Physics Form 5: Chapter 4 - Transistor as an Amplifier

An amplifier circuit

I_B = base current

I_C = collector current

I_E = emitter current

V_BB = base voltage 
V_CC =  collector voltage

The function of resistant R_B is to control and limit the base current.

How transistors function as an amplifier:

• The forward-bias from base to emitter narrows the base-emitter,BE depletion layer.
• The reverse-bias from base to collector widens the base-collector, BC depletion layer.
• The free electrons from emitter diffuse easily through the forward-biased BE junction into the p-type region.
• The base region is lightly doped and very thin, so it has a limited number of holes. Therefore, only a small percentage of the all electrons flowing through BE junction can combine with the available holes in the base region.
• Most of the electrons do not combine with holes but diffuse into BC depletion layer.
• Once here, these electrons are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative ions.
• Electrons now move through the collector region and into the positive terminal of the collector voltage source.
• This forms collector current which is larger than the base current.

Click on the diagram below to play!

 Conclusion:

When there is a small change in the base current, there will be a big change in the collector current.