NPN vs PNP

Bipolar Junction Transistors, or more simply BJTs, are 3-terminal electronic semiconductor devices. They are basically made of doped materials, and are often used in switching or amplifying applications.

In essence, there is a pair of PN junction diodes in every bipolar transistor. The pair is joined, which forms a sandwich that places a kind of semiconductor in between the same two types. Therefore, there can only be two types of Bipolar Sandwich, and these are the PNP and NPN.

BJTs are current regulators. Principally, the amount of the passing main current is regulated by allowing or restricting it, which is handled by, and in accordance to, a smaller current from the base. The smaller current is called the ‘controlling current’, which is the ‘base’. The controlled current (main) is either from the ‘collector’ to the ’emitter’, or vice-versa. It practically depends on the type of BJT, which is either PNP or NPN.

Nowadays, NPN bipolar transistors are the most commonly used of the two types. The main reason for this, is the NPN’s characteristic higher electron mobility compared to the hole mobility in semiconductors. Therefore, it allows greater amounts of current, and operates faster. Additionally, NPN is easier to build from silicon.

With the NPN transistor, if the emitter has a lower voltage than the one in the base, the current will flow from the collector to the emitter. There is a small amount of current that will also flow from the base to the emitter. The current flow through the transistor (from collector to emitter) is controlled by the voltage at the base.

The ‘base’, or the middle layer of the NPN transistor, is a P semiconductor, which is lightly doped. It is sandwiched between two N layers, in which the N type collector in the transistor is heavily doped. With the PNP, the transistor is ‘on’ when the base is pulled low, relative to the emitter, or in other terms, the small current leaving the base in common-emitter mode is amplified in the collector output.

Summary:

1. NPN has higher electron mobility than PNP. Therefore, NPN bipolar transistors are often more favored than PNP transistors.

2. NPN are easier to create from silicon than PNP.

3. The main difference of NPN and PNP is the base. One is just the opposite of the other.

4. With the NPN, a P-dope semiconductor is the base, while with the PNP, the ‘base’ is a N-dope semiconductor.

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