NPN and PNP Transistors

NPN and PNP Transistors Definition 

Semiconductors are basically used in manufacturing two devices that are diodes and transistors. With the help of diodes, amplification of the signal is not possible and that is the reason we use transistors. In a transistor, one semiconductor is sandwiched from both sides by another semiconductor. The transistor has three regions that are emitter, base, collector. The emitter emits the charges and these charges are collected by the collector. Thee are two types of transistors NPN and PNP transistor. In PNP transistor they are two differences between both P, that is doping and the size of both p is different.


PNP Transistor Definition 

A transistor in which there is one n-type semiconductor that is doped by two p-type semiconductors from both sides is called PNP transistor. The PNP transistor turns on when there is no current at the base of the transistor.

In PNP transistors instead of electrons, the emitter emits holes and they are collected by the collector. Here the collector and the emitter old of p-type and the base are of n-type. The base size is small and it is lightly doped, the emitter size is moderate (between the base size and the collector size) and the doping is strong, collector size is large as compared to base and emitter and the doping is moderate (between the base and emitter doping). 

Symbol of the transistor is  

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Here, which has an arrow is known as emitter, the base is always in the center. 

The arrow shows the direction of the electric current and the electric current always flows from positive to negative. To identify whether it is NPN or PNP we use this arrow.

If the arrow is pointing outside then it is NPN transistor and if it is pointing inwards then it is PNP transistor.


NPN Transistor Definition

A transistor, in which there is one p-type semiconductor that is doped by two n-type semiconductors from both sides are called NPN transistors. The NPN transistor turns on when the current flow through the base of the transistor. The current flows in the direction from the collector to the emitter. 

Usually, NPN transistors are used because it is easy to design them. In NPN transistors the majority charge carriers are electrons and in PNP transistors the majority charge carriers are holes. 

The mobility of electrons is better than the mobility of holes, therefore, NPN transistor works faster as compared to PNP transistors. So, we usually prefer NPN transistors for better results.


Transistor Action  

Each NPN junction has two terminals we connect a cell between them do there is a potential difference across it.

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There is a rule for the emitter and the base that is the battery connected should always be in forward biased. And in the base and collector circuit, it is always reverse biased. 

In the above circuit diagram, the base terminal is common for the emitter and the collector so, therefore, it is called a common base configuration. Similarly, we can have a common emitter configuration and a common collector configuration.

But mostly we use a common emitter configuration because the most popular use of transistor is to make the amplifier and the best amplification is done by a common emitter configuration. Below is the circuit diagram of common emitter configuration.

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After the circuit is connected the depletion layer of the emitter-base is decreased whereas the depletion region of the collector- base is increased.

The current flow is more in the emitter-base side current decreases in the collector- base side.

Resistance is decreased in the emitter-base side and becomes conductor, whereas resistance is increased in the collector-base side due to this the current majority stops in the collector-base side but the reverse current continues to flow.

The resistance is transformed from one side to another side; therefore, it is called a transistor.

More than 95℅ emitted electrons enter into the collector and only 5℅ recombine with the holes in the base, therefore, the collector current IC is much larger than the base current IB.

The transistor is also called a bipolar transistor because both the polarity carriers are present that are electrons and holes.


Solved Problems

1. In a collector-emitter connection, the current amplification factor is 0.9. If the emitter current is 1mA determine the value of the base current.

Solution- α = 0.9, IE = 1mA 

To find IB 

We know that,

α=IC / IE

IC= IE× α

IC= 0.9×1 = 0.9mA

Now, IE= IB +IC

IB= IE -IC

= 1-0.9

IB= 0.1mA

The base current is 0.1 mA.

2. In a collector base connection IE = 1mA, IC = 0.95mA. Calculate the value of IB.

Solution- We know that,

IE= IB + IC

IB= IE – IC

IB= 1 – 0.95

IB= 0.05 mA.

FAQ (Frequently Asked Questions)

1.Derive the Formula for Current Ratios of Collector Current to the Emitter Current and the Collector Current to the Base Current.

In a transistor, the current ratios are defined as follows,

α (alpha)- It is defined as the ratio of collector current to the emitter current.

α=IC / IE 

β(Beta) – also known as a beta factor. It is the ratio of collector current to the base current.

β =IC / I

we also have IE = IB + IC

Dividing this by IC 

IE/IC = IB/IC + 1

1/ α = 1/ β + 1

α = β/ 1+ β

2.Explain the Working of the Transistor as an Amplifier.

Transistor as an amplifier:

An amplifier is a device that increases the voltage or current of the signal. It magnifies the voltage or current signal.  With the help of a transistor amplifier is made. 

The transistor is connected in a common emitter configuration using an NPN transistor. To make the transistor working as an amplifier emitter-base junction should be forward biased and collector-base junction should be reverse biased. The transistor is a current sensitive device; therefore, it has large current gain when it is operated in an active region.