8)ELECTRONS

TRANSISTOR
Transistor are basic components of electric circuits.They are made from semiconductor materials and have three electrons called the base,the collector and the emitter.
There are two types of transistor:NPN and PNP

Each electrode of a transistor is connected to a semiconductor material which either releases electrons(N) or accepts electrons(P).
To understand how transistors work, we can compare them to the water pipe system in the picture below. A small electric current in the base(B) controls the flow of water(electrons) between the collector(C) and the emitter(E).

• When no electrons are flowing through the base,then no electrons can pass from the collector to the emiter.The transistor is in cut off.
• 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.
• 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 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 picture illustrate the concept of transistor gain.

In the fist circuit,only a small current is flowing,so the bell does not ring.In the second circuit,the transistor multiplies the flow of electrons though the base by one hundred,As a result,the current that passes through the collector is one hundred times stronger.
BASIC DEVICES MADE WITH ELECTRONIC COMPONENTS
We can build various devices with the electronic components discussed in the previous section.Here are two types:
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.
-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 button, current flows through the base, the transistor is activated,and LED turns on.The capacitor also begins to charge itselfs.
-When we release the button, the LED continues to produce light for some time,using current from the capacitor.When the capacitos runs down, the transistor cuts off and LED turns off.
INTEGRATED CIRCUITS
Integrated circuits consist of miniature electronic components, such as transistors, resistors and capacitors.