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What is a PNP Transistor?

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  • Written By: Jessica Reed
  • Edited By: Heather Bailey
  • Last Modified Date: 02 December 2016
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A PNP transistor is a common type of bipolar junction transistor made from a piece of semiconductor material with an excess of electrons sandwiched between two pieces of semiconductor material with an excess of holes — the areas where electrons are missing. It is used for switching or amplifying electrical signals. They are commonly used on circuit boards such as those found in computers. Bipolar junction transistors have come to replace the vacuum tube, which came before the transistor but is less efficient at its job.

The regions lacking electrons can be thought of as positive regions, known as p-type regions, and are represented by the two P's in "PNP." The area with extra electrons is known as the n-type region, located in the middle of the PNP transistor, and has extra electrons that want to find holes to fill. This area is represented by the N in "PNP."

Bipolar junction transistors, including the PNP transistor, have an emitter, a base, and a collector. The emitter is the section that gives off either electrons or holes. The base is responsible for regulating or controlling the flow of the charges — either electrons or holes — through the transistor. The collector collects the charges. Connecting the right voltage to specific spots on the transistor will control the flow of current within it.

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Two different currents are applied to the PNP transistor to make one half reversed-bias and the other half forward-bias. This means that the forward-bias half pushes the electrons forward toward the center and lets them pass easily while the reverse-bias half builds up resistance. It takes a lot of energy to move the electrons into this half of the PNP transistor. Both areas exist to better control current and help amplify signals.

Forward- and reverse-bias areas made from the three sections of the PNP transistor allow current to start flowing easily through one half and then meet resistance when it passes to the next half. When it finally gets enough energy to overcome this resistance and complete the circuit, the signal has been amplified. This is what allows a PNP transistor to amplify, or increase, a small electrical signal into a larger one.

Another common type of bipolar junction transistor is the NPN transistor. It is very similar to the PNP version. The main difference lies in the order of the semiconductor materials. Instead of using two positive materials with a negative material in the middle, it uses two negative materials with a positive material in the middle. Subtle differences do exist between the two, but for the most part either will work when building a circuit.

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