PNP vs NPN Transistors Guide

The definition, development and application of bipolar transistors cannot be understood simply from the combination of letters. This article briefly summarizes the above content. Whether you are a novice or a professional, I hope this article can bring you relevant inspiration.

Basics of Transistors

Scientists have discovered that materials with sufficient purity (such as silicon and germanium) have the function of signal amplification, and have begun new research on these materials to explore their future application possibilities. Transistors are made of semiconductor materials, which have insulating properties and have three external terminals including emitter, base and collector. The base acts as a control electrode to control the volt-ampere characteristic relationship of the other two electrodes, thereby changing the current at the output end. Since the signal at the input end is smaller than the signal at the output end, the transistor amplifies the signal.

Because of their fast response and accuracy, transistors are widely used in computers, mobile phones, and other electronic devices, which are becoming smaller, more efficient, and more powerful. The emergence of transistors has promoted the rapid development of electronic devices and continues to play an important role.

figure 1.Basics of Transistors

Types of Transistors

According to the history of transistors, transistors are mainly divided into two categories: BJT and FET. BJT is a bipolar junction transistor, which is further divided into NPN and PNP. FET is a field effect transistor. There are many differences between the two. BJT is current control, FET is voltage control, and there are differences in cost, power consumption, noise, etc.

According to use and function, there are low-noise amplification transistors, switching transistors, photosensitive transistors, etc.; According to structure, there are diffusion transistors, alloy transistors, etc.; According to capacity, there are small-power transistors, medium-power transistors, etc.; According to operating frequency, There are low-frequency transistor lamps and high-frequency transistor lamps; there are also different division methods.

In short, the classification of transistors, whether from the order of arrangement, or from the functional structure, or from the type of application, each type has its own characteristics, and the selection of the correct transistor depends on the specific circuit requirements and design needs, the correct choice of transistors is critical.

figure 2.Types of Transistors

What Is PNP and NPN?

Bipolar transistors are often referred to as triodes. There are two important classifications that need to be understood in detail: PNP and NPN.

The three-layer structure of the triode, in which the emitter is the outermost layer of the NPN tube, doped with N-type semiconductor material, is the output end of the NPN tube, and emits electrons to the base. The base is located in the middle area, is relatively thin, and is the control end of the NPN tube. The collector is the input end of the NPN tube and collects electrons from the base.

On the other hand, a PNP is the opposite of an NPN and is essentially the mirror image of an NPN.

Working Principle of PNP and NPN Transistor

The principle of operation of an NPN is to control the current in the emitter and collector by varying the base current, which is equivalent to presenting a positive bias when a positive voltage is applied from the base to the emitter, causing current to flow from the emitter to the base, but since the P-type material in an NPN (also known as the base) is very thin, only a small portion of this flow will go to the base, and most of the current will continue to flow through the base to the collector, and will be controlled by the base current.

In a PNP transistor, when a negative voltage is applied from the base to the emitter a positive bias is created, causing current to flow from the emitter to the base, but because the N-type material (also called the base) in a PNP is thin, most of the current will continue to flow through the base to the collector and will still be controlled by the base current, just as in an NPN.

figure 3.Working Principle of PNP and NPN Transistor

What Is The Difference Between PNP and NPN

Firstly, we need to understand what N and P stand for: N for negative and P for positive. PNP and NPN differ in many ways due to their internal ordering.

Different definitions: PNP is composed of an N-type semiconductor sandwiched between two P-type semiconductors, and NPN is composed of a P-type semiconductor in the middle and two N-type semiconductors on both sides.

Different directions of PN junction elements: The directions of the two PN junctions are inconsistent, PNP is a common cathode, and NPN is a common anode

Different structures: The emitter and collector of the NPN transistor are both N-type materials, the base is P-type material, and the electrons mainly flow from the emitter to the base; while the emitter and collector of the PNP are both P-type materials, the base is N-type material, and the electrons mainly flow from the emitter to the collector. In NPN, the current moves in the N-type region by electrons, and in PNP, the current moves in the P-type region by holes. Correspondingly, in NPN, holes play a secondary role in the movement of P-type regions, and in PNP, electrons play a secondary role in the movement of N-type regions.

Different current control: The current direction of the base of the NPN transistor flows from the base to the emitter, while the current of the PNP transistor flows from the emitter to the base;

Different output states: The output signals of the two are completely different, NPN is low level 0, and PNP is high level 1.

In terms of biasing, an NPN must be forward biased from base to emitter and reverse biased from base to collector, while a PNP must be reverse biased from base to emitter and forward biased from base to collector.

In terms of application, although NPN and PNP have similar functions and both perform similar basic tasks, such as the direction of current flow, proper circuit polarity, and circuit control methods, NPN is more cost-effective than PNP and is therefore more suitable for most simple circuit environments. PNP is less common than NPN and is suitable for some traditional relay-type control circuits, stabilizers, etc. However, in complex equipment, NPN transistor applications and PNP transistor applications can also be combined to use NPN and PNP in series. For example: When a capacitive switch is touched, the NPN controls the LED to turn on or off.

In conclusion, NPN and PNP have a lot of similarities and in many ways form a complementary situation, rational use of the differences between the two before the transistor can be used to maximise the role of the transistor.

figure 4.What Is The Difference Between PNP and NPN

How to Choose The Right  NPN and PNP Transistors

To ensure optimum performance, the correct transistor must be selected, and there are several factors to consider in selecting the correct transistor:

The first is the application environment, whether it is NPN or PNP, different environments require different characteristics of the transistor, the need for transistors to focus on different functions, so understand the application environment is better able to choose a good transistor, such as for sensitive applications, you need to take into account the noise characteristics of the transistor to minimize the impact of the environment on the transistor.

Secondly, the circuit conditions, the rated voltage and current of the circuit, as well as the frequency range of the circuit, is also one of the factors affecting the selection of transistors, both NPN and PNP need to be combined with the current and voltage conditions in order to stabilize the role, but also according to the application of high-frequency or low-frequency applications to select the appropriate transistor.

Thirdly, consider the cost, better features and lower price within the budget, in NPN and PNP, the former is less costly and more cost-effective compared to the latter.

In conclusion, choose an NPN transistor or a PNP transistor that must be suitable for your device, and by careful comparison and consideration, choose the right kind of transistor for your electronic circuit.

figure 5.How to Choose The Right  NPN and PNP Transistors

FAQ

 

Can NPN and PNP be used interchangeably?

Although the working principles are similar, their structures and bias directions are different, so they cannot be used interchangeably, but can be used in series.

 

Why is NPN more common than PNP?

NPN has higher performance and lower price than PNP, so more people choose it.

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