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What is Transistor and Its Functions and Characteristics[Video]

Author: Apogeeweb
Date: 15 Mar 2018
function of transistor

Warm hints: The word in this article is about 3200 words and the reading time is about 15 minutes.




I What is Transistor?

II Development of Transistors

 2.1 Vacuum Triode

 2.2 Point Contact Transistors

 2.3 Bipolar and Unipolar Transistors

 2.4 Silicon transistor

 2.5 Integrated circuits

 2.6 Field effect transistor (FET) and MOS transistor

 2.7 Microprocessor (CPU)

III Classification of Transistor

 3.1 How to classify the transistor

 3.2 Types of transistor and their characteristics

IV Main Parameters of Transistors

 4.1 DC Current Amplification Factor

 4.2 AC Current Amplification Factor

 4.3 Dissipation Power

 4.4 Characteristic Frequency (fT)

 4.5 Maximum Frequency (fM)

 4.6 Maximum Collector Current (ICM)

 4.7 Maximum Reverse Voltage

Frequently Asked Questions about Transistor and Its Functions and Characteristics

Book Suggestion


This article will mainly introduce what exactly a transistor is and its detailed characteristics and functions. A transistor is a kind of solid semiconductor device, which has many functions, such as detecting, rectifying, amplifying, switching, voltage stabilizing, signal modulating and so on. As a variable current switch, the transistor can control the output current based on the input voltage.


Unlike general mechanical switches (such as relays and switches), transistors use telecommunication signals to control their switching on and off, and the switching speed can be very fast, which can reach more than 100 GHz in the laboratory. In 2016, a team at Lawrence Berkeley National Laboratory broke the physical limit and cut the most sophisticated transistor process available from 14nm to 1nm, making a breakthrough in computing technology.

What is a Transistor? Definition, Function & Uses

Article Core

Introduction to transistors


Introduce what is a transistor and its functions and characteristics

English name



Discrete Semiconductor Products


Used as a detector, rectifier, amplifier, switch, voltage stabilizer, signal modulation


High response and high accuracy

I What is Transistor?

Transistors are semiconductor devices that are commonly used in amplifiers or electrically controlled switches. Transistors are the basic building block that regulates the operation of computers, mobile phones, and all the other modern electronic circuits.


Because of their high response and high accuracy, transistors can be used for a wide variety of digital and analog functions, including amplifiers, switches, voltage stabilizers, signal modulation and oscillators. Transistors can be packaged independently or in a very small area, accommodating part of 100 million or more transistor integrated circuits.

(Intel 3D transistor technology)

(Intel 3D transistor technology)

Strictly speaking, transistors refer to all single elements based on semiconductor materials, including diodes, transistors, field-effect transistors, thyristors, etc. which made from various semiconductor materials. Transistors mostly refer to crystal triode.

Transistors are divided into two main categories: bipolar transistors (BJT) and field-effect transistors (FET).

Transistor Structure

structure of transistor

The transistor has three poles: the three poles of the bipolar transistor are composed of N-type and P-type respectively: Emitter, Base and Collector; the three poles of field effect transistor are: Source, Gate, Drain


Due to the three polarities of the transistor, there are also three ways of using them: the grounded emitter (also called common emission amplifier/CE configuration), grounded base (also called common base amplifier / CB configuration) and grounded collector (also called common set amplifier / CC configuration/Emitter coupler).

II Development of Transistors

In December 1947, a team of Belle Labs, Shockley, Barding and Bratton, developed a point-contact germanium transistor, the advent of which was a major invention in the 20th century and the forerunner of the Microelectronics Revolution. With the advent of transistors, people were able to use a small, low-power electronic device instead of a tube with large volume and large power consumption. The invention of the transistor sounded the horn for the birth of the integrated circuit.


In the early 1910s, communications systems have begun to use semiconductors. In the early 1910s, communications systems have begun to use semiconductors. In the first half of the 20th century, ore radios were widely popular among radio lovers. They are used for detection by using such semiconductors. The electrical properties of semiconductors have also applied in telephone systems.

  • 2.1 Vacuum Triode

In February 1939, there is a great discovery of Bell laboratory ---- the silicon PN junction. In 1942, a student named Seymour Benzer of the Purdue University research group led by Lark Horovitz found that germanium single crystals have excellent rectifying properties that other semiconductors do not have. These two discoveries met the requirements of the United States government and set the stage for the subsequent invention of transistors.

  • 2.2 Point Contact Transistors

In 1945, the point-contact transistor invented by Shockley and other scientists became the forerunner of the human microelectronic revolution. For this reason, Shockley submitted the patent application for the first transistor for Bell. Finally, he obtained the authorization of the first transistor patent.

  • 2.3 Bipolar and Unipolar Transistors

In 1952, Shockley further proposed the concept of unipolar junction transistor based on the bipolar transistor in 1952, which is called junction transistor today. Its structure is similar to that of PNP or NPN bipolar transistor, but there is a depletion layer at the interface of PN material to form a rectifier contact between the gate and the source-drain conductive channel. At the same time, the semiconductor at both ends is used as the gate. The current between the source and drain is adjusted by the gate.

A detailed look at how an NPN bipolar junction transistor works and what it does


  • 2.4 Silicon transistor

Fairy Semiconductor that produces transistors has grown from a company of several people into a large company with 12,000 employees.

  • 2.5 Integrated circuits

After the invention of silicon transistors in 1954, the great application prospect of transistors has been more and more obvious. The next goal of scientists is to further connect transistors, wires and other devices efficiently.

  • 2.6 Field effect transistor (FET) and MOS transistor

In 1962, Stanley, Heiman and Hofstein, who worked in the RCA device Integration Research Group, found that transistors, MOS transistors, could be constructed by diffusion and thermal oxidation of conducting bands, high resistance channels and oxide insulators on Si substrates.

  • 2.7 Microprocessor (CPU)

At the beginning of Intel's founding, the company still focused on memory bars. Hoff integrated all the central processor functions on a single chip, plus memory. And it is the world's first microprocessor----4004 (1971). The birth of 4004 is marking the beginning of an era. From then on, Intel has become uncontrollable and dominant in the field of microprocessor research.

In 1989, Intel introduced 80486 processors. In 1993, Intel developed a new generation of processors. And in 1995, Intel released Pentium_Pro. Pentium II processor is released in 1997. In 1999, the Pentium III processor is released, and the Pentium 4 processor is released in 2000.

III Classification of Transistor

  • 3.1 How to classify the transistor

> Material used in the transistor

According to the semiconductor materials used in the transistor, it can be divided into silicon transistor and germanium transistor. According to the polarity of the transistor, it can be divided into germanium NPN transistor, germanium PNP transistor, silicon NPN transistor and silicon PNP transistor.


> Technology

According to their structure and fabrication process, transistors can be divided into diffusive transistors, alloy transistors and planar transistors.


> Current capacity 

According to current capacity, transistors can be divided into low-power transistors, medium-power transistors and high-power transistors.


> Operating frequency

According to operating frequency, transistors can be divided into low-frequency transistors, high-frequency transistors and ultra-high-frequency transistors.


> Package structure

According to the packaging structure, transistors can be divided into metal packaging transistors, plastic packaging transistors, glass shell packaging transistors, surface packaging transistors and ceramic packaging transistors, etc.


> Functions and usages

According to the functions and usages, transistors can be divided into low noise amplifier transistors, medium-high frequency amplifier transistors, switching transistors, Darlington transistors, high back voltage transistors, band-stop transistors, damping transistors, microwave transistors, optical transistor and magnetic transistor and many other types.

  • 3.2 Types of transistors and their characteristics

> Giant Transistor (GTR)

GTR is a high voltage, high current bipolar junction transistor (BJT), so it is sometimes called power BJT. 

Features: High voltage, high current, good switching characteristics, high driving power, but the driving circuit is complex; The working principle of GTR and ordinary bipolar junction transistors is the same.


> Phototransistor

Phototransistors are optoelectronic devices consisting of bipolar transistors or field-effect transistors. Light is absorbed in the active region of such devices, producing photo-generated carriers that pass through an internal electrical amplification mechanism and generate photocurrent gain. Phototransistors work at three ends, so they are easy to realize electronic control or electrical synchronization.


The materials used in phototransistors are usually GaAs, which are mainly divided into bipolar phototransistors, field-effect phototransistors and their related devices. Bipolar phototransistors usually have a high gain, but not too fast. For GaAs-GaAlAs, the magnification factor can be greater than 1000, the response time is longer than a nanosecond, which is often used as a photodetector and optical amplification.


Field-effect phototransistors (FET) respond quickly (about 50 picoseconds), but the disadvantage is that photosensitive area and gain are small, which is often used as an ultra-high-speed photodetector. There are many other planar optoelectronic devices associated, whose features are high response speed (response time is tens of picosecond) and are suitable for integration. These kinds of devices are expected to be applied in optoelectronic integration.


> Bipolar Transistor

A bipolar transistor is a kind of transistor commonly used in audio circuits. The bipolar results from the flow of current in two kinds of semiconductor materials. Bipolar transistors can be divided into NPN type or PNP type according to the polarity of operating voltage.


> Bipolar Junction Transistor (BJT)

"Bipolar" means that both electrons and holes are in motion at the same time they work. Bipolar Junction Transistor, also known as semiconductor triode, is a device that combines two PN junctions through a certain process. There are two combined structures of PNP and NPN. External elicitation of three poles: collector, emitter and base. BJT has an amplification function, which depends on its emitter current can be transmitted through the base area to the collector area.


In order to ensure this transport process, on the one hand, the internal conditions should be satisfied. It means that the impurity concentration in the emission region should be much larger than the impurity concentration in the base region, and the thickness of the base area should be very small. On the other hand, the external conditions should be satisfied. It means that the emission junction should be positive bias (plus positive voltage), and the collector junction should be inversely biased. There are many kinds of BJT, according to frequency, there are high and low-frequency tubes; according to power, there are small, medium and high power tubes; according to the semiconductor material, there are silicon and germanium tubes, etc. The amplifier circuit consists of a common emitter, common base and common collector.



> Field Effect Transistor (FET)

The meaning of "field effect" is that the principle of the transistor is based on the electric field effect of the semiconductor.


Field effect transistors are transistors that work on the principle of field effects. There are two main types of FET: Junction FET (JFET) and Metal-Oxide Semiconductor FETs (MOS-FET). Unlike BJT, FET consists of only one carrier, so it's also called a unipolar transistor. It belongs to voltage-controlled semiconductor devices which have the advantages of high input resistance, low noise, low power consumption, wide dynamic range, easy integration, no secondary breakdown, wide safe working area and so on.


The field effect is to change the direction or magnitude of the electric field perpendicular to the surface of the semiconductor to control the density or type of most carriers in the semiconductor conducting layer (channel). The current in the channel is modulated by voltage, and the working current is transported by most carriers in the semiconductor. Compared with bipolar transistors, FET is characterized by high input impedance, low noise, high limit frequency, low power consumption, simple manufacturing process and good temperature characteristics, which are widely used in various amplifiers, digital circuits and microwave circuits, etc. Metal MOSFETs based on silicon and Schottky barrier FET (MESFET) based on GaAs are two of the most important field-effect transistors. They are the basic devices of MOS large-scale integrated circuit and MES ultra high speed integrated circuit respectively. 



> Single Electron Transistor

A Single Electron Transistor is a transistor that can record a signal with one or a small number of electrons. With the development of semiconductor etching technology, the integration of large-scale integrated circuits is becoming higher and higher. Take dynamic random access memory (DRAM) as an example, its integration is growing at a rate of almost four times every two years, and it is expected that the single-electron transistor will be the ultimate goal.


At present, the average memory contains 200, 000 electrons, while the single electron transistor contains only one or a few electrons, so it will greatly reduce the power consumption and improve the integration of integrated circuits. In 1989, J.H. F.Scott-Thomas and others researchers discovered Coulomb Blocking Phenomenon. When there is a voltage applied, there will be no current passing through the quantum dot if the change in the amount of electric charge in a quantum dot that is less than one electron.


So the current-voltage relationship is not a normal linear relationship, but a step-shaped one. This experiment is the first time in history that the motion of an electron is controlled manually, which provides the experimental basis for the fabrication of a single electron transistor. 


> Insulated Gate Bipolar Transistor (IGBT)

Insulated Gate Bipolar Transistor combines the advantages of Giant Transistor-GTR and Power MOSFETs. It has good properties and a wide range of applications. IGBT is also a three-terminal device: gate, collector and emitter.

IV Main Parameters of Transistors

The main parameters of the transistor include current amplification factor, dissipation power, characteristic frequency, maximum collector current, maximum reverse voltage, reverse current and so on.

  • 4.1 DC Current Amplification Factor

DC current amplification factor, also called static current amplification factor or DC amplification factor, refers to the ratio of IC of transistor collector current to base current IB, which is usually expressed by hFE or β, when the static signal input is not changed.

  • 4.2 AC Current Amplification Factor

AC current amplification factor, also called AC amplification factor and dynamic current amplification factor, refers to the ratio of IC to IB in AC state, which is usually expressed by hFE or β. The hFE and β are closely related but also different. The two parameters are close at low frequency and have some differences at high frequency.

  • 4.3 Dissipation Power

Dissipation power, also known as the maximum allowable dissipation power of collector ---- PCM, refers to the maximum dissipation power of collector when the parameter of transistor does not exceed the prescribed allowable value.


The dissipation power is closely related to the maximum allowable junction and collector current of the transistor. The actual power consumption of the transistor is not allowed to exceed the PCM value when it is used, otherwise, the transistor will be damaged by overload.


The transistor whose dissipation power PCM is less than 1W is usually called a low power transistor, which is equal to or greater than 1W. The transistor less than 5W is called a middle power transistor, and the transistor whose PCM is equal to or greater than 5W is called a high-power transistor.

  • 4.4 Characteristic Frequency (fT)

When the operating frequency of the transistor exceeds the cutoff frequency fβ or fα, the current amplification factor β will decrease with the increase of frequency. The characteristic frequency is the frequency of the transistor at which the β value is reduced to 1.


The transistors whose characteristic frequency is less than or equal to 3MHZ are usually called low-frequency transistors. Transistors with fT greater than or equal to 30MHZ are called high-frequency transistors. Transistors with fT greater than 3MHZ and transistors less than 30MHZ are called intermediate frequency transistors.

  • 4.5 Maximum Frequency (fM)

The maximum oscillation frequency is the frequency at which the power gain of the transistor is reduced to 1.


In general, the maximum oscillation frequency of high-frequency transistors is lower than the common base cutoff frequency fα, while the characteristic frequency fT is higher than the common base cutoff frequency fα and lower than the common collector cutoff frequency fβ.

  • 4.6 Maximum Collector Current (ICM)

Maximum collector current (ICM) is the maximum current allowed through the transistor collector. When the collector current IC of the transistor exceeds ICM, the β value of the transistor will change obviously, which will affect its normal operation and even cause damage.

  • 4.7 Maximum Reverse Voltage

The maximum reverse voltage is the maximum operating voltage that the transistor is allowed to apply when it is in operation. It includes the collector-emitter reverse breakdown voltage, the collector-base reverse breakdown voltage and the emitter-base reverse breakdown voltage.

> Collector - Collector Reverse Breakdown Voltage

This voltage refers to the maximum allowable reverse voltage between the collector and emitter when the base circuit of the transistor is open, usually expressed in VCEO or BVCEO.


> Base - Base Reverse Breakdown Voltage

The voltage refers to the maximum allowable reverse voltage between the collector and the base when the transistor is fired, which is expressed in VCBO or BVCBO.


> Emitter - Emitter Reverse Breakdown Voltage

This voltage refers to the maximum allowable reverse voltage between the emitter and the base when the collector of the transistor is open, which is expressed in VEBO or BVEBO.


> Collector - Base Reverse Current (ICBO)

ICBO, also called collector reverse leakage current, refers to the reverse current between collector and base electrode when the emitter of the transistor is open. The reverse current is sensitive to temperature. The smaller the value is, the better the temperature characteristic of the transistor is.


> Collector - Emitter Reverse Breakdown Current (ICEO)

Reverse breakdown current ICEO between collector and emitter

ICEO is the reverse leakage current between the collector and emitter when the base of the transistor is open. The smaller the current is, the better the performance of the transistor is.


Frequently Asked Questions about Transistor and Its Functions and Characteristics

1. What is a transistor and how does it work?
A transistor is a miniature electronic component that can do two different jobs. It can work either as an amplifier or a switch: ... A tiny electric current flowing through one part of a transistor can make a much bigger current flow through another part of it. In other words, the small current switches on the larger one.


2. What are the main functions of a transistor?
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. Transistors are one of the basic building blocks of modern electronics. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.


3.What is the principle of transistor?
A transistor consists of two PN diodes connected back to back. It has three terminals namely emitter, base and collector. The basic idea behind a transistor is that it lets you control the flow of current through one channel by varying the intensity of a much smaller current that's flowing through a second channel.


4. What are the two main types of transistors?
Transistors are basically classified into two types; they are Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). The BJTs are again classified into NPN and PNP transistors.


5. How many types of transistors are there?
two types
There are two types of transistors, which have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter.


6. What is PNP and NPN transistor?
In an NPN transistor, a positive voltage is given to the collector terminal to produce a current flow from the collector to the emitter. In a PNP transistor, a positive voltage is given to the emitter terminal to produce current flow from the emitter to collector.


7. How transistors are measured characteristics?
The output characteristic of the transistor is determined by examining the change of the voltage between the collector-emitter terminals belonging to the collector current for different base currents. The experiment is started by pressing the “Output Characteristic” button on the mobile device.


8. What is a transistor in a CPU?
A transistor is a basic electrical component that alters the flow of electrical current. Transistors are the building blocks of integrated circuits, such as computer processors, or CPUs. Transistors in computer processors often turn signals on or off.


9. What is the purpose of an NPN transistor?
Definition: The transistor in which one p-type material is placed between two n-type materials is known as NPN transistor. The NPN transistor amplifies the weak signal enter into the base and produces strong amplify signals at the collector end.


10. What are transistors used for in a mobile phone?
They store an electric charge. They store data. They amplify the phone's incoming signal.

Book Suggestion

  • Transistor Circuit Techniques: Discrete and Integrated (Tutorial Guides in Electronic Engineering)

Thoroughly revised and updated, this highly successful textbook guides students through the analysis and design of transistor circuits. It covers a wide range of circuitry, both linear and switching.Transistor Circuit Techniques: Discrete and Integrated provides students with an overview of fundamental qualitative circuit operation, followed by an examination of analysis and design procedure. It incorporates worked problems and design examples to illustrate the concepts. This third edition includes two additional chapters on power amplifiers and power supplies, which further develop many of the circuit design techniques introduced in earlier chapters. Part of the Tutorial Guides in Electronic Engineering series, this book is intended for first and second year undergraduate courses. A complete text on its own, it offers the added advantage of being cross-referenced to other titles in the series. It is an ideal textbook for both students and instructors.

--Gordon J. Ritchie

  • Build Your Own Transistor Radios: A Hobbyist''s Guide to High-Performance and Low-Powered Radio Circuits

Create sophisticated transistor radios that are inexpensive yet highly efficient. Build Your Own Transistor Radios: A Hobbyist’s Guide to High-Performance and Low-Powered Radio Circuits offers complete projects with detailed schematics and insights on how the radios were designed. Learn how to choose components, construct the different types of radios, and troubleshoot your work. Digging deeper, this practical resource shows you how to engineer innovative devices by experimenting with and radically improving existing designs.

--Ronald Quan

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