Difference between NPN and PNP transistors

Transistors are electronic devices that are used to amplify and switch electronic signals. There are two types of transistors: NPN and PNP. These types differ in the arrangement of the semiconductor materials within the device. In this article, we will explore the differences between NPN and PNP transistors and how they work.

NPN Transistor:

An NPN transistor is a three-layer device consisting of two n-type semiconductor regions (collector and emitter) separated by a thin p-type semiconductor region (base). The NPN transistor is the most commonly used type of bipolar transistor. 

The collector and emitter of the NPN transistor are heavily doped n-type semiconductors, while the base is lightly doped p-type semiconductor. The collector-base junction is reverse-biased, while the base-emitter junction is forward-biased.

When a positive voltage is applied to the base with respect to the emitter, a small current flows into the base. This current causes a large current to flow from the collector to the emitter. The NPN transistor is said to be in the active region when the collector-base junction is reverse-biased, and the base-emitter junction is forward-biased. 

PNP Transistor:

A PNP transistor is also a three-layer device, but it consists of two p-type semiconductor regions (collector and emitter) separated by a thin n-type semiconductor region (base). 

The collector and emitter of the PNP transistor are heavily doped p-type semiconductors, while the base is lightly doped n-type semiconductor. The collector-base junction is forward-biased, while the base-emitter junction is reverse-biased.

When a negative voltage is applied to the base with respect to the emitter, a small current flows into the base. This current causes a large current to flow from the emitter to the collector. The PNP transistor is said to be in the active region when the collector-base junction is forward-biased, and the base-emitter junction is reverse-biased.

Differences between NPN and PNP Transistors:

• The main difference between NPN and PNP transistors is the direction of current flow. In an NPN transistor, current flows from the collector to the emitter, while in a PNP transistor, current flows from the emitter to the collector.

• Another difference is the direction of biasing. In an NPN transistor, the base-emitter junction is forward-biased, while the collector-base junction is reverse-biased. In a PNP transistor, the base-emitter junction is reverse-biased, while the collector-base junction is forward-biased.

• The conductivity of the majority carriers in each region of the transistor is different. In an NPN transistor, the collector and emitter are heavily doped with n-type material, while the base is lightly doped with p-type material. In a PNP transistor, the collector and emitter are heavily doped with p-type material, while the base is lightly doped with n-type material.

• The voltage required to bias an NPN transistor is positive, while the voltage required to bias a PNP transistor is negative.

• The current gain of an NPN transistor is positive, while the current gain of a PNP transistor is negative.