Introduction to Transistors
Presented: October 23, 2001
Chris Green
Carl Hanna
Ancil Marshall
Kwame Ofori
Overview
Introduction & History
Semiconductors
Operation of Transistors
Transistor Types
Applications
Examples
Questions
Conclusion
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
Background
Model of First Transistor
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.
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.
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.
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
Comparison of Energy Bands
Semiconductor resembles an insulator, but with a smaller
energy band.
Small energy band makes it a marginal conductor
Simple Semiconductors: Diodes
Diode is the simplest semiconductor.
Allows current to flow in one direction only.
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
Forward-Bias Example
Charge Diffusion aided by Supply Current
Current is allowed through easily
P-N Junction
(Depletion Region / Offset voltage = 0.7V)
-
+
--
“p” ++ “n”
-
+
--
(positive charges +++ (negative charges
-
Dominate) ++ dominate)
--
+++
Diode Electric Field
Supplied Current
Diffusion (hole) Current
Reverse-Bias Example
Charges cannot diffuse unless supplied
current flows towards “n”
(Depletion Region)
-
+
--
“p” ++ “n”
-
+
--
(positive charges +++ (negative charges
-
Dominate) ++ dominate)
--
+++
Diode Electric Field
Supplied Current
Diffusion (hole) Cuurent
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
Bipolar Junction Transistors (BJT)
collector collector
Three Layers in a BJT p n
Collector
Base (very thin) base n p
has fewer doping atoms
Emitter i P+ i n+
Two Types of BJT’s
PNP (figure on left)
emitter emitter
operates with outgoing base current
NPN (figure on right)
operates with incoming base current
BJT Schematic Representation
collector
iB
p
base n Corresponds to:
i P+
emitter
collector
n
p Corresponds to:
i n+
emitter
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
BJT Operation Regions
Operation IB or VCE BC and BE Mode
Region Char. Junctions
Cutoff IB = Very Reverse & Open Switch
small Reverse
Saturation VCE = Small Forward & Closed Switch
Forward
Active VCE = Reverse & Linear
Linear Moderate Forward Amplifier
Break-down VCE = Large Beyond Overload
Limits
Cutoff NPN BJT
Collector current
C
V2 n
Base current
B Reverse Biased
+++ p
Reverse biased
n
V1
Emitter current
E
Saturated NPN BJT
Collector current
C
V2 n
----
Base current Forward biased
B
++ p
- Forward biased
-
n
V1
Emitter current
E
Active Linear NPN BJT
Collector current
C
V2 n
---
Base current
B Reverse Biased
---
++ p
---
Forward biased
n
V1
Emitter current
E
Possible Uses for BJT’s
Can act as Signal Current Switch (Cutoff
Mode)
Can act as Current Amplifier (Active Region)
I c I B
Where:
Beta = intrinsic amp property (20 - 200)
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)
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
n-channel p-channel n-channel p-channel
Depletion type Enhancement type
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.
MOSFET (n-channel Enhancement-Mode)
Device Structure
Substrate, source connected to ground
The drain-body n+p junction is reverse-biased.
The body-source pn+ junction is reverse-biased.
Enhancement MOSFET acts as an open circuit with no gate
voltage.
n-channel Enhancement Mode
(Regions of operation)
Cutoff region
VGS VT
Voltage controlled
resistor.
IDS
VGS
VT
Characteristic Curve
n-channel Enhancement Mode
(Regions of operation)
Saturation region
VDS ≥ VGS-VT, VGS > VT
Constant-current IDS
Ohmic Saturation
source. IDSS
VGS
VGS VTH
VDS
Characteristic curves
n-channel Enhancement Mode
(Regions of operation)
Breakdown region
VDS > VB
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.
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
JFET
JFET
n-channel
p-channel
D D
G G
S S
n-channel p-channel
JFET (Physical and circuit representations)
JFET (Regions of Operations)
Cutoff region
VGS -VP. IDS
Resistance controlled by VGS
IDSS
VDS
VP
Transfer characteristic
in saturation region
(| VDS |>|VP|)
JFET (Regions of Operations)
Saturation region
VDS ≥ VGS +VP, VGS
> -VP. IDS VGS = 0V
Constant- current
source. IDSS
Ohmic Saturation
region region VGS
VGS = VP
VDS
-VP
Idealized output characteristic
JFET (Regions of Operations)
Breakdown regions.
VDS > VB.
JFET (Physical representation of the regions)
Illustration of depletion layer growth and pinch-off voltage
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 Switch operation
Active Linear Region Amplifier operation
Saturation Region
MOSFET
Cutoff Region Switch operation
Ohmic or Triode Region
Saturation (Active Region) Amplifier operation
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
Vcc VCE
IC =
RC RC
I-V Characteristic Curves
Vcc VCE
IC =
RC RC
Load-line curve
Q
Transistors as Amplifiers
•BJT – common emitter mode
•In Linear Active Region
•Significant current Gain
Example
let Gain, = 80
VB = 2V
VE = 1.3V
Find IC and VC
Transistors as Amplifiers
VBE = VB – VE = 0.7V
IB = VBB – VB 4-2
RB = 40,000
= 50 mA
IC = x IB = 80 x 50 mA
= 4mA
VC = Vcc – IC x RC
= 12 – (4x10-3)(1x103)
=8V
VCE = VC – VE = 8 – 1.3
= 6.7 V
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
Transistors as Switches – BJT Inverter
Use of the cutoff and saturation regions in the I-V curves.
VCE = Vcc - (IC)(RC)
Vout = VCE
Transistors as Switches – BJT Inverter
•Vin Low •Vin High
•Cutoff region •Saturation region
•No current flows •VCE small
•Vout = VCE = Vcc •Vout = small
•Vout = High •Vout = Low
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
Transistors as Switches- MOSFET Inverter
•Vin Low •Vin High
•Cutoff region •Ohmic region
•No Voltage drop across •VDS small
RD •Vout = small
•Vout = VDD •Vout = Low
•Vout = High
Transistors as Switches- CMOS Inverter
•Employs a p-channel, Qp, and an n-channel, Qn MOSFET
•Vin = Low •Vin = High
•Qn = off •Qn = on
•Qp = on •Qp = off
•Vout = High •Vout = Low
References
•Rizzoni - Principles and Applications of Electrical
Engineering, 2nd Edition
•www.HowStuffWorks.com
•www.williamson-labs.com