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2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 1
Transistor and their Characteristics –
Lesson-1 Junction Transistor
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 2
1.1. Junction Transistor Junction Transistor Definition Definition
The transferred-resistance or transistoris a multi-junction device that is capable of• Current gain• Voltage gain• Signal-power gain
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 3
Bipolar junction transistorBipolar junction transistorInvented in 1948 by Bardeen, Brattain and Shockley
Contains three adjoining, alternately doped semiconductor regions: Emitter (E), Base (B), and Collector (C)
The middle region, base, is very thin compared to the diffusion length of minority carriers
Two kinds: npn and pnp
2008 Junction Transistor Unit 4 Lesson 1-" , Raj Kamal, 4
Bipolar junction transistor – BJT Definition
The Bipolar junction transistor is an active device that works as a voltage-controlled current source and whose basic action is control of current at one terminal by controlling voltage applied at other two terminals.
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 5
Bipolar Junction Transistor Representation
NPN PNP
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 6
2.2. Transistor Current Transistor Current ComponentsComponents
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 7
BJT circuit Current Components BJT circuit Current Components
As shown, the currents are positive quantities when the transistor is operated in forward active mode.
IE = IB + IC and VEB + VBC + VCE = 0 VCE = VEC
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 8
Meaning of Forward biasingMeaning of Forward biasing
When the p side is applied +ve and n side negative in a junction and applied voltage is grater than a threshold 0.65 V for Silicon (Si)
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 9
Meaning of Reverse biasingMeaning of Reverse biasing
When the p side is applied -ve and n side +ve in a junction and applied voltage is between 0 to a breakdown voltage
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 10
3.3. Transistor as AmplifierTransistor as Amplifier
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 11
BJT Common BJT Common Base Base
configurationconfiguration
BJT Common BJT Common EmitterEmitter
configurationconfiguration
BJT Common BJT Common Collector Collector
configurationconfiguration
PNPPNP PNP
IBIE IC
Three configurations
2008 Junction Transistor Unit 4 Lesson 1-" , Raj Kamal, 12
Six Problems
• Draw circuits of CB, CE, CC configurations for npn and pnp transistors• Show current directions
2008 Junction Transistor Unit 4 Lesson 1-" , Raj Kamal, 13
npn transistor in a simple circuit, known as‘common-emitter’ amplifier
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 14
Characteristics ofCharacteristics of Transistor Amplifier in three Transistor Amplifier in three regions of operations regions of operations
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 15
Input or driving Point Input or driving Point Characteristics of Common Characteristics of Common Base transistor AmplifierBase transistor Amplifier
Input or driving Point Characteristics of Common Emitter transistorAmplifier
Active region
Saturation region Saturation region
Active region
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 16
Output Collector Output Collector Characteristics of Common Characteristics of Common Base transistor AmplifierBase transistor Amplifier
Output Collector Output Collector Characteristics of Common Characteristics of Common
Emitter transistorEmitter transistorAmplifierAmplifier
Saturation region Cut-off region Cut-off regionSaturation region
Active region Active region
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 17
First Approximation Relationship First Approximation Relationship between between IICC and and IIEE
IC IE
BJT Common BJT Common Base Base
configurationconfiguration
2008 Junction Transistor Unit 4 Lesson 1-" , Raj Kamal, 18
The collector current is being controlled by the B-E voltage, orthe current in the one part of the device is being controlled bythe voltage in another part - transistor action
Since the B-E junction is forward biased, holes from the baseare injected into the emitter. However, these injected holesdo not contribute to the collector current and are thereforenot part of the transistor action
To design a useful device, we need mathematical expressionsfor the minority carrier concentrations shown in the figureabove.
There are three modes of operation we must consider
• Forward-active (B-E FB, B-C RB)• Cut-off (B-E RB, B-C RB)• Saturation (B-E FB, B-C FB)
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 19
BJT biasing modesBJT biasing modes
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 20
In the cut-off region, the base emitter and collector base junctions of transistor are reverse biased IC IE 0
CutCut--off defined as region of characteristics Ioff defined as region of characteristics ICC = 0= 0
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 21
Saturation- region – defined as region of characteristics left of VCB = 0 and IC increase
exponentially with VCB increases toward 0
In the saturation region region, the base emitter and collector base junctions of transistor are forward biased
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 22
Active - region – defined as region of characteristics right of VCB = 0 and IC first increases exponentially with VCB increases toward 0.7 V in Si transistor and then becomes constant In the active region region, the base emitter and
collector base junctions of transistor are forward and reversed biased respectively IC IE ≠ 0 and is in mA IC
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 23
Operation in the forward-activemode
B-E junction is forward biased so electronscan be injected from the emitter to the base, B-Cjunction is reverse biased.
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 24
4.4. Transistor Transistor Construction Construction
2008 Junction Transistor Unit 4 Lesson 1-" , Raj Kamal, 25
BJT structure - three regions, two p-n junctions, threeterminals (emitter, base, collector)
ThinThin
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Construction Construction Devices can be p-n-p, or n-p-n structures Width of the base is small compared to the
minority carrier length and is about 1/150 of total width, number of free carriers are small as doping level is 1/10 th or less compared to collector)
Emitter is normally heavily doped, the collector has light doping
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 27
SummarySummary
We learnt (i) pnp transistor (ii) npn transistor (iii) Current Components (iv) Three configurations-
Common base, common emitter and Common collector
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 28
We learnt (v) Three regions- cut-off,
saturation and active (vi) Transistor construction
2008 Junction Transistor Unit 4 Lesson 1- " , Raj Kamal, 29
End of Lesson 1End of Lesson 1