Difference Between NPN and PNP Transistor

Main Difference – NPN vs. PNP

NPN and PNP are two types of transistors. Transistors are semiconductor devices, made of doped p-type and n-type junctions. The main difference between NPN and PNP transistor is that, in NPN transistors, a p-type semiconductor is sandwiched between two n-type semiconductors whereas, in PNP transistors, an n-type semiconductor is sandwiched between two p-type semiconductors.

What is an NPN Transistor

The structure of an NPN transistor is shown below:

Difference Between BJT and FET - an_npn_BJT

An NPN BJT transistor

Three terminals are connected to each of the doped semiconductors. The terminal in the middle in the above diagram is called the base (B). The terminal on the left forms the emitter (E) and the terminal on the right forms the collector (C).

In an NPN transistor, the base is connected to a negative potential while the collector is connected to a positive potential, such that the base-collector junction is connected under a large reverse bias. The emitter and collector are connected in forward bias. This causes electrons to flow from the emitter towards the base. This is a large flux, and the electrons cross all the way across the base and into the collector.

The circuit symbol for an NPN transistor is shown below:

Difference Between NPN and PNP Transistor - An_NPN_Transistor_Symbol

A circuit symbol of an NPN transistor

What is a PNP Transistor

PNP transistors work under the same principles as does an NPN transistor, so the properties of a PNP transistor are mostly the reversed versions for an NPN transistor.

For instance, the base is connected to a positive terminal with respect to the collector, so that the base-collector junction is reverse biased. As a result, holes cannot flow from the collector into the base. The base-emitter junction is connected in forward bias; so that, holes can enter from the emitter into the base. They flood into the base and manage to cross the depletion region between the base and the collector and enters into the collector.

One of the fundamental differences between NPN and PNP transistors is that the majority carriers in a PNP transistor are holes. Holes travel slightly slowly compared to electrons, so the switching times for PNP transistors is slower compared to an NPN transistor.

Difference Between NPN and PNP Transistor - A_PNP_Transistor

A PNP transistor

The circuit symbol for a PNP transistor is shown below:

Difference Between NPN and PNP Transistor - A_PNP_Transistor_Symbol

Circuit symbol of a PNP transistor

The image below shows what a bipolar junction transistor looks like:

Difference Between NPN and PNP Transistor - A_Transistor

A transistor

In most cases, it is not possible to differentiate between NPN and PNP transistors by just looking at them. They could be connected to a multimeter, and by looking at under which polarity the transistor conducts current, it is possible to determine the two types.

Difference Between NPN and PNP Transistor

Structure:

An NPN transistor consists of a p-type semiconductor sandwiched between two n-type semiconductors.

PNP transistor consists of an n-type semiconductor sandwiched between two p-type semiconductors.

Majority Carriers:

The majority carriers in an NPN transistor are electrons.

The majority carriers in a PNP transistor are holes.

Switching Time:

An NPN transistor has a faster switching time compared to a PNP transistor.

Image Courtesy:

“The basic operation of an NPN BJT in Active mode” by Inductiveload (Own drawing, done in Inkscape) [Public Domain], via

Untitled image by E2m (Own work) [Public Domain], via

“A diagram of the structure of an PNP BJT, showing the collector-emitter voltage…” by Inductiveload (Own work Based on File:Pnp-structure.png, by User:Heron at the English Wikipedia) [], via

Untitled image by E2m (Own work) [Public Domain], via

“Transistor, resistor” by Windell Oskay (Own work) [], via (Modified)

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