# PNP Transistor: Definition & Equations

Instructor: Babita Kuruvilla

Babita has an electrical engineering degree and has taught engineering students and college students preparing for medical and dental college admissions tests.

In this lesson, we learn about pnp transistors - how do they function and why are they so useful in modern electronics. We will also explore their unique characteristics by deriving mathematical equations.

## At the Concert!

You are at the concert dancing away to the music pouring through the loudspeakers. The singer's voice and the instrumental music is carried far enough that the songs can be heard at least half a mile from the venue.

Ever wondered how the modern loudspeakers work? They have circuits with components called transistors of the type pnp or npn that can amplify small signals. Let's explore how a pnp transistor works. Here, the cathodes of the diodes point towards each other.

## What's a pnp Transistor?

A pnp transistor is a semiconductor piece in which an n-region is sandwiched between two p-regions, giving us a device with three distinct regions and two pn junctions:

• Emitter-Base Junction - Where the very positively doped emitter meets with the negatively doped base
• Collector-Base Junction - Where the positively doped collector meets with the more negatively doped base

A pnp transistor can be represented using two diodes pointing towards each other. A great mnemonic to remember how the diodes are facing in the pnp transistor is 'Pointed iNto Pointed.'

## npn vs. pnp Transistors

A pnp transistor is a dual of an npn transistor, and so they are complementary to each other. Their bias voltages and currents are in opposite directions. While current flow is because of the movement of holes in pnp transistors, it's due to the movement of electrons in npn transistors.

The pnp transistor symbol shows an arrow from the emitter to the base. What does it mean? The arrow indicates the direction of the conventional current flow from the emitter into the base.

A pnp transistor is ON when the value of the voltage source connected to the base is low and turned OFF when it is high.

## How Does a pnp Transistor Work?

To understand how a pnp transistor works, let's follow its currents in active-forward operation mode.

Note: As we go through the description, it is important to keep in mind that current is defined as the flow of positive charge. Although we now know that it is the movement of electrons that results in current, historically it was thought that current was due to the movement of positive charges.

### Voltages

Using an external voltage source, the base-collector junction is reverse-biased, meaning that the base is held at a higher potential than the collector - (VBC); ie., the voltage at B is greater when biased against C. Because it is reverse-biased, there is no diffusion, and no current flows between the two terminals.

(Note: An easy way to determine whether a diode is forward-biased is to check if the positive terminal of the voltage source is connected to the p-region of the diode. If it is connected to the n-region, then the junction is reverse-biased.)

On the other hand, the base-emitter junction is forward-biased so that the voltage VEB applied at the emitter is higher than at the base. Typically, the VBE of a silicon npn transistor is 0.7V and a germanium transistor 0.3V.

### Currents

Holes from the positive terminal of the voltage source VEB push holes in the emitter (p-region) down their diffusion gradient into the base. Because the emitter is highly doped, it has a lot of electrons that diffuse into the base region.

At the same time, electrons flow from the negative terminal of the voltage source VEB into the base (n-region). These electrons push those electrons near the emitter-base junction into the emitter. The injections of both holes and electrons in the base-emitter junction account for the emitter current (IE).

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