9.4.1 Transistors

1. A transistor is a double p-n junction semiconductor with three terminals, 
   1. the emitter (e), 
   2. the base (b) 
   3. the collector (c).
2. Figure below shows the illustration of a transistor. It looks like a combination of 2 p-n junction diodes.
   
3. In a transistor, the emitter emits charge carriers (free electrons or holes).
4. The charge carriers move towards the base.
5. Under certain condition, large amount of the charge carriers will pass through the thin base layer and to be collected by the collector.
   

Types of the Transistors

1. There are 2 types of transistors:
   1. npn transistor
   2. pnp transistor
2. Figure 2 below shows the illustration of the npn and pnp transistor and Figure 3 below shows the symbol of both npn and pnp transistor.
3. For the symbol of the transistor, the arrow shows the direction of current. Take note that, for the emitter and base, the current always flow from the positive terminal to the negative terminal.

(Figure 2: Illustration of the npn and pnp transistor)

(Figure 3: Symbol of the npn and pnp transistor)

How a Transistor Work?

(Figure 4)

1. In the Figure 4 above, there are 2 circuits in the connection:
   1. the base circuit
   2. the collector circuit
2. The base circuit is forward bias whereas the collector circuit is reverse bias (This will be discuss in "The Connection of a Transistor").
3. Table below shows the response of bulb 1 (B₁) and bulb 2 (B₂) when switch 1 (S₁) and switch 2 (S₂) are closed.

S1	S2	B1	B2
Open	Open	Does not light up	Does not light up
Close	Open	Light up	Does not light up
Open	Close	Does not light up	Does not light up
Close	Close	Light up	Light up

Connection of Transistor

1. The terminals of a transistor must be connected to the terminals of a cell correctly to avoid damaging the transistor.
2. Transistor should be connected in such a way that
   1. the emitter-base circuit is forward bias
   2. the collector-base circuit is reverse bias.

Current in a Transistor

1. The current flows in the base, emitter and collector is called the base current (I_B), the emitter current(I_E) and the collector current(I_C) respectively.
2. Figure below shows the direction of the current in an npn transistor.
   
   
3. In general, IE is related to IB and IC through the formula

Another thing that you need to know about the 3 currents is