The document discusses the bipolar junction transistor (BJT). It describes how the BJT was invented in 1947 by scientists at Bell Labs. The BJT consists of three terminals - the emitter, base, and collector - and comes in two types, p-n-p and n-p-n. The document explains the basic operation and principles of both types of BJT, including how current flows when junctions are forward or reverse biased in different modes. It also provides examples of calculating currents given bias conditions and current gains. Finally, it summarizes the key current-voltage relationships and characteristics of BJTs in common base, common emitter, and common collector configurations.

1. BIPOLAR JUNCTION TRANSISTOR
By
Prankit Mishra
FST, ICFAI University

2. INTRODUCTION
 A semiconductor triode is known as bipolar junction
transistor (BJT)
 The transistor was invented by a team of three men at Bell
Laboratories in 1947.
 Dr. William Shockley,
 Dr. John Bardeen,
 Dr. Walter H. Brattain.
 It can be used as amplifiers and logic switches
 The BJT consists three terminals: emitter (E), base (B),
and collector(C)

3.  Based on its construction, a junction transistor can be
classified into two types:
 p-n-p transistor and
 n-p-n transistor
 A junction transistor consists of a silicon (or germanium)
crystal in which a layer of n-type silicon is sandwiched
between two layers of p-type silicon, called a p-n-p
transistor.
 Alternatively, a transistor may consist of a layer of p-type
between two layers of n-type material, called an n-p-n
transistor
 The three layer of transistor are Emitter (E), Base (B) and
Collector (C) shown in the figure
 The Emitter (E) layer is heavily doped; the base (B) and
collector (C) layers are lightly doped.

5. BASIC OPERATION OF TRANSISTOR
 There are three mode of operations:
 If transistor is operating active mode, it can be used as
amplifier.
 The transistor can be used as logical switch if it operates in
cut-off and saturation mode.
Bias Mode E-B Junction C-B Junction
Saturation Forward Forward
Active Forward Reverse
Inverted
active
Reverse Forward
Cutoff Reverse Reverse

6. OPERATING PRINCIPLES OF P-N-P TRANSISTOR
Consider the active mode, that is E-B junction is forward
biased and C-B junction is reverse biased (One p-n junction
of a transistor is forward biased, while the other is reverse
biased) shown in fig.

7.  First take forward biased p-n junction ( E-B junction).
 The depletion region has been reduced in width due to the
applied bias, resulting in a heavy flow of majority carriers (hole)
from p-type to n-type.
 The emitter current IE consists two components: IpE (due to hole
diffusion from E to B) and InE (due to electron diffusion from B
to E)
IE = IpE + InE
 Because the emitter is heavily doped than the base, the emitter
current due to diffusion of holes from E to B. Therefore IE ≈ IpE
 The holes emitted from the emitter are injected into the n-type
base region, some of these are recombine with the electrons in
the base. It accounts for a small base current (IB)
 Most of them reach the C-B junction where they are instantaneously
swept out by the strong electric field in the depletion layer of C-B
junction

8.  Then the holes are collected by the collector.
 The fraction (  ) of injected holes which are finally collected
by the collector.  is usually just less than 1
IC =  IE
 Now consider the reverse biased p-n junction
 This reverse current (ICO) consists two components:
 one is InCO (due to the motion of electrons from p to n region across
collector junction) and
 the other one is IpCO (due to motion of holes from n to p region across
collector junction)
 The total collector current is
where α is called large signal current gain.
Typical range of α is 0.95 to 0.998
E
COC
COEC
I
II
III






10. From Kirchhoff's Current law, IE = IC + IB
Substituting this in IC ≈  IE we get

IC =  ( IC + IB ) or, IC =  . IB
1 - 
or, IC =  IB
where,  =  / (1- ) = common emitter current gain.
Also,  =  / ( 1+  ) If  = 0.995,  = 199
Usually  is very close to unity and  lies in the range of
25 – 350.

11. OPERATING PRINCIPLES OF N-P-N TRANSISTOR
 The operation of the npn transistor is exactly the same if the
roles played by the electron and hole are interchanged

12. IE = IS [ e VBE / VT ]
Based on KCL: IE = IC + IB
IC =  IB
IC =  IE
 = [  /  + 1 ]
IE = IB(  + 1)
IE = IS [ e VEB / VT]
NPN PNP
 = [ / 1 -  ]

13. EXAMPLE
Calculate the collector and emitter currents, given the base current and current gain.
Assume a common-base current gain and a base current of .
Also assume that the transistor is biased forward in the forward active mode.
Solution: The common-emitter current gain is
The collector current is
And the emitter current is

14. EXAMPLES
 EXAMPLE 1
 Given IB = 6.0A and
IC=510 A
Determine ,  and IE
 EXAMPLE 2
 NPN Transistor
 Reverse saturation
current Is = 10-13A with
current gain,  = 90.
Based on VBE = 0.685V,
determine IC , IB and IE
 EXAMPLE 3
 PNP Transistor
  = 60, IC= 0.85mA
 Determine , IE and IB

15. TRANSISTOR I-V CHARACTERISTICS
 Basically there are three types of circuit connections
(called configurations) for operating transistor.
 Common base (CB)
 Common emitter (CE)
 Common collector CC)
 The term common is used to denote the electrode
that is common to the input and output of the circuit

17. Summary of the BJT current‐voltage relationships in the active mode

18. TRANSISTORS CHARACTERISTICS ( COMMON BASE)
Common Base Connection

19. Input characteristics of
common base transistors

20. Typical output characteristics (CB) of an n-p-n transistors
IC versus VCB plot with various IE as parameter is known as
common‐base output characteristics

21. Common emitter circuit
Input characteristics of an
n-p-n transistors(CE)

22. Typical common emitter output characteristics of
an n-p-n transistors