What is Transistor?

Transistors are semiconductor devices which means both conductive allowing current to pass through them or non-conductive which block current from flowing through it.  It can be used as an electronic switch to turn a current on and off.

There are different types of transistors.  A very common is the “Bipolar junction transistor” or “BJT”.  It looks like this:


It has three pins: Base (b), collector (c) and emitter (e).  The schematic symbol for a transistor act like a switch looks like this:


The circuit diagram below shows how it is connected and worked.


1st diagram : Connecting a transistor in non-conductive state

It is a simple circuit consisting of a approximately 3 volt power supply connected to an LED with the collector and the emitter of this transistor connected in series.  Everything is connected.  We expected the current will flow from the power supply through the LED back to the transistor and to the power supply.  In this case, the LED should be lit.  However, the LED does not lit because no current is going through the circuit.  Why is this?  It is because the transistor is in it’s non-conductive state and something should change to turn the transistor ON enabling it’s conductive state.


2nd diagram : Connecting a transistor in conductive state

We need to apply a voltage to the base of the transistor.  So, let’s move on to the second circuit diagram above.  It is exactly the same circuit except now there is a separate power supply which supplies positive voltage to the base of this transistor.  A voltage is now apply to the base of this transistor and it has turned this transistor into it’s conductive state.  Now, the current from the first power supply is now able to flow from the first power supply through the LED, and through the transistor and back to this power supply.


Turn the LED off by disconnecting the switch

We put a switch to the second power supply which supply voltage to the base of the transistor.  If we turn the second power supply off by disconnecting the switch, the current will stop going through the 1st circuit as well.  It is because the transistor is back to it’s non-conductive state because voltage is not being apply to the base of the transistor.

Why transistors are so common and useful nowadays?  Transistors can act like very fast switches that we can control to switch on and off at very fast speed.  We can switch them on and off at a rate of millions of times in a second.  This works commonly in binary data which act like zero and one state.  There are billions of transistors in your computer’s processor and it can understand those transistor switches as zeros and ones and encode them into usable data and instructions. Without transistor, we simply wouldn’t have the luxury of computers and other digital devices.




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