Transistors are the cornerstone of technology, enabling everything from basic circuit operations to the advanced functionalities of modern computing systems. To understand the operation of this electrical component, NPN transistor vs PNP transistor comparison is vital.

Introduction

Looking back a century or so ago, the strides we have made today might seem unimaginable. It is all due to our understanding of semiconductor materials that enabled us to get something like a transistor. By understanding the structure, operation, and applications of transistors, we can appreciate their importance in modern electronics.

It is a fundamental building block in modern electronic devices. These devices replaced the vacuum tubes in the first generation of computers and reduced the sizes of the second generation of computers to a great extent.

Besides this, transistors enabled advancements in numerous fields, including telecommunications, medical devices, consumer electronics, and even space exploration. They have had a transformative impact on the development of virtually all modern electronics.

What is a Transistor

A transistor is a semiconductor device, consisting of only a crystal of silicon or germanium and is used to amplify or switch electronic signals and electrical power.

Terminals of Transistor

There are mainly three terminals or regions in a transistor.

Emitter (represented as E)
Base (represented as B)
Collector (represented as C)

Doping Level of Terminals

The concentration of doping is different in different terminals of a transistor. For instance,

Emitter (E) is doped with a high concentration of impurities to increase the injection of carriers (holes/electrons).
Base (B) is lightly doped to minimise the recombination of carriers within the base.
Collector (C) is moderately doped to allow efficient carrier collection and high-voltage handling.

Size of Each Terminal

The three terminals or regions of a transistor are of different sizes. This difference in size is of great value in a transistor. The order of size is as follows,

Collector (C) > Emitter (E) > Base

Emitter (E) is smaller in size and efficiently injects carriers.
Base (B) is the smallest or thinnest in size ( $\approx 10^{-4} \text{ m}$ ) and ensures efficient control over carrier flow.
Collector (C) is the largest of all and accumulates carriers.

Role of Each Terminal

Each terminal in a transistor serves a specific task. For instance,

Emitter (E) plays the role of a source in the circuit.
Base (B) controls the flow of carriers between the emitter and collector.
Collector (C) helps to dissipate heat.

Types of Transistor

Based on the construction, there are various types of transistors. These include,

Bipolar Junction Transistor (BJT)
Field Effect Transistor (FET)
Darlington Transistor
Phototransistor
Unijunction Transistor (UJT)

Here, we shall only discuss about BJTs.

Bipolar Junction Transistors (BJTs)

A BJT is a type of transistor that uses both electrons and holes as charge carriers. It is widely employed in amplifiers and switching circuits.

Classification of BJTs

BJTs are further divided into two main types.

NPN Transistor
PNP Transistor

What Type of Biasing is Present in BJTs?

In both NPN transistor and PNP transistor, the base-emitter junction is always forward-biased, and the base-collector junction is always reverse-biased in active mode.

1. NPN Transistor

In an NPN transistor, a thin p-type semiconductor layer is sandwiched between two n-type layers.

Current Relationships in NPN Transistors

In this configuration, the hierarchy of currents follows this pattern:

$I_E > I_C > I_B$

Here,

$I_E$ = emitter current
$I_C$ = collector current
$I_B$ = base current

It is given as,

$I_E = I_C + I_B$

This equation is called the fundamental equation of the transistor. A detailed breakdown of all these currents is given below.

i. Emitter Current ( $I_E$ )

The emitter in an NPN transistor is the source of the majority of the current. It supplies electrons (majority carriers) to the base.
Most of these electrons are swept into the collector, but a small fraction flows into the base.

ii. Collector Current ( $I_C$ )

The collector current is slightly smaller than the emitter current but much larger than the base current.
This is because the collector receives nearly all the electrons injected by the emitter, except for the small fraction used by the base for control purposes.

iii. Base Current ( $I_B$ )

The base current is the smallest of the three. It is only a small fraction of the emitter current ( $I_B \approx \frac{1}{\beta} I_C$ ,where $\beta$ is the current gain of the transistor).
This small current controls the much larger collector current, allowing the transistor to amplify signals.

Current Gain ( $\beta$ )

It is defined as,

“the ratio between collector current $I_C$ to the base current $I_B$ .”

Mathematically, it is represented as;

$\beta = \frac{I_C}{I_B}$

It is a large quantity and is of the order of hundreds.

Operation

A small current flowing into the base allows a much larger current to flow from the collector to the emitter. The base-emitter junction is forward-biased, while the base-collector junction is reverse-biased. Electrons flow from the emitter to the collector through the base, creating the amplification effect.

Key Features

Suitable for high-speed and high-power applications.
Commonly used in digital and switching circuits.

2. PNP Transistor

In a PNP transistor, a thin n-type semiconductor layer is sandwiched between two p-type layers.

Current Relationships in PNP Transistors

For a PNP transistor, the order and relationships of current remain the same, but the directions of current and the type of majority carriers (holes instead of electrons) are reversed. The correct relationship remains:

$I_E = I_C + I_B$

Here,

$I_E > I_C > I_B$

Operation

A small current flowing out of the base enables a larger current to flow from the emitter to the collector. The base-emitter junction is forward-biased, while the base-collector junction is reverse-biased. Holes move from the emitter to the collector through the base, enabling amplification.

Key Features

Operates effectively at lower voltages.
Typically used in low-power and analog circuits.

Why Do $I_E > I_C$ in a Transistor?

The reason the emitter current is always greater than the collector current lies in Kirchhoff’s Current Law and the nature of transistor operation. As we know, in a transistor;

$I_E = I_C + I_B$

Since, $I_B > 0$ but extremely small. Hence,

$I_E > I_C$

This shows that a small portion of the emitter current ( $I_B$ ) is lost to the base. Hence, not all of the emitter current reaches the collector.

NPN Transistor vs PNP Transistor Comparison

Revolutions Brought by Transistor

Here are a few key areas where transistors have made a significant difference:

Miniaturisation: Transistors have allowed electronic devices to become much smaller and more portable. This miniaturisation has been crucial for the development of smartphones, laptops, and wearable technology.
Energy Efficiency: Transistors consume far less power than vacuum tubes, leading to more energy-efficient devices. This is especially important in devices that rely on battery power, like mobile phones and laptops.
Speed and Performance: Transistors can switch on and off much faster than vacuum tubes, greatly increasing the processing speeds of computers. This has been fundamental in the advancement of computing power, allowing for complex calculations and data processing.
Cost Reduction: As the production of transistors became more efficient, the cost of electronic components significantly dropped. This made electronic devices more affordable and accessible to the general public.
Reliability: Unlike vacuum tubes, which were prone to burnout and failure, transistors are much more reliable and durable, contributing to longer device lifespans.
Integration: The invention of the integrated circuit (IC) took advantage of transistors, allowing thousands or even millions of transistors to be embedded in a single chip. This paved the way for modern computers, smartphones, and microelectronics.

Through their diverse applications, transistors have shaped the technological landscape of the modern world. It powers everything from everyday gadgets to advanced scientific equipment.

Conclusion

Transistors are the fundamental building block of modern electronics. They have revolutionised technology in ways that appeared unimaginable a century ago.

Their ability to amplify and switch electrical signals has made them crucial in countless applications, from everyday devices like smartphones and laptops to complex systems used in telecommunications and space exploration.

By replacing bulky vacuum tubes, transistors enabled the miniaturisation of electronic circuits, boosting performance, energy efficiency, and reliability.

Understanding the operations of different transistor types, like NPN and PNP, helps us grasp the significant impact they have on both the practical and theoretical aspects of modern technology.

Frequently Asked Questions (FAQs)

What is a transistor?

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is a key component in modern electronic devices.

What are the main types of transistors?

The main types include Bipolar Junction Transistors (BJTs), Field Effect Transistors (FETs), Darlington Transistors, Phototransistors, and Unijunction Transistors. BJTs are further classified into NPN and PNP transistors.

What is the difference between NPN and PNP transistors?

In an NPN transistor, electrons are the majority carriers, whereas in a PNP transistor, holes are the majority carriers. This results in reversed current directions for the two types.

What are the key components of a transistor?

A transistor consists of three terminals: the emitter (E), the base (B), and the collector (C), each with different doping levels and sizes that influence their function.

Why is the emitter current always greater than the collector current?

The emitter current ( $I_E$ ) is always greater because a small portion of the emitter current is diverted to the base ( $I_B$ ), as per Kirchhoff’s Current Law. The relationship is given by,

$I_E = I_C + I_B$

What is current gain ( $\beta$ ) in a transistor?

Current gain ( $\beta$ ) is the ratio of the collector current ( $I_C$ ) to the base current ( $I_B$ ), typically a large value in the hundreds, indicating the amplification capability of the transistor.

How does an NPN transistor work?

In an NPN transistor, when a small current flows into the base, it allows a much larger current to flow from the collector to the emitter, amplifying the signal.

What are the main applications of transistors?

Transistors are used in amplifiers, switching circuits, digital logic, and as components in virtually all modern electronic devices, including computers, smartphones, and medical equipment.

NPN Transistor vs PNP Transistor Comparison | 101 Basics of Transistors Simplified

Table of Contents

Introduction

What is a Transistor

Terminals of Transistor

Doping Level of Terminals

Size of Each Terminal

Role of Each Terminal

Types of Transistor