PNP vs. NPN: What Are the Important Differences?

Transistors are one of the most important inventions in electronics and have revolutionized technology since their creation in the late 1940s. As a fundamental building block, transistors are used to amplify, switch and control the flow of electrical signals. Nearly all modern electronic devices contain transistors, from simple circuits to complex processors with billions of microscopic transistors.

Two of the most common types of transistors are PNP (positive-negative-positive) and NPN (negative-positive-negative). While they perform similar functions, PNP and NPN transistors have some important differences in how they are constructed, operate and are used. In this article, we‘ll take an in-depth look at PNP vs NPN transistors, explore their characteristics, and discuss how to choose the right one for your needs.

Transistor Basics

Before diving into the specifics of PNP and NPN, let‘s briefly review what transistors are and how they work. A transistor is a semiconductor device with three terminals – the emitter, base and collector. By applying different voltages to these terminals, a transistor can act as an electronic switch or amplifier.

The two main transistor types are bipolar junction transistors (BJTs), which include PNP and NPN, and field-effect transistors (FETs). BJTs rely on both electron and hole charge carriers, while FETs use only one type of charge carrier. This article focuses mainly on PNP and NPN bipolar transistors.

PNP Transistor Overview

A PNP consists of two P-type semiconductor regions separated by a thin N-type layer. P-type silicon is created by adding impurities or "doping" the material with elements that have fewer valence electrons than silicon, such as boron. This creates "holes" or missing electrons in the crystal structure.

The basic operation of a PNP transistor is:

A small current flowing out of the base allows a larger current to flow from the emitter to the collector
Applying a more negative voltage to the base (relative to emitter) turns the PNP transistor on and allows current
Making the base voltage more positive turns the transistor off and blocks current flow
PNP is described as "current sinking" since the emitter supplies current to the load

So in summary, a PNP transistor switches on when the base is pulled low (more negative) and switches off when the base is pulled high (more positive). The transistor acts like a closed switch when on and an open switch when off.

NPN Transistor Overview

An NPN is constructed from two N-type layers with a P-type layer sandwiched in between. N-type semiconductor material is doped with elements that have more valence electrons than silicon, such as phosphorus. This creates an excess of free electrons in the material.

The NPN operates as follows:

A small current flowing into the base allows a larger current to flow from the collector to the emitter
Applying a voltage to the base that is more positive than both the emitter and collector turns the NPN on
Lowering the base voltage (making it more negative) switches the transistor off
NPN is known as "current sourcing" since the collector supplies current to the load

An NPN transistor therefore turns on when the base is pulled high and switches off when the base is pulled low – the opposite behavior of a PNP. Like the PNP, the NPN acts as a closed switch when on and an open switch when off.

PNP vs NPN: Key Differences

Now that we understand the fundamentals of PNP and NPN transistors, let‘s examine the crucial differences between the two:

Voltage and Current Polarity

The most significant difference is the polarity of the voltages and current flow:

PNP: Requires a negative voltage on the base (relative to emitter) to turn on. Current flows from emitter to collector.
NPN: Needs a positive base voltage (relative to emitter) to switch on. Current flows from collector to emitter.

This means the supply voltages and current directions are reversed for PNP vs NPN circuits. A PNP is typically used in circuits with a negative ground (earth) while NPN is more common in circuits with a positive ground.

Switching and Amplification

Both PNP and NPN can be used for switching and amplification but their behavior is opposite:

PNP: Switches on when base is pulled low, amplifies when emitter-base voltage is increased
NPN: Switches on when base is pulled high, amplifies when collector-base voltage is increased

In switching applications, the base is driven fully on or off to make the transistor act like a closed or open switch. For amplification, the base voltage/current is varied to control the larger collector-emitter current.

Use Cases and Applications

PNP and NPN are both widely used in different types of analog and digital circuits such as:

Switches and relays
Logic gates and flip-flops
Amplifiers and drivers
Current sources and regulators
Oscillators and multivibrators

Historically PNP transistors were more common in early germanium devices, but today NPN is more widely used, especially in integrated circuits. This is because NPN switches on with positive voltages which are easier to generate and interface with.

Some typical applications of each type:

PNP: High-side switches, current sources, complementary pairs with NPN
NPN: Low-side switches, push-pull amplifiers, logic gates, Darlington pairs

The choice of PNP or NPN depends on factors like the power supply voltages, speed requirements, available space, cost and noise immunity of the circuit.

Cost and Availability

In general, NPN transistors are more readily available and lower cost than PNP, especially for power devices. This is due to NPN being easier to manufacture in high volumes and more commonly used in consumer electronics.

However, for small-signal and RF applications, PNP may be preferred for its lower noise and better high-frequency performance. Specialty PNP devices can be more expensive than NPN equivalents.

Many circuits use a complementary pair of PNP and NPN transistors to efficiently drive loads in both directions or create a push-pull output stage. So it‘s common to see the two types used together.

*Example PNP and NPN transistor packages showing pinouts and schematic symbols*

PNP vs NPN Comparison Table

Here is a quick reference comparing the main characteristics of PNP and NPN transistors:

Characteristic	PNP	NPN
Semiconductor Layers	P-N-P	N-P-N
On State	Base more negative than emitter	Base more positive than emitter
Current Flow	Emitter to collector	Collector to emitter
Circuits	Negative ground/earth	Positive ground/earth
Packaging	Uncommon vs NPN	Most common BJT type
Cost	Higher for high power & RF	Generally lower cost

Other Transistor Types

Beyond standard PNP and NPN bipolar transistors, there are many other types of transistors used in modern electronics:

MOSFET (Metal-oxide-semiconductor field-effect transistor)
JFET (Junction gate field-effect transistor)
IGBT (Insulated-gate bipolar transistor)
Unijunction transistor
Schottky transistor
Phototransistor

Each of these transistor types has unique properties that make them suited for different applications. MOSFETs are by far the most widely used today, especially in digital logic and memory. JFETs and MOSFETs operate by a different mechanism than PNP/NPN bipolars.

IGBTs combine a MOSFET and bipolar to handle very high voltages and currents. Unijunction transistors only have one PN junction and are used in triggering and timing circuits. Schottky transistors are designed for high-speed switching. And phototransistors detect light for optical circuits.

Historical Usage of PNP and NPN

The first transistor invented at Bell Labs in 1947 was actually a crude PNP point-contact device made of gold foil on a germanium base. Throughout the 1950s, germanium PNP transistors were more common because they were easier to manufacture than NPN and had better performance.

However, silicon proved to be a superior semiconductor material. By the 1960s, silicon NPN transistors surpassed PNP in usage, especially for logic circuits in the early integrated circuits. PNP remained popular for analog amplifiers and high-power switching.

With the rise of CMOS logic in the 1970s and 80s, NPN became the dominant transistor type, with lateral and vertical PNP used in specialized circuits. RF and microwave frequency PNP devices are still used today for low-noise applications.

In the 1990s and 2000s, MOSFETs displaced NPN transistors for most digital logic, while NPN remains in use for general purpose amplification, interface circuits and power stages. The invention of complementary BJT (CBiPolar) processes allowed PNP and NPN to be fabricated on the same chip.

Interesting Facts About PNP and NPN

Here are some fascinating facts and trivia about PNP and NPN transistors:

The first PNP transistors were about 1/50th of an inch in size. Today the smallest transistors are only a few nanometers wide!
There are two types of bipolar PNP/NPN packages – through-hole and surface-mount. Through-hole parts have leads that insert into holes drilled in a circuit board. Surface-mount transistors solder flat to the surface of the board.
The metal casing and epoxy packaging material of some power transistors is designed to conduct heat away from the semiconductor die. This allows the transistor to handle more power.
A bipolar transistor‘s current gain (hfe) is the ratio of collector current to base current. This key parameter is typically between 50-1000 and varies with temperature and collector current.
A Darlington pair (often a single 3-pin package) contains two bipolar transistors that multiply their current gains, allowing a very small signal to control a large load current.
Most bipolar transistors are silicon, but other semiconductor materials like silicon-germanium and gallium-arsenide are used for higher speed devices.
Multi-emitter BJTs exist that have one collector and base but 2, 3 or 4 separate emitter regions. These are useful for logic and interface circuits.
Before transistors, vacuum tubes were the only devices capable of amplification and switching. The first transistor radios in the 1950s were much smaller, cheaper and more efficient than tube radios.

Choosing Between PNP and NPN

For many circuit designs and applications, either a PNP or NPN transistor can be used. The main consideration is whether the load will be switching between the positive supply and the load (use an NPN) or between the load and ground (use a PNP). It‘s also important to check that the transistor can handle the required voltage, current and power.

Here are some guidelines to help choose between PNP and NPN:

PNP is often used for high-side load switching, sourcing current and for low-noise applications
NPN is preferred for sinking current on the low-side of a load, high-speed circuits and general purpose use
Consider a complementary PNP/NPN pair for efficient push-pull output stages
Select a transistor with specifications that meet or exceed the load requirements with a safety margin
Check the cost, availability and package options for PNP vs NPN and go with the most economical choice
Consult reference designs, app notes and ask for advice if you are unsure which type is best for your circuit
It‘s a good idea to breadboard and test the circuit to validate the PNP or NPN choice before committing to a pcb layout
Remember that in general, NPN is lower cost and more widely available, except for certain niche applications

Useful Resources

To learn more about PNP vs NPN transistors, check out these resources:

I hope this in-depth article gave you a good understanding of the differences and similarities between PNP and NPN transistors. While these two BJT types operate on the same principles, their opposite polarity and current flow gives them distinct use cases. Knowing the nuances of PNP vs NPN is key to designing efficient and robust transistor circuits.

The invention of the bipolar junction transistor in the 1940s launched the field of microelectronics and transformed the world. Over 70 years later, the humble PNP and NPN transistors, along with their FET cousins, are still the core enabling technology behind every computer, smartphone and embedded system. So the next time you use any electronic device, remember it likely contains millions or billions of these tiny semiconductor switches!