How Does a PNP Transistor Work?

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How Does a PNP Transistor Work?
(Georgia State University Physics Department)

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What is a pnp bipolar transistor?

A PNP bipolar transistor is a semiconductor device with three terminals, or sections, referred to as the collector, base and emitter, respectively. A transistor can be viewed as a valve, in that the current, or voltage, in one lead controls the current established between itself and the other two leads. In a bipolar transistor, the current in the base controls the current established between all three leads. Transistors typically function as switches or amplifiers.

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SatCure http://www.satcure-focus.com/

PN Junctions

A PNP transistor is formed from three layers of doped semiconductor regions. P stands for positive, and n for negative.

In a semiconductor that has been doped to form a p-type, the holes are the majority carriers, and the electrons are minority carriers. An applied voltage causes the electrons to move to the left and the holes to the right. These holes recombine with free electrons from the external circuit. The electrons are so few in number that their effect is negligible, and we say that there is a flow of holes, so that p-types have excess positive charge.

In a semiconductor that has been doped to form an n-type, there are more free electrons than there are holes. The free electrons are therefore the majority carriers, and the holes are the minority carriers. Again, when a voltage is applied, the free electrons move to the left, and the holes to the right. These holes are filled in by electrons from an external circuit. We say that there is a flow of free electrons, so that n-types have an excess negative charge.

When you combine the two types of semiconductors, you form what is called a pn junction.

Forward Biased Diode
Forward Biased Diode

The Diode or Ebers-Moll Model (Conventional Current)

Diodes are composed of pn junctions. In a diode that is forward biased, the positive terminal of the battery is connected to the p side, and the negative terminal of the battery is connected to the n side. This means the charge from the battery repels the holes and electrons so that the current flows in the same direction as the applied voltage, once a threshold is reached.

In a diode that is reverse biased, the negative terminal of the battery is connected to the p side, and the positive terminal of the battery is connected to the n side. This means the charge from the battery repels the holes and electrons in a direction opposite from the applied voltage, so that no current can flow. If the voltage is so high as to cause the current to flow in the wrong direction, the diode will break.

You can therefore model a PNP bipolar transistor as a combination of two diodes. The base-collector part is conducting or forward biased, with the current flowing in the same direction as the applied voltage. However the base-emitter portion is reverse biased--the current must flow from the emitter to the base, not vice-versa. Finally, in a PNP transistor circuit, the emitter must be more positive in voltage than the collector.

Ohio State University Physics Department
Ohio State University Physics Department

References

Getting Started in Electronics by Forrest Mims III Malvino Electronic Principles by Albert Malvino The Art of Electronics by Paul Horowitz and Winfield Hill

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