISTORS
( BACKGROUND )

• In 1925, the fundamental principle of FET
  transistors was establish by Lilienfield.
• In 1955, the first successful FET was made.
• Types of Transistors
• MOSFET (metal-oxide-semiconductor field-effect
  transistors)
• JEFT (Junction Field-effect transistors)

24
MOSFET
(Types)

• Four types
• n-channel enhancement mode
• Most common since it is cheapest to manufacture
• p-channel enhancement mode
• n-channel depletion mode
• p-channel depletion mode

25
MOSFET
(n-channel Enhancement-Mode)

• Device Structure
• Three terminals
• Gate, Drain, and Source
• Analogous respectively to the base, collector,
  and emitter.
• Substrate electrically connected to the source.

26
MOSFET
(n-channel Enhancement-Mode)

• Device Structure
• Substrate, source connected to ground
• The drain-body np junction is reverse-biased.
• The body-source pn junction is reverse-biased.
• Enhancement MOSFET acts as an open circuit with
  no gate voltage.

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n-channel Enhancement Mode
(Regions of operation)

• Cutoff region
• VGS lt VT.

Cutoff region
28
n-channel Enhancement Mode
(Regions of operation)

• Ohmic region
• VDS lt 0.25 (VGS-VT), VGSgtVT
• Voltage controlled resistor.

29
n-channel Enhancement Mode
(Regions of operation)

• Saturation region
• VDS VGS-VT, VGS gt VT
• Constant-current source.

30
n-channel Enhancement Mode
(Regions of operation)

• Breakdown region
• VDS gt VB

31
Comparison
(n-channel and p-channel)

• p-type charge carrier.
• Direction of drain current is opposite.
• VDS and VGS are negative.
• n-channel, p-channel behave the same way.

32
Depletion MOSFET

• Addition of an n-type region between the oxide
  layer and p-type substrate.
• Thus, depletion MOSFETs are normally on.
• VT, threshold voltage, is negative.
• Unlike enhancement MOSFET, depletion MOSFET
• Allows positive and negative gate voltages.
• Can be in the saturation region for VGS 0

33
JFET

• JFET
• n-channel
• p-channel

34
JFET
(Physical and circuit representations)
35
JFET
(Regions of Operations)

• Cutoff region
• VGS lt -VP, -VP is the threshold voltage.
• VDS 0

36
JEFT
(Regions of Operations)

• Ohmic region
• VDS lt 0.25(VGS VP), VGS gt -VP.
• Resistance controlled by VGS

37
JFET
(Regions of Operations)

• Saturation region
• VDS VGS VP, VGS gt -VP.
• Constant- current source.

38
JFET
(Regions of Operations)

• Breakdown regions.
• VDS gt VB.

39
JFET
(Physical representation of the regions)

• Illustration of depletion layer growth and
  pinch-off voltage

40
Transistors as Amplifiers and Switches

• Use the I-V characteristic curves of BJT and
  MOSFET
• Use the regions of operation of these transistors
  
• BJT
• Cutoff Region
• Active Linear Region
• Saturation Region
• MOSFET
• Cutoff Region
• Ohmic or Triode Region
• Saturation (Active Region)

Switch operation
Amplifier operation
Switch operation
Amplifier operation
41
I-V Characteristic Curves

• Operating Point for BJT
• For each, IB there is a corresponding
• I-V curve.
• Selecting IB and VCE, we can find the
• operating point, or Q point.

• Applying KVL around the base-emitter
• and collector circuits, we obtain
• IB IBB
• VCE Vcc ICRC

42
I-V Characteristic Curves
Load-line curve
Q
43
Transistors as Amplifiers

• BJT common emitter mode
• In Linear Active Region
• Significant current Gain

Example let Gain, b 80 VB 2V
VE 1.3V Find IC and VC
44
Transistors as Amplifiers
VBE VB VE 0.7V IB VBB VB 4 -
2 RB 40,000 50
mA IC b x IB 80 x 50 mA 4mA VC Vcc
IC x RC 12 (4x10-3)(1x103) 8
V VCE VC VE 8 1.3 6.7 V

45
Transistors as Switches

• Basis of digital logic circuits
• Used in microprocessors
• Input to transistor gate can be analog or digital
• Common names are
• TTL Transistor Transitor Logic
• CMOS Complementary Metal Oxide Semiconductor

46
Transistors as Switches BJT Inverter
Use of the cutoff and saturation regions in the
I-V curves. VCE Vcc - (IC)(RC)
Vout VCE
47
Transistors as Switches BJT Inverter

• Vin Low
• Cutoff region
• No current flows
• Vout VCE Vcc
• Vout High

• Vin High
• Saturation region
• VCE small
• Vout small
• Vout Low

48
Transistors as Switches- MOSFET

• Advantages over BJT logic gates
• Normally Off. Does not require much current from
• input signal
• Easy Fabrication Economical for large scale
• production
• CMOS consumes very little power. Used in pocket
• calculators and wrist watches
• Disadvantages over BJT logic gates
• Cannot provide as much current as BJT
• Switching speed is not as fast

49
Transistors as Switches- MOSFET Inverter

• Vin High
• Ohmic region
• VDS small
• Vout small
• Vout Low

• Vin Low
• Cutoff region
• No Voltage drop across RD
• Vout VDD
• Vout High

50
Transistors as Switches- CMOS Inverter

• Employs a p-channel, Qp, and an n-channel, Qn
  MOSFET

• Vin Low
• Qn off
• Qp on
• Vout High

• Vin High
• Qn on
• Qp off
• Vout Low

51
References

• Rizzoni  -  Principles and Applications of
  Electrical
• Engineering, 2nd Edition
• www.HowStuffWorks.com
• www.williamson-labs.com