Chapter 10
BJT Fundamentals
Haris Mehmood
BJT
BJT : Bipolar Junction Transistor
It is a semiconductor device with three regions
These three regions are doped alternately
Alternately: PNP or NPN
BJT Regions
Base: relatively narrow middle region of BJT
Emitter: more heavily doped region
Collector: relative to emitter, less heavily doped region
BJT Symbols
Plus(+) and minus(-) signs show voltage polarity
DC voltages or DC currents in BJT are represented by
capital letters and capital subscript
BJT Symbols
𝐼𝐸 = DC emitter current
𝐼𝐵 = DC base current
𝐼𝐶 = DC collector current
BJT Symbols
𝑉𝐵𝐸 = DC voltage difference between base and emitter
𝑉𝐵𝐶 = DC voltage difference between base and collector
𝑉𝐶𝐸 = DC voltage difference collector and emitter
BJT DC Currents and DC Voltages
Using KCL (currents: sum of leaving = sum of entering)
𝐼𝐸 = 𝐼𝐵 + 𝐼𝐶
Using KVL (voltages: sum in a loop = 0)
𝑉𝐸𝐵 + 𝑉𝐵𝐶 + 𝑉𝐶𝐸 = 0
BC547B
How to determine its type?
• Datasheet
NPN general purpose transistors,
especially suited for use in
• audio amplifiers,
• low noise input stages of tape
recorders,
• HI-FI amplifiers,
• signal processing circuits of
television receivers.
Different Circuit Configurations or Biasing Modes for BJT
BJT is a three terminal device
One terminal can be used for input
Second terminal can be used for output
What about third terminal ?
Primarily BJT is used in three different configurations that will be introduced one by one
Common Base Configuration
Common base configuration
Base is part of both input and output terminal
Common base : (base is common to both emitter and collector)
Based upon the requirement, you
decide which terminal should be
input and which terminal should be
output
Common Emitter Configuration
Common emitter configuration
Emitter is part of both input and output terminal
Common emitter : (emitter is common to both base and collector)
Common Collector Configuration
Common collector configuration
Collector is part of both input and output terminal
Common collector : (collector is common to both base and emitter)
Common Base Mode (𝑉𝐵𝐶 𝑣𝑠. 𝐼𝐶 ) at Different values of 𝐼𝐸
You can set different values
of emitter current. You may
observe following relation
between 𝑉𝐵𝐶 𝑎𝑛𝑑 𝐼𝐶 under
common base configuration
of BJT
Common Emitter Mode (𝑉𝐸𝐶 𝑣𝑠. 𝐼𝐶 ) at Different values of 𝐼𝐵
You can set different values of
base current. You may observe
following relation between
𝑉𝐸𝐶 𝑎𝑛𝑑 𝐼𝐶 under common
emitter configuration of BJT
Different Biasing Modes of BJT
Biasing Modes
Energy band diagram (NPN transistor)
BJT operating regions
Electrostatics
For PNP BJT , following notations are used;
𝑁𝐴𝐸 = emitter doping
𝑁𝐷𝐵 = base doping
𝑁𝐴𝐶 = collector doping
𝑁𝐴𝐸 ≫ 𝑁𝐷𝐵 ≫ 𝑁𝐴𝐶
Electrostatics
For PNP BJT , following relation exists for doping profiles
𝑁𝐴𝐸 = emitter doping, 𝑁𝐷𝐵 = base doping, 𝑁𝐴𝐶 = collector doping
𝑁𝐴𝐸 ≫ 𝑁𝐷𝐵 ≫ 𝑁𝐴𝐶
𝑊𝐵 = Total width of base ,
𝑥𝑛𝐸𝐵 = E-B depletion width lying in n-type base
𝑥𝑛𝐶𝐵 = C-B depletion width lying in n-type base
𝑊 = Portion of base that is not depleted (Quasineutral base width)
𝑊 = 𝑊𝐵 − 𝑥𝑛𝐸𝐵 − 𝑥𝑛𝐶𝐵
Electrostatics (Energy band diagram)
Electrostatics
Quiz # 3 (Time: 20 minutes)
1. Find the total current density of an ideal p+-n Si step junction diode on
which the voltage VA of 11.05(kT/q) has been applied at the temperature
of T = 400 K, ND = 1018 cm-3, µp= 800 cm2/V-sec, τp= 1 millisecond and
ni = 1013 cm-3. (7 marks)
𝑞 𝑉𝐴 𝐷𝑁 𝑛𝑖2 𝐷𝑝 𝑛𝑖2
𝐼 = 𝐼𝑜 (𝑒 𝑘𝑇 − 1) 𝐼𝑜 = 𝑞𝐴( + )
𝐿𝑁 𝑁𝐴 𝐿𝑝 𝑁𝐷
2. What happens to the minority carrier diffusion equation if the device has
no R-G process occurring and no light being fall upon the device
(3 marks).
Exercise 10.1
Career Activity in PNP BJT under active biasing mode
Diffusion Currents flowing in PNP BJT under active biasing mode
𝐼𝐸 𝑎𝑛𝑑 𝐼𝐶
𝐼𝐸𝑝 = Hole diffusion current flowing across E-B junction
𝐼𝐸𝑛 = Current associated with electron injection from base to emitter
𝐼𝐸 = 𝐼𝐸𝑝 + 𝐼𝐸𝑛
𝐼𝐶𝑝 = Hole diffusion current flowing across C-B junction
𝐼𝐶𝑛 = It arises due to minority carrier electrons that stray into C-B depletion
region and then swept into base
𝐼𝑐 = 𝐼𝐶𝑝 + 𝐼𝐶𝑛
Performance Parameters
1. Emitter Efficiency
Increasing the hole current (IEp) across the E-B junction while keeping IE
constant decreases the electron current (IEn)
Overall current gain increases
Performance Parameters
2. Base Transport Factor
The fraction of minority carriers injected into the base that successfully
diffuse across the base region and enter the collector is known as base
transport factor
Performance parameters
3. Common base DC current gain
Performance parameters
4. Common emitter DC current gain
Problem 10.9
