1. DEPARTMENT OF COMPUTER SCIENCE
TRANSISTOR BASICS
MAHESHTALA COLLEGE
DEPARTMENT OF COMPUTER SCIENCE
PALLAB KUMAR DAS

4. INTRODUCTION The basic of electronic system now a days is
semiconductor device.
The famous and commonly use of this device is BJTs
(Bipolar Junction Transistors).
It can be use as amplifier and logic switches.
BJT consists of three terminal:
 collector : C
 base : B
emitter : E
Two types of BJT : pnp and npn

5. NPN PNP 3 layer semiconductor device consisting:
2 n- and 1 p-type layers of material  npn transistor
2 p- and 1 n-type layers of material pnp transistor
The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material
A single pn junction has two different types of bias:
forward bias
reverse bias
Thus, a two-pn-junction device has four types of bias.
NPN
PNP

6. Transistor currents The arrow is always drawn on the emitter
The arrow always point
toward the n-type
The arrow indicates the
direction of the emitter
current:
pnp:E B
npn: B E
IC=the collector current
IB= the base current
IE= the emitter current

7. Transistor Operation
The basic operation will be described using the pnp transistor. The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged.
One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.
Forward-biased junction
of a pnp transistor
Reverse-biased junction
of a pnp transistor

8. Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated.
Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.
A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes.
The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.

9. Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material.
Applying KCL to the transistor :
IE = IC + IB
The comprises of two components – the majority and minority carriers
IC = ICmajority + ICOminority
ICO – IC current with emitter terminal open and is called leakage current.

10. npn BJTs – Operation Modes
Forward & reverse polarized
pn junctions
Different operation modes:

11. BJTs – Basic configurations

12. npn Common Emitter circuit
Emitter is grounded.
Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=b*IB.
Increasing IB, VBE slowly increases to 0.7V but IC rises exponentially.
As IC rises ,voltage drop across RC increases and VCE drops toward ground. (transistor in saturation, no more linear relation between IC and IB)

13. Common Emitter characteristics
Collector current controlled by the collector circuit. (Switch behavior)
In full saturation VCE=0.2V.
Collector current proportional to Base current
The avalanche multiplication of current through collector junction occurs: to be avoided
No current flows

14. BJTs – Current & Voltage Relationships
Operation mode: vBE is forward & vBC is reverse
Einstein relation
The Shockley equation
IES–saturation I ( A); VT=kT/q -thermal V (26meV)
D – diffusion coefficient [cm2/s]
m – carrier mobility [cm2/Vs]
The Kirchhoff’s laws
It is true regardless of the bias
conditions of the junction
Useful
parameter
the common-emitter current gain
for ideal BJT b is infinite

15. BJTs – Current & Voltage Relationships
Useful
parameter
the common-base current gain
for typical BJT a is ~0.99
The Shockley equation
once more
If we define the scale current
A little bit of math… search for iB
Finally…

16. BJT as Switch Vin(Low ) < 0.7 V BE junction not forward biased
Cutoff region
No current flows
Vout = VCE = Vcc
Vout = High
Vin(High)
BE junction forward biased (VBE=0.7V)
Saturation region
VCE small (~0.2 V for saturated BJT)
Vout = small
IB = (Vin-VB)/RB
Vout = Low

17. End of Chapter