NPN vs PNP Transistors: What’s the Difference and Which One to Use

Tech Explorers
11 min readMar 13, 2023

NPN vs. PNP Transistors: Understanding the Difference and Functionality

Transistors are an incredible revolution to the world; if this didn’t exist, you wouldn’t see the process of new technology. Men have discovered electricity, which can harm small devices like computers, cameras, and many tools that use a small amount of power. Also, we are familiar with AC, which can damage an appliance when given directly; in such cases, a small instrument called Transistor came into the field of electronics.

To make the devices applicable to such a large amount of current or voltage, we have to use DC currents and voltages, so in this case, Transistors play a vital role that can control the CURRENT and VOLTAGE according to our needs.

Hence, the question arises here of how it controls electricity; it’s discussed step by step.

Who invented the Transistor?

The scientists responsible for the invention in 1947 that led to the creation of transistors were John Bardeen, Walter Brattain, John Bardeen, and William Shockley. Bardeen, a scientist with a PhD in math and physics at Princeton University, was an expert in semiconductors’ electron conductivity properties. Brattain, PhD, was an expert on the structure of the crystal structures of the solid at the surface and solid-state Physics. Shockley, PhD, was the director of transistor research at Bell Labs.

Their original patent title of the Transistor is “Semiconductor amplifier; Three-electrode circuit component made up of semiconductive material.’ In 1956, the team received the Noble Prize in Physics for the invention of the Transistor. In 1977, John Bardeen was awarded the Presidential Medal of Freedom.

What is meant by Transistor?

The Transistor is “The semiconductor device, made of silicon and Germanium.”


“Transistor is a semiconductor device used to amplify or switch the electronic signals and electrical power.”

Transistors are still utilized in electronic devices at present. Where did the word “transistor” come from? The term is derived directly from the words “transfer” and “resistor” since an electrical current inside the Transistor is transmitted across resistors, which are which is a component with two terminals in electronics.

Types of transistors?

There are two main types.

  1. BJT
  2. FET

BJT Transistor:

NPN and PNP transistors are Bipolar junction transistors (BJT); BJT controls current, which means they only amplify flow. A current at the base terminal of BJT allows a large (current) at the emitter and collector terminal. NPN and PNP transistors have the same function of {AND Capability}.


Remember that NPN points the arrow to the outer. PNP leads the shaft to the inner.

If you want to remember this, here is a trick for you;

For NPN;




For PNP;

P= Points


P= Permanently

How does NPN Transistor work?

Its working is explained according to the figure below.

NPN Transistor

When we connect the supply to the LED, and the current passes through, it won’t glow because the flow is not passing through the emitter terminal. The current got blocked at the collector, as shown in the figure above.
To make the LED glow, we must close the switch to complete the circuit. BJT’s transistor only works when sufficient current is supplied to the base terminal, as shown below.

NPN Transistor

In PNP, When we provide a positive voltage to the collector terminal, current flows from the collector terminal to the emitter terminal when there is enough base current.

An NPN can be ON when a sufficient current to the base terminal is provided, connecting the positive terminal to the supply.

How does the PNP transistor work?

The working of PNP is described in the figure below.

PNP transistor

Initially, the switch is open; hence when the voltage is applied to the emitter side of the PNP current, it will not pass through the collector terminal of the PNP, so the LED won’t be able to glow.

PNP Transistor working

When the switch is closed, the current passes from a positive terminal to the negative terminal of the supply, and hence the current creates this path. Accordingly, the current is passed through the collector terminal of PNP, so the LED glows.

When the positive voltage is applied to the emitter terminal, current flows from the emitter to the collector and when a sufficient amount of negative current is applied to the base terminal.

The only difference between PNP and NPN is that;

  1. In NPN, the Load is connected to the Collector terminal.
  2. In PNP, the Load is connected with the Emitter terminal.
  3. In NPN positive terminal of supply is connected with the base terminal.
  4. In PNP negative terminal of supply is connected with the base terminal.

Difference between NPN and PNP


The FET (field effect transistor) is a three-terminal device that uses an electric field to control current flow through the machine. Today most integrated, including the computer, use FETs.

FET Working

The three-terminal of FET ;

  1. Gate
  2. Drain
  3. source

In FET, current flows from the drain through the source, and in this way, the current is controlled by applying a voltage between the gate and the source. This voltage generates an electric field within the device, so the flow of current is monitored by controlling this voltage.

Simply put, the flow of current is monitored by controlling the electric field. That’s why it is known as a Field-Effect Transistor or FET. So it is said the Field-Effect Transistor is a voltage control device.

Difference between FET and BJT Transistors ;

  1. BJTs are current controlling devices.
  2. FETs are voltage-controlling devices.
  3. BJTs are Bi-polar devices.
  4. FETs are Uni-polar devices.
  5. BJT’s Input impedance is very high.
  6. FETs have high input impedance.
  7. BJTs are larger than FETs
  8. FETs are smaller than BJT. That’s why they can be found in integrated circuits.
  9. FET uses low power consumption than BJT.

Application Of Field-Effect Transistor:

FETs are used almost everywhere where BJTs are used. Some of the claims are as under.

  1. FETs are used in amplifiers.
  2. They can also be used in the analogue switch.
  3. FET is used in integrated circuits.
  4. It is used as a buffer amplifier.
  5. It can be used as an oscillator.

For more info, you can check Wikipedia.

Transistors as a switch:

How to use it as a switch?

The switch is a device used to manually turn On or Off the lights or any loads in houses. Similarly, if we want to turn the fan on, we use the switch to turn it On or off. But in electronic PCV or other devices, the switch is made automatically. First, let’s know about transistor work,

What is the result of the transistor?

Transistor has two uses: The first use of the transistor-transistor is it works as switching. On and Off work is work taken from it. The transistor is used as a switch.The second use of a transistor is that it also works as an amplifier for any signals in a circuit.

How can transistors be used for switching purposes in a course?

First of all, choose a transistor. As in the given diagram, the value of the Transistor is S8550. If you are using any transistor, use it according to its value. In the chart, the transistor has three pins, and the other diagram shows the symbol known as the NPN Transistor.

Below are the steps of how to select the transistor.

Differentiate Between Transistors Types:

  • If the arrow is downward, it’s known as the NPN.
  • If the same arrow is upward, it would be known as the PNP.

Pins: If we talk about transistor pins, every transistor has different pins. It has Ammeter, Collector, and a base.

How to differentiate between these Pins?

For the differentiation of pins, a multimeter is used. The switch could be divided into Ammeter, Collector, and Base through that device. If the device, a multimeter, is not available. You can find it through google, just put the value of the transistor in the search bar. It will show you which is an ammeter, collector, and base.

Switching using transistor:

Consider a lamp and a battery in the circuit where the transistor is attached. For making the transistor for automatic switching, the ammeter and collector terminals are sorted by applying a low voltage to the base. Once they get sorted, the lamp will be turned On.

Transistor as a switch

The figure shows that the negatives terminal is directly connected to the lamp. When the positive terminal is attached to the Transistor, the base of a transistor is given a low voltage or signal supply. It will make the ammeter and the collector get sorted, or in simple words, the current will move directly from the ammeter to the collector and then to the lamp. Once the supply current is Off, the terminal will shut down.

For example, the water motors.

When the water level in the tank is low, the water motor starts automatically, and when the tank is complete, the water motor stops because of the sensor in it when water touches the sensor. The circuit gets completed and gives supply to the base of the Transistor, the ammeter and collector are sorted, and then the Load is On or Off.

Uses of Transistors:

In this topic, you will come to know where Transistor Is used?

The Transistor is used in every electronic device, such as radio, tv, computer, calculator, and telephone. In all these devices, the motherboard is mended in them.

Why are Transistors operated?

The Transistor is wielded because, in electronics, it provides us with two types of work.

1: It is used for switching purposes. For example, a bulb or switch is used annually for its On and Off.

In the same way, in every electronic form, the flow of the current Transistor is present.

2: Transistor is similarly used for Amplification as it converts weak signals to solid signals, for example, Mic, water tank, fire alarm, and power supply.


  • In water tanks, the Transistor is fixed when the water tank gets full. The sensor recognizes the jingles in the alarm.
  • In the fire alarm, the Transistor is fixed when the fire takes place, the light or smoke of the fire is detected, and the signal is activated.
  • In the power supply circuit, more than one Transistor is used.
  • To make a device with more transistors, its circuit is complicated and expensive, so a small IC is used. Approximately 2000 or 3000 electronic transistors can be placed in one IC.

In some IC, you can place up to one lac transistor. Its benefit is it’s cheap and easy. You only have to check the s datasheet for all the information.

As you know, computer language is binary code, and transistors play a vital role in conversion from binary to common words.

Exploring the Latest Transistor Technologies

As technology advances, developing new and improved transistors is becoming increasingly important. Transistors are the basic building blocks of modern electronic devices and are essential for functioning devices such as computers, smartphones, and televisions. This article will discuss the latest developments in five types of transistors: Graphene, Nanoscale, Organic, Neuromorphic and Quantum.

Graphene Transistors: High Frequencies and Low Power

Graphene is an excellent conductor of electricity and has been proposed as a replacement for silicon in transistors. Researchers have been developing graphene-based transistors that use less power to operate at high frequencies. These transistors have the potential to revolutionize the electronics industry by enabling faster and more efficient electronic devices. TechRadar says, “Graphene transistors could allow electronic devices to be much faster, more efficient and smaller, making them ideal for use in smartphones, tablets, and other portable devices.” However, there are still some challenges to overcome in developing these transistors, including the difficulty of mass production and the high manufacturing cost.

Nanoscale Transistors: The Future of Miniaturization

As electronics become smaller and more compact, the development of nanoscale transistors is becoming increasingly important. These transistors are just a few atoms wide and could lead to faster and more efficient electronics. Researchers are developing nanoscale transistors using various materials, including carbon nanotubes and graphene. According to a recent article in Wired, “Nanoscale transistors have the potential to revolutionize electronics, making them smaller, faster, and more powerful than ever before.”

Organic Transistors: Flexibility and Wearability

organic field-effect transistor image credit wikipedia

Organic transistors are made of organic materials and have the potential to be used in flexible and wearable electronics. Researchers are currently exploring ways to improve the performance of these transistors and make them more reliable. According to a recent review in Nature, “Organic transistors have the potential to revolutionize the field of electronics by enabling the development of flexible, low-cost, and environmentally friendly electronic devices.”

Neuromorphic Transistors: Artificial Intelligence and Machine Learning

Neuromorphic transistors are designed to mimic the behaviour of biological neurons and synapses. These transistors could be used in artificial intelligence and machine learning applications, enabling computers to process information more efficiently and human-likely. According to a recent article in The Verge, “Neuromorphic transistors have the potential to revolutionize the field of artificial intelligence by enabling computers to learn and process information more efficiently and naturally.”

Quantum Transistors: The Future of Computing

Quantum transistors use the principles of quantum mechanics to control the flow of electrons. These transistors have the potential to be much more potent than traditional transistors and could be used in quantum computers. According to a recent article in TechCrunch, “Quantum transistors have the potential to revolutionize the field of computing by enabling the development of powerful new computers that can solve problems that are currently beyond the capabilities of traditional computers.”



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