PNP Transistor: How Does it Work? (Symbol & Working Principle)

What is a PNP Transistor

A PNP bipolar junction transistor has a thin N-type semiconductor base between emitter and collector regions made from P-type semiconductors. Its three terminals are emitter, base and collector. The PNP transistor contains the two junctions described in the linked PN junctions diodes overview, but two separate back-to-back diodes cannot reproduce transistor action because they do not share a thin base.

The two PN junctions, sometimes drawn as back-to-back diodes to show polarity, are the base-emitter and base-collector junctions. That drawing is not a functional equivalent circuit.

The heavily doped emitter injects holes into the thin, lightly doped base. Most holes diffuse across the base to the collector-base depletion region, where the electric field sweeps them into the collector. Emitter-base voltage controls the injection rate.

The figure below shows the polarity of the two junctions. It is a junction analogy, not an equivalent model for gain or collector current.

PNP Transistor Symbol and Construction

A PNP transistor has the complementary doping sequence to an NPN transistor. The NPN structure uses two N-type semiconductors around a P-type base, while the PNP structure uses two P-type semiconductors around an N-type base. The regions remain asymmetric in both devices.

The figures show the simplified PNP layer sequence and a more representative device structure.

Holes are the majority carriers in P-type semiconductors. Both holes and electrons contribute to PNP terminal currents, but hole injection from the emitter dominates forward-active collector current.

In the simplified layer drawing, the middle N-type region is the base, while the P-type regions are emitter and collector. A fabricated transistor is not a symmetric three-layer slab.

The emitter is heavily doped, the base is thin and lightly doped, and the collector is more lightly doped than the emitter so it can withstand reverse voltage. Depletion width extends farther into the more lightly doped side of each junction. The collector junction area is often larger for carrier collection and heat dissipation.

Electrons are the majority carriers in the N-type base, but the base’s thin geometry and light doping limit recombination. This lets most injected holes reach the collector.

The symbol’s arrow is on the emitter and points in the conventional emitter-current direction during forward-active operation. For a PNP transistor it points inward towards the base.

How Does a PNP Transistor Work

In the shown active-mode bias, the positive terminal of a voltage source V_EB connects towards the P-type emitter and the negative terminal towards the N-type base. This forward biases the emitter-base junction.

The positive terminal of V_CB connects towards the N-type base and the negative terminal towards the P-type collector. This reverse biases the collector-base junction. The junction voltages, rather than supply names alone, define the operating region.

Forward bias narrows the emitter-base depletion region, while reverse bias widens the collector-base depletion region. The electric field in the latter collects holes that diffuse across the base.

Forward bias injects holes from the heavily doped emitter into the base. A smaller electron current flows from base to emitter because of the asymmetric doping.

Injected holes diffuse through the thin base. A small fraction recombines with electrons in the base, while most reaches the collector-base depletion region.

The terminal current associated with hole injection and the smaller electron component is Emitter current (I_E). Conventional emitter current points into the PNP symbol.

Holes that do not recombine reach the collector-base depletion region and are swept into the collector. Their main terminal-current contribution is Collector current (I_C).

PNP Transistor Circuit

The circuit of the PNP transistor is as shown in the below figure.

Compared with an NPN circuit, a complementary PNP circuit reverses voltage polarities and conventional current directions.

In forward-active operation, a small conventional current leaves the PNP base while a larger current flows from emitter to collector. The base must be below the emitter by the forward base-emitter voltage, and the collector must remain below the base to keep the collector-base junction reverse biased.

If the base is not sufficiently below the emitter, the emitter-base junction does not inject appreciable carriers. A silicon junction often conducts near 0.6-0.8V forward bias, but the required voltage varies continuously with current, device and temperature. This is forward bias, not reverse bias.

Load resistor R_L helps set collector current, while base resistor R_B limits base drive. The supply, load and collector-emitter voltage determine the actual current.

Kirchhoff’s current law gives emitter-current magnitude as the sum of base- and collector-current magnitudes under the sign convention used below.

   

   

PNP Transistor Switch

An ideal OFF switch carries no current and an ideal ON switch has no voltage drop. A transistor only approximates these states because it has leakage current and a nonzero saturation voltage.

A PNP transistor can amplify in forward-active mode or switch between cut-off and saturation. It is often used as a high-side switch with its emitter connected towards the positive supply.

The PNP device turns on when its base is driven below its emitter enough to forward bias the base-emitter junction. The needed voltage is not a fixed 0.7V threshold and is not reverse bias.

The absolute base voltage does not determine the state. The transistor is near cut-off when base voltage is close to emitter voltage; a base at 0V can turn it on if the emitter is at a sufficiently positive voltage.

To turn the transistor ON, drive the base below the emitter and provide enough base current for the required collector current. In saturation it approximates a closed switch but retains a collector-emitter saturation voltage.

PNP vs NPN Transistor

PNP and NPN transistors use opposite carrier types and voltage polarities. In forward-active mode, both types have a forward-biased base-emitter junction and a reverse-biased base-collector junction; the polarities reverse between them. The protected legacy table below states the PNP junction biases incorrectly and its blanket slower-versus-faster comparison is not universal. Switching speed depends on the specific device, circuit and drive conditions.