Transistors

Transistors:

A junction transistor id formed by sandwiching either 'p' type semiconductor in between two 'n' type semiconductors or 'n' type in between two 'p' semiconductor. The 1st type of transistor is called n-p-n and 2nd type is called p-n-p transistor. In the actual design the middle layer is very thin generally 1 μm. The middle layer is called base it is lightly doped with impurities and heavily doped layer is called emitter and moderately doped layer is called collector. Thus a transistor is a 3 terminal device.

 

p-n-p

 

n-p-n

 

 
 

These are the circuit symbols of a transistor. The emitter base junction is always forward biased and the collector base junction is always reversed biased. The arrow on the symbols shows the direction of current emitter base junction. An n-p-n transistor's emitter contains the largest number of electrons base contain largest number of holes, and collector contains electrons more than base but less than emitter.

Biasing of a transistor: Suitable potentials should be applied across the two junctions to operate the transistor. This is called biasing. A transistor can be operated in 3 different modes. Common emitter (CE), Common base (CB) mode and Common collector (CC) mode. In CE, CB, CC mode, the emitter, the base and the collector are kept at zero potential respectively. 

The emitter current is I_E, the base current is I_B and the collector current is I_C.Using Kirchhoff's law we can write I_E = I_B + I_C. I_B may be 1% to 5% of I_E.
 There are two parameters of a transistor called alpha and beta, defined as:
⍺ = I_C/I_E  and   β = I_C/I_B.
 The relation between  ⍺ and β is β = ⍺/(1-⍺)
Thats all for today, if you have any question email me at shirshendubhowmick@theinec.com.