TRANSISTORS

   

    Diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junctions, and three terminal devices forming the basis of a Bipolar Junction
Transistor, or BJT for short.

Transistors are three terminal active devices made from different semiconductor materials
that can act as either an insulator or a conductor by the application of a small signal voltage.
The transistor's ability to change between these two states enables it to have two basic
functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then
bipolar transistors have the ability to operate within three different regions:

1.Active Region - the transistor operates as an amplifier and Ic = β.Ib
2.Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
3.Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0

Transistors (MOSFET , BJT or JFET) has two major applications

1. use it as a switch
2. use it as an amplifier (to amplify current or voltage)

we will now talk about using it as a switch.

why do we use transistor as a switch ? why not any other device ?

So here’s the answer

Its because of the property of base terminal to control the current from collector to emitter. (in case of BJT)

The input voltage is always applied to “base” of the transistor and the output voltage is taken out from the “collector” terminal to the ground (in case of a common emitter BJT)

Now,

There are two conditions,

• If I don’t apply any voltage to the base terminal, the base current (Ib) will be zero and the transistor will remain in cutoff region since collector current (Ic) will be zero and the output voltage (Vout) will be equal to Vcc(+5 volts) and transistor will act as open switch. i.e (logic 1)

• If I apply input voltage (Vin) to the base terminal, the base current (Ib) will flow. But if I apply “specifically very high input voltage” (very high Vin) , to the base terminal, the base current will increase and thus the collector current current will increase. This increase in collector current will be very high (Ic will be very high) thus the transistor will go in saturation region.

since collector current (Ic) is very high and output voltage (Vout) will be zero

transistor now will act as closed switch.

So basically, the conceptual meaning is that the base acts as a controller and it allows the transistor to be open or close.

Base controls the flow of current from collector to emitter terminal.

That is a property of a “switch” hence transistor proves that it can be used as a switch.

what I wrote here is for your conceptual understanding of why transistor is used as a switch. I didn’t explained it in detail as you can find many websites explaining the working of transistor.