For proper working of a transistor, the voltage at the base region must be more positive than that of the emitter region. The voltage at the collector region, in turn, must be more positive than that of the base region(to put it vaguely collector>base>emitter).
When voltage is applied to transistor, the emitter supplies electrons, which is pulled by the base from the emitter as it is more positive than the emitter.
This movement of electrons from emitter to collector creates a flow of electricity through the transistor. The current passes from the emitter to the collector through the base.
Thus, adjustment of voltage in the base region modifies the flow of the current in the transistor by changing the number of electrons in the base region.
In this way, small changes in the base voltage can cause large changes in the current flowing out of the collector.
Let us compare transistor with valve to understand working of transistor.
All transistors have three "elements":
- Emitter - analogous to the cathode of a valve. The emitter only emits electrons in a NPN device.
- Base - the controlling terminal. A current at the base controls the current through the transistor.
- Collector - basically, collects the emitted electrons. Somewhat analogous to the plate of a valve.
Using working of pump, functioning of transistor can be explained easily.
Let us consider working of pump on water current, which is similar to working of transistor on current.
There are three openings in the pump, which have been labeled as follows
1. Opening "B", similar to Base of transistor.
2. Opening "C", similar to collector of transistor.
3. Opening "E", similar to emitter of transistor.
Suppose, the main tank passes water into the opening "C"(same as source supplying voltage to collector) but the water cant flow ahead as there is a big plunger on the way, which is blocking the outlet to "E". Now, at this stage, if more water is passed out of main tank to opening "C", it will burst our pump (just the same way if we increase the voltage to a real transistor).
Now, if we pour water into opening "B" this water flows along the "Base" pipe and pushes plunger upwards, allowing some water to flow from opening "C" to opening "E". At this point of time, some water from "B" also gets added to the water running from "C to E". If more water is poured into "B", the plunger moves up further due to which a great amount of water current flows from "C" to "E".
Thus, by controlling amount of water flowing into "B", flow of water from "C" to "E" can be controlled.
Therefore, we can control a BIG flow of current with a SMALL flow of current.
If we continually change the small amount of water flowing into "B" then we cause corresponding changes in the LARGE amount of water flowing from "C" to "E". So 1mA flowing into "B" would allow 100mA to flow from "C" to "E".
The amount of current that flows from "C" to "E" is limited by the "pipe diameter". So, no matter to what extend plunger is pushed inside opening "C". Water flowing from "C to E" can`t exceed beyond a certain level.
The transistor can be used to switch the current flow on and off. If we put sufficient current into "B" the transistor will allow the maximum amount of current to flow from "C" to "E". The transistor is switched fully "on".
If the current into "B" is reduced to the point where it can no longer lift the black plunger thing, the transistor will be "off". Only the small "leakage" current from "B" will be flowing. To turn it fully off, we must stop all current flowing into "B".
In a real transistor, any restriction to the current flow causes heat to be produced. To get rid of this heat, the transistor might be clamped to a metal plate, which draws the heat away and radiates it to the air. Such a plate is called a "heat sink"(explained above in section 14-7).