NPN vs PNP Transistor
Transistors are 3 terminal semiconductor devices used in electronics. Based on the internal operation and structure transistors are divided into two categories, Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET). BJT’s were the first to be developed in1947 by John Bardeen and Walter Brattain at Bell Telephone Laboratories. PNP and NPN are just two types of bipolar junction transistors (BJT).
The structure of BJTs is such that a thin layer of P-type or N-type semiconductor material is sandwiched in between two layers of an opposite type semiconductor. The sandwiched layer and the two outer layers create two semiconductor junctions, hence the name Bipolar junction Transistor. A BJT with p-type semiconductor material in the middle and n-type material at the sides is known as an NPN type transistor. Likewise, a BJT with n-type material in the middle and p-type material at the sides is known as PNP transistor.
The middle layer is called the base (B), while one of the outer layers is called the collector (C), and the other emitter (E). The junctions are referred to as base – emitter (B-E) junction and base-collector (B-C) junction. The base is lightly doped, while the emitter is highly doped. The collector has a relatively lower doping concentration than the emitter.
In operation, generally BE junction is forward biased and BC junction is reverse biased with a much higher voltage. The charge flow is due to diffusion of carriers across these two junctions.
More about PNP Transistors
A PNP transistor is constructed with an n-type semiconductor material with a relatively low doping concentration of donor impurity. The emitter is doped at a higher concentration of acceptor impurity, and the collector is given a lower doping level than the emitter.
In operation, BE junction is forward biased by applying a lower potential to the base, and BC junction is reverse biased using much lower voltage to the collector. In this configuration, the PNP transistor can operate as a switch or an amplifier.
The majority charge carrier of the PNP transistor, the holes, has a relatively low mobility. This results in a lower rate of frequency response and limitations in current flow.
More about NPN Transistors
The NPN type transistor is constructed on a p -type semiconductor material with a relatively low doping level. The emitter is doped with a donor impurity at a much higher doping level, and the collector is doped with a lower level than the emitter.
The biasing configuration of the NPN transistor is the opposite of the PNP transistor. The voltages are reversed.
The majority charge carrier of NPN type is the electrons, which has a higher mobility than the holes. Therefore, the response time of an NPN type transistor is relatively faster than the PNP type. Hence, NPN type transistors are the most commonly used in high frequency related devices and its ease of manufacture than the PNP makes it mostly used of the two types.
What is the difference between NPN and PNP Transistor?
- PNP transistors have p-type collector and emitter with an n-type base, while NPN transistors have n-type collector and emitter with a p-type base.
- Majority charge carriers of PNP are holes while, in NPN, it is the electrons.
- When biasing, opposite potentials relative to the other type are used.
- NPN has a faster frequency response time and a greater current flow through the component, while PNP has low frequency response with limited current flow.