Transistor – Bipolar Junction Transistor

Transistor – Bipolar Junction
TransistorTransistor
Dipak Raut
Electronics & Telecommunication
International Institute of Information Technology, I²IT
www.isquareit.edu.in

1.1 Types
Two types of transistor
1. Uni-polar Junction Transistor –
Current conduction is due to one type of charge carriers,
majority carriers.
Example – Field Effect Transistor
2. Bi-polar Junction Transistor –
Current conduction is due to both types of charge
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Current conduction is due to both types of charge
carriers, holes & electrons.
Types of BJT’s are NPN & PNP which are called
current operated device

1.2 Construction
NPN & PNP Type
 NPN transistor is formed by sandwiching a single p-region
between two n-regions.
 PNP transistor is formed by sandwiching a single n-region
between two p-regions.
 The middle region of each transistor type is called base of
transistor.
transistor.
 The remaining two layers are emitter & collector
 The process by which impurities are added to pure semiconductor
is called doping
 Transistor has two pn junctions
1.Emitter-base (JE) 2. Collector-base (JC)
 Doping priority : Base --->Collector--->Emitter--->
 Area-wise Layers : Base--->Emitter --->Collector

1.3 V-I Characteristics
 To understand complete electrical behavior of transistor it is necessary to
study interrelation between respective current & voltages. The relationship
can be plotted graphically which are commonly known as a characteristics
of transistor. Input & output characteristics are important in transistor
 Input characteristics: Graph plot of VBE Vs IB
Following are observations from input characteristics
1. Input Resistance : Ri = ΔVBE / ΔIB ‫׀‬ VCE=Constant
2. After cut in voltage , the IB increases rapidly with small increase
2. After cut in voltage , the IB increases rapidly with small increase
in VBE, Thus dynamic input resistance is small in CE configuration.
3. For fixed value of VBE , IB decreases as VBE is increased.
 Output Characteristics: Graph plot of VCE Vs IC
1. β = IC/IB = Output current / Input current
2. Output Resistance : Ro = ΔVCE / ΔIC ‫׀‬ IB=Constant
3. For fixed value of IB , IC increases slowly as VCE is increased.

1.4 Region of Operation
 Three operating regions – Active, Saturation & Cutoff
 Three configurations – CB , CC & CE
 We will study regions in CE configuration
 The output characteristics of CE consists of three regions
1. Active regions – For transistor works in active region JE is
forward biased while JC is reverse biased. Ic rise more sharply with increase
in VCE in linear region of output characteristics.
2. Saturation regions – In this region emitter-base junction (JE)
2. Saturation regions – In this region emitter-base junction (JE)
& collector base junction (JC) is in forward biased. The saturation
value of VCE , designated VCE(SAT), usually ranges between 0.1v to 0.3v.
3. Cut-Off region – The region below IB = 0 is called cut-off region
of transistor where JE & JC both junctions are in reverse biased.
 To identify operating region of transistor
1.For saturation : IB > IC/βdc
2. For cut-off : IB = 0
3. For Active : VCE > VCE(SAT)

Region
Emitter-Base
Junction
Collector-Base
Junction
Application
Active Forward biased Reverse biased Amplifier
Saturation Forward biased Forward biased On-Switch
Cut-Off Reverse biased Reverse biased Off-Switch
Inverse active
Reverse biased Forward biased
---

1.5 Advantages
1. Small size & light weight
2. Low operating voltage
3. Low cost
4. Low power consumption
5. Higher efficiency
6. Long life
7. Low sensitive to mechanical shock & vibration
7. Low sensitive to mechanical shock & vibration

1.6 Application
1. Transistor can be used as switch
2. Transistor can be used as amplifier as current or voltage
amplifier
3. As a switch , transistor is used in SMPS
4. Transistors are used in oscillator circuit & feedback amplifiers

References
1. “Electronics Devices” by Thomas. L. Floyd, 9th Edition, Pearson

Transistor – Bipolar Junction Transistor

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