PNP Transistor

What Is a Transistor?

A transistor is a three-terminal semiconductor device, which has three terminals, namely, emitter (E), Base (B) and Collector (C). Semiconductors are the material that is key for the manufacturing of diodes and transistors. Transistors are the devices that have revolutionized the electronics field and are discovered by John Bardeen, Walter Brattain, and William Shockley.

Transistors are of two types:

• Bipolar Junction Transistors (BJT)
• Field Effect Transistors (FET)

The field-effect transistors are a type of transistor that uses an electric field to control the current. Field-effect transistors are known as unipolar transistors. This device has three terminals: source, gate, and drain.

Bipolar Junction Transistors: These transistors are commonly known as junction transistors. These were the first type of transistors produced in 1947 by Bell Labs. Bipolar Junction Transistors are of two types:

• NPN transistors
• PNP transistors

NPN transistors consist of p-type semiconductors sandwiched between two n-type semiconductors. This article lets us learn and understand in detail about PNP transistors.

PNP Transistors

PNP transistors representation is shown in the figure below.

This bipolar PNP junction transistor is formed with three layers of semiconductor material, with two P-type regions and one N-type region. It includes three terminals:

• Emitter
• Collector
• Base

Emitter – emitter part in a transistor lets it supply majority charge carriers. The emitter is always forward biased with respect to the base. Hence the majority of charge carriers are supplied to the base. The emitter of a transistor is heavily doped and moderate in size.

Collector – the majority of the charge carrier supplied by the emitter is collected by the collector. The collector-base junction is always reverse biased. The collector area is moderately doped and has the capacity to collect the charge carrier supplied by the emitter.

Base – The centre section of the transistor is known as the base. The base forms two circuits, the input circuit with the emitter and the output circuit with the collector. The emitter-base is forward biased and offers low resistance to the circuit. The collector-base junction is in reverse bias and offers higher resistance to the circuit. The base of a transistor is lightly doped and very thin, due to which it offers the majority charge carrier to the base.

Depletion region –The depletion regions are formed at the emitter-base junction and the base-collector junction.

Construction of PNP Transistor

P-type semiconductors, which represent the emitter and collector, are doped heavily than N-type semiconductors, which represent the base. Hence, the depletion region at both junctions penetrates towards the N-type layer.

In PNP transistors, in this type of transistor, majority charge carriers are holes, and minority charge carriers are electrons. The emitter emits holes and is collected at the collector.

In a PNP transistor, the base current which enters into the collector is amplified. The flow of current is typically controlled by the base. Current flows in the opposite direction in the base. In a PNP transistor, the emitter emits “holes”, and these holes are collected by the collector.

The base region features a large number of free electrons. But, the width of the middle layer is very small and is lightly doped. So significantly less free electrons are present in the base region.

Working of PNP Transistor

Emitter current is created when the emitter-base junction is forward biased, the emitter pushes the holes towards the base region. When electrons move into the N-type semiconductor or base, they combine with the holes. The base is lightly doped and is comparatively thin. Hence only a few holes are combined with the electrons and the remaining are moved towards the collector space charge layer. This phenomenon generates the base current. The current is carried by holes in p-n-p transistors.

Read more about Murphy’s law.

Stay tuned with BYJU’S for more such interesting articles. Also, register to “BYJU’S – The Learning App” for loads of interactive, engaging Physics-related videos and unlimited academic assist.