Construction of Transistor

Transistor Construction:

A transistor is constructed by placing a oppositely doped semiconductor material between two similarly doped semiconductors. Or placing n-type material between two p-type material which forms the pnp-transistor or by placing a p-type material between two n-type semiconductor which forms npn-transistor.

The above diagram shows the schematic construction of a PNP transistor. As you can see an N-type silicon (green layer) is sandwiched between two P type materials (red layer). The left part is indicated by P+ which means its highly doped P-type material. This highly doped portion is called Emitter, that is the piece of semiconductor that supplies majority carriers for the transistor to function. At the extreme right is moderately doped P type material which is called as the Collector. This portion collects the majority charge carriers that is been emitted by Emitter and that manage to cross the collector. The middle region is denoted by n- because it's doped with N-type impurities. The minus'-' sign indicates it's doped very very less compared to the emitter and collector. The middle region is called the Base, and it's this region that serves as a gate, regulating flow of charge from Emitter to collector.
The doping of base is just one tenth of that of collector. In a real transistor, the width of base is very thin. The total width of the transistor will be 150 times that of the width of the base.
In a similar way by sandwiching a lightly doped P region between highly and moderately doped N region we get a NPN transistor as shown below.

 Transistor Operation

To see how transistor perates we will look at how an PNP transistor works. NPN transistor works the same way as PNP, but with voltage and currents reversed.
Below is the diagram of PNP transistor. One can see that its emitter is held more positive than base and base more positive than collector. When emitter-base is forward biased and collector based is reversed bias, the transistor is said to be in active region. It's in the active region that transistor acts as a amplifer and so on. So let's study about it.
See the image below for active biasing of PNP transistor.
Now let's sat we remove the voltage between base and collector hence the circuit looks as follows.
Let's analyze it.

Common Base Configuration:

It has following properties:

• low input impedance
• high output impedance
• high voltage gain
• unity (or less) current gain

Common Emitter Configuration:

As well as being used as a switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor, NPN Transistors can also be used to produce a circuit which will also amplify any small AC signal applied to its Base terminal.

If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a Common Emitter Amplifier is produced. One such Common Emitter Amplifier configuration is called a Class A Amplifier. A Class A Amplifier operation is one where the transistors Base terminal is biased in such a way that the transistor is always operating halfway between its cut-off and saturation points, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of the AC input signal superimposed upon the DC Biasing voltage. Without this "Bias Voltage" only the positive half of the input waveform would be amplified. This type of amplifier has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.
With reference to the common emitter configuration shown below, a family of curves known commonly as the Output Characteristics Curves, relates the output collector current, (Ic) to the collector voltage, (Vce) when different values of base current, (Ib) are applied to the transistor for transistors with the same β value. A DC "Load Line" can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value of Vce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point


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