bjt
TRANSCRIPT
Introduction to Transistors
Balaji
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”
(positive chargesDominate)
-- --
- --
- -
++ ++
+ + ++ +
+ + +
“n”
(negative charges dominate)
P-N Junction
(Depletion Region / Offset voltage = 0.7V)
Diode Electric Field
Supplied Current
Diffusion (hole) Current
Reverse-Bias Example
Charges cannot diffuse unless supplied current flows towards “n”
“p”
(positive chargesDominate)
-- --
- --
- -
++ ++
+ + ++ +
+ + +
“n”
(negative charges dominate)
(Depletion Region)
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)
Three Layers in a BJT Collector Base (very thin)
has fewer doping atoms Emitter
Two Types of BJT’s PNP (figure on left)
operates with outgoing base current NPN (figure on right)
operates with incoming base current
p
P+
n
emitter
collector
base
n
n+
p
emitter
collector
i i
BJT Schematic Representation
p
P+
n
emitter
collector
base
i
n
n+
p
emitter
collector
i
iB
Corresponds to:
Corresponds to:
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 Operation RegionRegion
IB or VCE
Char. BC and BE BC and BE JunctionsJunctions
ModeMode
Cutoff IB = Very small
Reverse & Reverse
Open Switch
Saturation VCE = Small Forward & Forward
Closed Switch
Active Linear
VCE = Moderate
Reverse &Forward
Linear Amplifier
Break-down
VCE = Large Beyond Limits
Overload
Cutoff NPN BJT
n
p
n
V2
V1
+++
C
B
E
Emitter current
Collector current
Base current
Reverse biased
Reverse Biased
Saturated NPN BJT
n
p
n
V2
V1
+ +
- - - -
C
B
E
Emitter current
Collector current
Base current Forward biased
Forward biased--
Active Linear NPN BJT
n
p
n
V2
V1
+ +
- - -
- - -
- - -
C
B
E
Emitter current
Collector current
Base current
Forward biased
Reverse Biased
Possible Uses for BJT’s
Can act as Signal Current Switch (Cutoff Mode)
Can act as Current Amplifier (Active Region)
Where: Beta = intrinsic amp property (20 - 200)
Bc II
FIELD-EFFECT TRANSISTORS
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)
( BACKGROUND )
MOSFET
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
(Types)
Depletion typen-channel p-channel
Enhancement typen-channel p-channel
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
Cutoff region VGS < VT.
(Regions of operation)
IDS
VGS
VT
Characteristic Curve
Cutoff region
n-channel Enhancement Mode
Ohmic region VDS < 0.25 (VGS-VT),
VGS>VT
Voltage controlled resistor.
(Regions of operation)
IDS
VGSVT Characteristic Curve
n-channel Enhancement Mode
Saturation region VDS ≥ VGS-VT, VGS >
VT
Constant-current source.
(Regions of operation)
IDSS
Ohmic SaturationIDS
VDS
VGS
VGS VTH
Characteristic curves
Breakdown region VDS > VB
n-channel Enhancement Mode
(Regions of operation)
Comparison
p-type charge carrier. Direction of drain current is
opposite. VDS and VGS are negative. n-channel, p-channel behave the
same way.
(n-channel and p-channel)
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
G
S
D
G
S
n-channel p-channel
JFET (Physical and circuit representations)
JFET
Cutoff region VGS < -VP, -VP is the threshold voltage. VDS = 0
(Regions of Operations)
JEFT
Ohmic region VDS < 0.25(VGS + VP), VGS > -VP.
Resistance controlled by VGS
VP
IDS
VDS
Transfer characteristic in saturation region
(| VDS |>|VP|)
IDSS
(Regions of Operations)
JFET
Saturation region
VDS ≥ VGS +VP, VGS > -VP.
Constant- current source.
(Regions of Operations)
IDSS
Ohmic region
Saturation region
IDS
-VP
VDS
VGS = 0V
VGS
VGS = VP
Idealized output characteristic
JFET
Breakdown regions. VDS > VB.
(Regions of Operations)
JFET (Physical representation of the regions)
Illustration of depletion layer growth and pinch-off voltage
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)
Transistors as Amplifiers and Switches
Switch operationAmplifier operation
Switch operation
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
IC = Vcc VCE
RC RC
I-V Characteristic Curves
IC = Vcc VCE
RC RC
QLoad-line curve
Transistors as Amplifiers
•BJT – common emitter mode•In Linear Active Region•Significant current GainExamplelet Gain, = 80 VB = 2V VE = 1.3V
Find IC and VC
VBE = VB – VE = 0.7VIB = VBB – VB 4 - 2 RB 40,000 = 50 AIC = x IB = 80 x 50 A = 4mA
VC = Vcc – IC x RC
= 12 – (4x10-3)(1x103) = 8 V
VCE = VC – VE = 8 – 1.3 = 6.7 V
Transistors as Amplifiers
=
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 •Cutoff region•No current flows•Vout = VCE = Vcc
•Vout = High
•Vin High •Saturation region•VCE small•Vout = small
•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 •Cutoff region•No Voltage drop across RD•Vout = VDD
•Vout = High
•Vin High •Ohmic region•VDS small•Vout = small
•Vout = Low
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
References
•Rizzoni - Principles and Applications of Electrical Engineering, 2nd Edition•www.HowStuffWorks.com•www.williamson-labs.com