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NAME OF FACULTY : MR. Harekrishna Avaiya
DEPARTMENT: E.C. (PPI-1ST)
BASIC ELECTRONICS
Transistors• History• Transistor Types
• BJT: A bipolar (junction) transistor
is a three-terminal electronic device
constructed of doped semiconductor material
and may be used in amplifying or switching
applications • FET :The field-effect transistor (FET) relies on an electric field to control the shape and hence the conductivity
of a channel of one type of charge carrier
in a semiconductor material
• Power transistorsSub: BE Topic: Transistors
What is a Transistor?
A Transistor is an electronic device
composed of layers of a semiconductor
material which regulates current or
voltage flow and acts as a switch or gate
for electronic circuit.
Sub: BE Topic: Transistors
History of the Transistor
John Pierce –supervised the Bell Labs team which built the first transistor (1947)
First Solid State Transistor – (1951) Gordon K. Teal (left) and Morgan Sparks at Bell Laboratories, 1951
Akio Morita, who founded a new company named Sony Electronics that mass-produced tiny transistorized radios (1961)
Sub: BE Topic: Transistors
Applications Switching Amplification Oscillating Circuits Sensors
Sub: BE Topic: Transistors
Composed of N and P-type Semiconductors
• N-type Semiconductor has an excess of electrons
– Doped with impurity with more valence electrons than silicon
P-type Semiconductor has a deficit of electrons (Holes)
– Doped with impurity with less valence electrons than silicon
Sub: BE Topic: Transistors
P-N Junction (Basic diode):
- Bringing P and N Semiconductors in contact
P Type N Type - Creation of a Depletion Zone
Sub: BE Topic: Transistors
Reverse Biased => No Current
Applying –Voltage to Anode increases
Barrier Voltage & Inhibits Current Flow
• Applying Voltage to Cathode
• Barrier Voltage to Anode allows current flow
Forward Biased =>
Current
Sub: BE Topic: Transistors
Types Of TransistorsNPN: transistor where the majority current carriers are electrons
The majority current carriers in the PNP transistor are holes
Sub: BE Topic: Transistors
Transistor Operation
Sub: BE Topic: Transistors
Transistors
• Semiconductor device
• First Active circuit element - gain > 1
• Discuss the Bipolar Junction Transistor only
• See Simpson Chapter 5 for more detail.
Sub: BE Topic: Transistors
Bipolar Junction Transistors
• NPN Bipolar Junction transistor shown (PNP also possible)
• 3 terminals: Emitter, Base, Collector
• Contains 2 p-n junctions: emitter-base junction, collector-base junction
• Can be thought of as two back-to-back diodes, but operating characteristics are very different
• Base region (P-type here) must be thin for transistor action to work
Base
Emitter
Collector
Sub: BE Topic: Transistors
Modes of operation
• Use 2 voltage supplies to bias the two junctions (forward or reverse)• 3 basic modes: cutoff, active and saturation, correspond to three different bias conditions.
Base
Emitter
Collector
“OFF”
“ON”
Sub: BE Topic: Transistors
Active Mode• Collector-Base junction is reverse biased• Emitter-Base junction is forward biased• iC=βiB (β=100 - 500) Active circuit element - gain > 1 !!• How does collector current flow when collector-base junction is reverse biased?
Emitter
CollectorBase
+
-
iC
iB
iE
VCE
VBE
Sub: BE Topic: Transistors
Active Mode
• What's happening?• Emitter-base is forward biased; collector-base reverse biased.• Forward bias of emitter injects electrons into thin base region• Majority shoot through the base into the collector region where they encounter the voltage source on the collector and produce a current.• Electrons combine with holes in the base region and form negative ions which impede the flow• Drawing off negative charge via the base lead reduces this effect (“making the base smaller”) - so the base current controls the flow of electrons into the collector• Nobel Prize 1956; Shockley, Bardeen & Brattain
Direction of current flow
+-
Sub: BE Topic: Transistors
Circuit Symbols and Conventions
• BJTs are not symmetric devices• Doping and physical dimensions are different for collector and emitter
• Collector largest, connected to heat sink as most power dissipated there• Emitter region smaller, and more heavily doped to provide an abundance
of charge carriers• Base region is very thin (~50nm) to enhance probability that electrons will
cross it• PNP devices also exist - diode senses are reversed, so bias voltage polarities must also be reversed
Sub: BE Topic: Transistors
Emitter
CollectorBase
+
-
iC
iB
iE
VCE
VBE
i-v Characteristics
• Simplest model for low frequencies (DC condition) "Ebers-Moll". • Relates collector current IC to base-emitter voltage VBE:
• IS=Saturation Current• Similar to Diode Law• Recall IB=IC/β
VBE
IC
TBE
TBE
VVS
VVSC
eI
eII/
/ 1
• Collector current is controlled by the base-emitter voltage VBE
Sub: BE Topic: Transistors
BJT Amplifier
Note labelling scheme:iC=IC+ic
"Common emitter" configuration
Sub: BE Topic: Transistors
Thank You