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What is a PNP Transistor?

Author: Apogeeweb
Date: 26 Apr 2022
pnp vs npn transistor


Ⅰ Introduction 

Ⅱ What is a PNP Transistor?

Ⅲ PNP Transistor Symbol

Ⅳ PNP Transistor Construction

Ⅴ How a PNP Transistor Works

Ⅵ PNP Transistor Configuration

Ⅶ PNP Transistor Circuit

Ⅷ Applications of PNP Transistors

Ⅸ Benefits of PNP Transistors

Ⅹ How to Identify a PNP Transistor 

Ⅺ PNP vs NPN Transistor

Ⅻ Frequently Asked Questions About PNP Transistor


Ⅰ Introduction

PNP transistors are Bipolar Junction Transistors (BJT). The PNP transistor has a completely different structure than the NPN transistor. In the PNP transistor structure, the two PN-junction diodes are reversed with regard to the NPN transistor, so that two P-type doped semiconductor materials are separated by a thin layer of N-type doped semiconductor material.


The majority current carriers in a PNP transistor are holes, while electrons are the minority  current carriers  . The polarity  of all supply voltages  applied to the PNP transistor are inverted. The current sinks into the base terminal in PNP,  Because the PNP is a current-controlled device, the modest base current may regulate the huge emitter-collector current.


Ⅱ What is a PNP Transistor?

PNP transistors are transistors that have one n-type material doped with two p-type materials. It is a device that is powered by current. The modest quantity of base current regulated both the emitter and collector currents. In the PNP transistor, two crystal diodes are linked back to back. The emitter-base diode is on the diode's left side, while the collector-base diode is on the diode's right side.


The majority of the carriers in the PNP transistors make up the current in the hole. The movement of holes inside the transistor generates current, while the flow of electrons in the transistor's leads generates current. The PNP transistor switches on when a tiny current flows through its base. In a PNP transistor, current passes from the emitter to the collector.


The letter of the PNP transistor indicates the voltage required by the transistor's emitter, collector, and base. The base of a PNP transistor has always been negative in proportion to the emitter and collector. In a PNP transistor, electrons are drawn from the base terminal. Before reaching the collector ends, the electricity that enters the base is amplified.


Ⅲ PNP Transistor Symbol

The letters PNP stand for PNP Transistor. The symbol for a PNP transistor is illustrated in the diagram below. The current flows from the emitter to the collector in a PNP transistor, as represented by the inward arrow.


PNP Transistor Symbol

PNP Transistor Symbol


PNP Transistor Construction

The diagram below depicts the structure of a PNP transistor. The emitter and base junctions are biased forward, whereas the collector and base junctions are biased reverse. Electrons are drawn to the battery by the forward biased emitter, causing current to flow from the emitter to the collector.


Doped semiconductors can be found in three different parts of a transistor. There is an emitter on one side and a collector on the other. The term "base" refers to the area in the center. The three components of the transistor are detailed in detail below.


PNP Transistor Construction

PNP Transistor Construction



It is the emitter's responsibility to provide charge carriers to the receiver. In order to supply a large number of charge carriers, the emitter is constantly forward biased when compared to the base.



The base of a transistor is the part in the middle that connects the emitter and collector via two PN-junctions. Because the base-emitter junction is forward biased, the emitter circuit has a low resistance. The collector circuit has a high resistance due to the reverse bias of the base-collector junction.



The collector is the portion of the emitter on the opposite side that gathers the charges. The collector is always prejudiced in the opposite direction when it comes to collecting.


Because it has two PN-junctions, the transistor is comparable to two diodes. The junction between the emitter and the base is referred to as an emitter-base diode or emitter diode. A collector-base diode, also known as a collector diode, is the junction between the collector and the base.


Ⅴ How a PNP Transistor Works

A voltage source (VEBpositive )'s terminal is connected to the Emitter (P-type), while the negative terminal is connected to the Base terminal (N-type). As a result, the Emitter-Base junction is biased forward.


Furthermore, the positive terminal of a voltage source (VCB) is linked to the Base terminal (N-type), while the negative terminal is attached to the Collector terminal (P-type). As a result, the Collector-Base junction is biased in reverse.


How a PNP Transistor Works

How a PNP Transistor Works


Because it is connected in forward bias, the depletion region at the Emitter-Base junction is narrow as a result of this sort of bias. Because the Collector-Base junction is in reverse bias, the depletion zone at the Collector-Base junction is quite large.


The Emitter-base junction is biased forward. As a result, a substantial number of holes from the emitter pass through the depletion region and enter the Base. At the same time, only a few electrons reach the Emitter from the base and recombine with the holes.


The amount of electrons present in the Base layer is equal to the number of holes lost in the emitter. However, the amount of electrons in the base is relatively modest due to the fact that it is a very lightly doped and thin region. As a result, practically all Emitter holes will cross the depletion area and penetrate the Base layer.


The current will flow via the Emitter-Base junction due to the movement of the holes. This current is referred to as Emitter current (IE). To flow the Emitter current, the holes are the predominant charge carriers.


The leftover holes that do not recombine with electrons in the Base will proceed to the Collector. Because of the perforations, the Collector current (IC) travels via the Collector-Base area.


Ⅵ PNP Transistor Configuration

PNP Transistor Configuration

(Note: For a PNP transistor, the arrow represents the emitter and the typical current flow, "in.")


The accompanying diagram depicts the structure and terminal voltages of an NPN transistor. The PNP Transistor has characteristics that are very similar to their NPN bipolar cousins, except that the polarities (or biasing) of the current and voltage directions are reversed for any of the three possible configurations discussed in the first tutorial, Common Base , Common Emitter, and Common Collector.


Because the Base terminal of a PNP transistor is always biased negatively with respect to the Emitter, the voltage between the Base and Emitter (VBE) is now negative at the Base and positive at the Emitter.


In addition, the Emitter supply voltage is positive in relation to the Collector ( VCE ). As a result, for a PNP transistor to conduct, the Emitter must always be more positive than both the Base and the Collector.


PNP Transistor Connection

PNP Transistor Connection


As depicted, the voltage sources are coupled to a PNP transistor. This time, the Emitter is connected to the supply voltage VCC via a load resistor, RL, limiting the maximum current flowing through the device attached to the Collector terminal. The Base voltage VB is biased negative in relation to the Emitter and is connected to the Base resistor RB, which is utilized to limit the maximum Base current once again.


In order for the Base current to flow in a PNP transistor, the Base must be more negative than the Emitter (current must leave the base) by approximately 0.7 volts for a silicon device or 0.3 volts for a germanium device, with the formulas used to calculate the Base resistor, Base current, or Collector current being the same as those used for an equivalent NPN transistor and is given as.



IC = β·IB

IB = IC / β


The basic difference between an NPN and a PNP transistor is the proper biasing of the transistor junctions, as current and voltage polarities are always opposed to each other. So, in the aforementioned circuit, Ic = Ie - Ib since current must leave the Base.


In general, PNP transistors may substitute NPN transistors in most electronic circuits; the main difference is the polarity of the voltages and current flow directions.


Ⅶ PNP Transistor Circuit

PNP Transistor Circuit

The Output Characteristics Curves of a PNP transistor are essentially similar to those of an equivalent NPN transistor, with the exception that they are rotated by 180o to accommodate for reverse polarity voltages and currents (that is for a PNP transistor, electron current flows out of the base and collector towards the battery). To determine the operating points of PNP transistors, the same dynamic load line can be drawn onto the I-V curves.


Ⅷ Applications of PNP Transistors

  • PNP transistors are used to source current, i.e. current flows out of the collector.
  • PNP transistors are used as switches.
  • These are used in the amplifying circuits.
  • PNP transistors are used when we need to turnoff something by push a button. i.e. emergency shutdown.
  • Used in Darlington pair circuits.
  • Used in matched pair circuits to produce continuous power.
  • Used in heavy motors to control current flow.
  • Used in robotic applications.


Ⅸ Benefits of PNP Transistors

  • To source current, PNP Transistors are used.
  • Because it generates a signal that is referenced to the negative power supply rail, it simplifies the circuit design.
  • In comparison to NPN Transistors, they produce less noise.
  • It's smaller than other transistors and can be used in Integrated Circuits, just like the others.


Ⅹ How to Identify a PNP Transistor 

PNP transistors are generally identified by their structure. When comparing the structures of NPN and PNP transistors, we see various discrepancies. Another way to recognize a PNP transistor is that it is normally in OFF for positive voltage and ON for tiny output current and negative voltage at its base with respect to the emitter. However, to identify them most efficiently, we use another technique that involves calculating the resistance between the three terminals, such as the base, emitter, and collector.


For recognizing both NPN and PNP transistors, we have some standard resistance values. Each pair of terminals must be tested for resistance values in both directions, resulting in a total of six tests. This method is quite beneficial for quickly identifying the PNP transistor. We can now observe how each pair of terminals operates.


Terminals for Emitter-Base

The emitter-base area functions like a diode, but it only conducts in one direction.


Terminals for Collector-Base

The collector-base area also functions as a diode, conducting current in only one way.


Terminals for Emitter-Collector

The emitter-collector area has the appearance of a diode, yet it does not conduct in any direction.


Let us now look at the resistance value table to identify both NPN and PNP transistors, as illustrated in the table below.


Between Transistor Terminals PNP NPN
Collector Emitter RHIGH RHIGH
Collector Base RLow RHIGH
Emitter Collector RHIGH RHIGH
Emitter Base RLow RHIGH
Base Collector RHIGH RLow
Base Emitter RHIGH RLow


Then we can define a PNP Transistor as generally "OFF," but a modest output current and a negative voltage at its Base (B) relative to its Emitter (E) will turn it "ON," allowing a big Emitter-Collector current to flow. When Ve is substantially bigger than Vc, PNP transistors conduct.


In other words, a Bipolar PNP Transistor will only conduct if both the Base and Collector terminals are polarized against the Emitter.


Ⅺ PNP vs NPN Transistor

The following table summarizes the main distinctions between PNP transistors and NPN transistors:

  PNP Transistor NPN Transistor
Structure It has one N-type and two P-type semiconductors. It has two N-type and one P-type semiconductor.
Direction of current The current will flow through the emitter to the collector. The current will flow through the collector to the emitter.
Majority charge carrier Holes Electron
Minority charge carrier Electrons Holes
Switching time  Slower Faster
Junction biasing Emitter-base junction is in reverse bias and collector-base junction is in forward bias. Emitter-base junction is in forward bias and collector-base junction is in reverse bias.
Collector-emitter voltage Negative Positive
Emitter arrow Pointed in Pointed out


Ⅻ Frequently Asked Questions About PNP Transistor

1. Where are PNP transistors used?

Amplification circuits employ PNP transistors. Darlington pair circuits employ PNP transistors. Robotic applications make advantage of PNP transistors. PNP transistors are used to control current flow in high-power applications.


2. How can PNP transistor be controlled?

To begin, in order to switch on the PNP transistor, the voltage on the base must be lower than the voltage on the emitter. It's customary for a basic circuit like this to connect the emitter to the plus from your power supply. This manner, you can tell what voltage is on the emitter.


3. How does a PNP transistor turn on?

PNP and NPN Transistors' Terminal Resistance Values


Then we can define a PNP Transistor as generally "OFF," but a modest output current and a negative voltage at its Base (B) relative to its Emitter (E) will turn it "ON," allowing a big Emitter-Collector current to flow.


4. Can I replace PNP with NPN?

If you remember one simple rule, you can use NPN and PNP transistors interchangeably. A bipolar transistor is effectively two diodes connected back to back, with the base serving as the common connection.


5. How does a PNP junction work?

A PNP transistor is a bipolar junction transistor composed of an N-type semiconductor sandwiched between two P-type semiconductors. A PNP transistor has three terminals: a Collector (C), an Emitter (E), and a Base (B) (B). The PNP transistor functions similarly to two PN junction diodes connected back to back.


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