How transistors work

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit.A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals.
Bipolar transistor has three terminals 
1)Base-control signal
2)Emitter
3)Collector
A small current in the base of the transistor can control large current in the Collector and Emitter terminals.
Generally Bipolar junction Transistor can be classified in to two types
1)NPN Transistor, 2) PNP transistor
NPN transistor means Emitter-N type material, Base- P type material, Collector-N type material,
similarly for PNP transistor-Emitter-P type material, Base- N type material, Collector-P-type material,



Fig: showing NPN and PNP transistor
Note: if NPN or PNP is not mentioned, arrow mark towards base indicates PNP transistor.Arrow mark away from the base indicates NPN transistor.
For transistor to work-
1) Emitter- Base junction should be forward biased-Junction has low resistance,
2)Collector-Base junction should be reverse biased-Junction has high resistance.

In transistor Emitter is heavily doped, because it has to supply majority carries.and Collector and Emitter terminals should not be interchanged.and size of Collector is large as compared to base because it has to accept majority carriers.
Transistor can be operated for two purpose
1)Transistor as a Switch
2)Transistor as a amplifier

Transistor as a switch

Transistors are commonly used in digital circuits as electronic switches which can be either in an "on" or "off" state.
Because a transistor’s collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the collector.


fig showing transistor as switch.
so with help of base current we can control the large collector current.

Practical application of transistor as switch

Light/ dark activated switch



   


Here photo resistor or  LDR(Light Dependent Resistor) is used as sensor-property of LDR is resistance will decrease when light is applied(few hundred ohms) and resistance will increase in darkness(in mega ohms).
LDR is connected in series with 100 Kilo ohm resistor it forms simple voltage divider connection.
so in darkness voltage drop across Base-Emitter junction increases. so transistor Base-Emitter voltage is above cut off voltage and now transistor starts conducting and current flows through the LED.

similarly when light falls on LDR. Its resistance decreases and voltage across Base-Emitter junction of transistor decreases now transistor will not conduct and current through the LED is zero. and LED will not glow.so by this simple circuit we have made an automatic light which will glow only in darkness and automatically turns off in light. 

Transistor as an amplifier

fig:circuit of transistor as an amplifier
The common-emitter amplifier is designed so that a small change in voltage (V_in) changes the small current through the base of the transistor; the transistor's current amplification combined with the properties of the circuit mean that small swings in V_in produce large changes in V_out.
the transistor can be used as a current and voltage amplification.
current gain  h_fe = Ic / Ib
A fairly low gain transistor might have a current gain of 20 to 50, while a high gain type may have a gain of 300 to 800 or more
Voltage gain  = change in collector voltage / change in base voltage.
so transistor as amplifier is explained,

Transistor Characteristics:

 Transfer Characteristics
 Input Characteristic.
 Output Characteristic.

Transfer characteristics:


X axis: Base current(mA)
y axis:  collector current(MA)

The graph of I_CE / I_BE shown (right) is called the TRANSFER CHARACTERISTIC and the slope of the graph shows the h_fe for that transistor.

Input Characteristics:

X axis: Base-Emitter voltage(V)
y axis:  Base current(MA)



The INPUT CHARACTERISTIC (right), a graph of base emitter current I_BE against base emitter voltage. Initially the transistor is not conducting because the voltage is below cut in voltage or knee voltage. when the voltage is above knee voltage base current flows.

 Output Characteristic.

X axis: Collector-Emitter voltage(V)
y axis:  collector current(MA)

 shows the OUTPUT CHARACTERISTIC whose slope gives the value of output conductance.for small values of base current the corresponding collector current is low and if base current is increased, collector current is also increased.

These are the various characteristics of transistor.

Conclusion

Most of the analog and digital electronic  circuits and logic gates  are build using transistors.so knowing the working of transistor is very important