TRANSISTOR
Mr. Kishor.S.Bonde
Department of Physics
Moolji Jaitha College, Jalgaon. (MS)
M.J.COLLEGE, JALGAON
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
Introduction
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.
Transistor Structure BJT is bipolar because both holes (+) and electrons (-) will take part in the
current flow through the device
N-type regions contains free electrons (negative carriers)
P-type regions contains free holes (positive carriers)
2 types of BJT
NPN transistor
PNP transistor
P
P
P
N
N
N
PNP and NPN transistor structure
IB(µA)
IB(µA)Ic(mA)
IE(mA) IE(mA)
IC(mA)
Arrow shows the current flowsArrow shows the current flows
NPN transistor:- A n p n transistor consist of thin p-type material sandwitched between two n-type matériel
PNP transistor:- A p n p transistor consist of thin n-type material sandwitched between two p-type matériel
The transistor regions are:
Emitter (E) – It is more heavily doped than other two region because it supplies charge carriers to the base send the carriers into the base region and then on to the collector
Base (B) – It is very thin(10-6m) as compare to emitter & collector & is very lightely doped. acts as control region. It can allow none , some or many carriers to flow
Collector (C) – Its main function is to collect the majority charge carriers through the base.
Collector
Base
Emitter
EB
C
PNP Schematic Symbol
Emitter
Base
Collector
Memory aid: NPN
means Not Pointing iN.
EB
C
NPN Schematic Symbol
N
P
NThe collector is lightly doped. C
The base is thin and
is lightly doped.B
The emitter is heavily doped. E
NPN Transistor Structure
TRANSISTOR BIASING
• Fundamental operation of pnp transistor and npn transistor is similar except for: – role of electron and hole, – voltage bias polarity, and – Current direction
For proper working of transistor, it is essential to apply voltage of correct polarity across its two junction The main condition;1) Emitter junction is always forward biased2) Collector junction is always reverse biased
Transistor configuration
Transistor configuration –is a connection of transistor to get variety operation.
3 types of configuration:
Common Base.
Common Emitter.
Common Collector.
Common-Base Configuration Base terminal is a common point for input and output.
Input – EB
Output – CB
Not applicable as an amplifier because the relation between input current gain (IE) and output current gain (IC) is approximately 1
Current amplification factor
It is the ratio of collector current to emitter current is called Current
amplification factor (α)
α =
Common-Emitter Configuration Emitter terminal is common for input and output circuit
Input – BE
Output – CE
Mostly applied in practical amplifier circuits, since it provides good voltage, current and power gain
Common-Collector Configuration The input signal is applied to the base terminal and the output is taken
from the emitter terminal.
Collector terminal is common to the input and output of the circuit
Input – BC
Output – EC
Input = Output
I-V Characteristic for CE configuration : Input characteristic
Input characteristic: input current (IB) against input voltage (VBE) for several output voltage (VCE)
From the graph IB = 0 A VBE < 0.7V (Si) IB = value VBE > 0.7V (Si)
The transistor turned on whenVBE = 0.7V
I-V Characteristic for CE configuration : Output characteristic
Output characteristic: output
current (IC) against output
voltage (VCE) for several
input current (IB)
3 operating regions:
Saturation region
Cut-off region
Active region
I-V Characteristic for CE configuration : Output characteristic
Saturation region – in which both junctions are forward-biased and IC increase linearly with VCE
Cut-off region – where both junctions are reverse-biased, the IB is very small, and essentially no IC flows, IC is essentially zero with increasing VCE
Active region – in which the transistor can act as a linear amplifier, where the BE junction is forward-biased and BC junction is reverse-biased. IC increases drastically although only small changes of IB.
Saturation and cut-off regions – areas where the transistor can operate as a switch
Active region – area where transistor operates as an amplifier
Current Relationships
Relations between IC and IE :α = IC
IE
Value of α usually 0.9998 to 0.9999, α ≈ 1
Relations between IC and IB :β = IC @ IC = βIB
IB
Value of β usually in range of 50 400 The equation, IE =IC + IB can also written in β
IC = βIB
IE = βIB + IB => IE = (β + 1)IB
The current gain factor , α and β is: α = β @ β = α .
β + 1 α - 1
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