TRANSISTORS: Are basic components of electrical circuits.They are made from semiconductor materials and have three electrodes called the base,the collector and the emitter.
There are two transistor: NPN and PNP.

1. When no electrons are flowing through the base,then no electrons can pass from the collector to the emitter.The transistor is in cut off.
2. When many electrons are flowing through the base,the route between the collector and the emitter will be completely open.The transistor is in saturation.
3. When the flow of electrons through the base is between the cut off and saturation levels,it will be proportional to the flow of electrons between the collector and the emitter.The transistor is in the active region.

Transistor are the basic components of the microprocessors and memory circuits that are used in computers.The following pictures illustre the concept of transistor gain.











8.2.BASIC DEVICES MADE WITH ELECTRONIC COMPONENTS:
We can build various devices with the electronic components discussed in the previous section.Here are two examples:

TIMERS: A timer is a device that operates for a certain period of time and then shuts itself off automatically. We can use the circuit on the right as an example:

1. At first,the LED is off,since there is no current flowing through the base.The transistor is in cut off.When we push the botton,current flows through the base,the transistor is activated,and the LED turn on.The capacitor also begins to charge itself.
2. When we released the botton,the LED continues to produce light for some time,using current from the capacitor.When the capacitor runs down,the transistor cuts off and the LED turns off.

INTEGRATED CIRCUITS: Integrated circuits consist of miniature electronic components,such as transistor,resistors and capacitors.For example,the following UL-N2803A integrated circuit contains eight pairs of Darlington transistors.