Understanding PNP Transistors: Characteristics, How They Work, And More

Although NPN transistors are more popular than the PNP type, they have unique properties that make them suitable for specific applications. Just like NPNs, this transistor is a BJT type, meaning it has two different charge carriers and two semiconductors intersecting at two junctions. Let’s look at how these devices work and what separates them from NPN transistors.

What Is a PNP Transistor? Definition and Types

PNP transistors are BJTs with three doped semiconductor layers consisting of two P-type semiconductors that sandwich an N-type semiconductor. With P-types on both sides, this device has holes as the majority charge carriers and electrons as the minority.

The internal structure of a PNP transistor

The development of PNP transistors was necessitated by the need to have complementary NPN counterparts to create symmetry in digital logic circuits and amplifiers.

PNP Transistor Symbol and Terminals

Like NPN transistors, PNPs have the same three parts/terminals, which are:

• Emitter

• Base

• Collector

These three layers form junctions of depletion regions and are equivalent to two diodes that forward bias to the inner area, which is the base.

A comparison between the diode models of PNP and NPN transistors

So when you hear PNP and NPN transistors have opposite polarities, this diagram should make it easier to understand why. Essentially, the charge carriers create different biasing and power supply polarities.

The device’s symbol is also different because its arrow points inward from the emitter to the base, not outward like in NPNs. This arrow indicates the direction of the conventional current, which flows into the emitter from the circuit.

Circuit symbols of PNP and NPN transistors

PNP Transistor Construction

The three semiconductors used to make this device have different properties.

• Emitter: This part lets the majority of charge carriers into the transistor, so it is heavily doped to feature lots of holes. It should be forward-biased toward the base to provide minimal resistance.

• Collector: As the name suggests, this section collects holes (majority charge carriers) coming from the emitter, so it is moderately doped and has enough capacity to collect these charges. The collector-base junction is always reverse-biased in active mode for amplification.

• Base: Base layers are usually thin and carry the minority charge carriers (electrons in PNPs). Also, they are more lightly doped than the other two parts.

How a PNP Transistor Works

PNP transistors also operate in three modes.

Active Mode

In this mode, the base-emitter junction is forward-biased, while the base-collector junction is reverse-biased, which is created using the circuit below.

PNP transistor circuit configuration in active mode

The voltage source VBE is positive at the emitter and negative at the base. This forward-biases the junction, allowing electrons to flow from base to emitter and conventional current from emitter to base.

VCE, on the other hand, reverse-biases the collector-base junction, which should block the flow of current, but this is not the case. Because the emitter is heavily doped and the base is lightly doped, the holes from the emitter combine with all electrons, leaving a lot more free holes. The remaining ones force their way into the collector due to the electrostatic force formed with the negative voltage from VCE connected to the collector terminal.

Therefore, electrons move from collector to emitter and conventional current (holes) from emitter to collector. But there is also some current flowing out from the base, so the collector current formula is:

IC = IE - IB

This configuration is known as a common-base PNP amplifier because both the emitter and collector share the base terminal. Essentially, all this current flows back into the transistor via the emitter terminal, hence the arrow in the symbol.

Saturation and Cutoff Modes

Saturation mode turns on the PNP transistor by forward-biasing both junctions, making the device operate as an on switch. On the other hand, these transistors shut off or act as off switches when both junctions are reverse-biased.

There is a fourth mode known as reverse active, which occurs when the emitter-base junction is reverse-biased and the base-collector junction is forward-biased. This allows current to flow in the opposite direction but is not recommended because it can damage the transistor.

Key PNP Transistor Formulas

Collector Current (IC)

This can be calculated using the formula IC = β × IB where:

• β is the transistor current gain or common-base current gain (represents the amplification factor)

Base Current (IB)

The base current is calculated using the formula IB = IC/β

Emitter Current (IE)

The emitter current is a summation of the base and collector currents (IC + IB). This explains why the arrow in the PNP transistor symbol points inward from the emitter.

Collector-Emitter Voltage (VCE)

You can calculate the collector-emitter voltage by replacing this formula with the values for the collector supply voltage, collector current, and collector resistor.

VCE = VCC – IC x RC

Transistor Current Gain (β)

A PNP transistor’s gain represents the ratio of the collector current to the base current, and you can calculate it using this formula.

β = IC/ IB

Why Use PNP Transistors?

Comparing their charge carriers, NPN transistors are more efficient. From a general overview, PNP transistors operate just like their NPN counterparts but in the opposite polarity. However, the majority charge carriers (holes) are not as mobile as electrons, which means NPN transistors have a speed advantage. They are also cheaper than PNP devices.

So why use them?

Their primary application is in transistor matching with NPNs because their output characteristics are similar but rotate by 180° due to the reverse polarity voltage and current. Transistor matching is handy for building devices like class B power amplifiers.

A push-pull class B amplifier circuit

This amplifier uses a complementary pair of transistors (NPN and PNP) with near-identical characteristics. For instance, it can have a TIP3055 NPN transistor matched to a TIP2955 PNP, (60V, 15A, and 90W each), which have a beta and DC current-gain matched to within 10%.

The NPN transistor conducts only through the positive half as the PNP type handles the negative half, making it possible to push the required power through the loudspeaker in both directions of the stated nominal power and impedance. The resulting current will be in the order of several amps higher while only using two transistors.

This circuit is also suitable for making a reversible H-Bridge motor control circuit to regulate current flow through the motor in either direction (forward and reverse motion).

Applications of PNP Transistors

• High-side current mirror/active load circuits

• Complementary amplifier/driver circuits (class AB and B output stages)

• Low dropout voltage regulators

• High-side switches (driver applications)

PNP vs. NPN Transistors

Advantages of PNP Transistors

• Can operate in both AC and DC circuits

• Complement NPN transistors to form simple amplification circuits

• Generates low noise

Disadvantages of PNP Transistors

• Not suitable for low-power applications

• More susceptible to temperature variations

• More expensive than NPN transistors

Key Drivers That Are Increasing Demand for PNP Transistors

The increasing demand for power and consumer electronics, as well as RF circuits, such as in IoT applications, is the primary driver behind the high demand for PNP transistors. EVs, for instance, fall in the high-power electronics category, which falls squarely under the area where PNP transistors excel.

Restraints in PNP Transistor Usage

Other transistor types, specifically IGBTs and FETs, are better for digital electronics because they have a high input impedance, generate less noise, consume little power, and are more thermally stable. So they give PNP BJTs a lot of competition.

IGBTs, for instance, are better for high-power applications and are tinier, so they have replaced PNP transistors in some applications. Generally, their disadvantages and limitations, which include compatibility issues, are their restraints.

Emerging Technologies in PNP Transistor Construction

Although they have limitations, advancements in PNP transistor construction are solving some of their issues. For instance, manufacturers have found ways to make these devices provide better thermal performance and handle higher current levels.

Final Words

PNP transistors are not as commonly used as their NPN counterparts, but they have unique switching and amplification capabilities that make them ideal for matching with their opposite polarity equivalents.

But ensure you use opposites with similar or near similar properties to output similar amplitude signals through the positive and negative halves. We have several of these devices that you can use to make such circuits, as well as H-bridge motor drivers. Even if you want complete motor drivers, you can get them at reasonable prices.

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