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Mar 15 2018

What is Transistor and Its Functions and Characteristics

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


This article will mainly introduce what exactly a transistor is and its detailed characteristics and functions. 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, transistor can control the output current based on the input voltage. Unlike general mechanical switches (such as relays ans 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.

Article Core

Introduction to transistors


Introduce what is transistor and its functions and characteristics

English name



Discrete Semiconductor Products  


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


High response and high accuracy


I.What is transistor?4. Silicon transistor1. how to classify the transistor3. Dissipation Power
II. Development of Transistors5. Integrated circuits2. Types of Transistor and their characteristics4.Characteristic Frequency (fT)
1. Vacuum Triode6. Field effect transistor (FET) and MOS transistorIV. Main Parameters of Transistors5.Maximum Frequency (fM)
2. Point Contact Transistors7. Microprocessor (CPU)1. DC Current Amplification Factor6. Maximum Collector Current (ICM)
3. Bipolar and Unipolar TransistorsIII. Classification of Transistor2. AC Current Amplification Factor7. Maximum Reverse Voltage


I.What is transistor?

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

Because of its 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).

structure of transistor

structure of transistor

The transistor has three poles: the three poles of 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: 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 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 early 1910s, communications systems have begun to use semiconductors. The first half of the 20th century, ore radios that are widely popular in radio lovers are used for detection by using such semiconductors. The electrical properties of semiconductors have also applied in telephone systems.

  • 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 Purdue University research group led by Lark Horovitz found that germanium single crystals have excellent rectifying properties that other semiconductors do not. These two discoveries met the requirements of the United States government and set the stage for the subsequent invention of transistors.

  • 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.

  • 3. Bipolar and Unipolar Transistors

In 1952, Shockley further proposed the concept of unipolar junction transistor based on 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.

  • 4. Silicon transistor

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

  • 5. Integrated circuits

After the invention of silicon transistor 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.

  • 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, that is, MOS transistors, could be constructed by diffusion and thermal oxidation of conducting bands, high resistance channels and oxide insulators on Si substrates.

  • 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. PentiumII processor is released in 1997. In 1999, the Pentium III processor is released, and the Pentium 4 processor is in 2000.

III. Classification of Transistor

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

  • 2. Types of Transistor 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 nanosecond, which is often used as photodetector and optical amplification. Field-effect phototransistors (FET) respond quickly (about 50 picoseconds), but the disadvantage is that photosensitive area and gain is small, which is often used as a 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. This kind of devices are expected to be applied in optoelectronic integration.

> Bipolar Transistor

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 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, that is, 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. That is to say, 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 semiconductor material, there are silicon and germanium tubes, etc. The amplifier circuit consists of 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 that 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 transistor that can record a signal with one or a small amount of electrons. With the development of semiconductor etching technology, the integration of large-scale integrated circuits is becoming more and more high. 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 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. 

> Insulate Gate Bipolar Transistor (IGBT)

Insulate Gate Bipolar Transistor combines the advantages of Giant Transistor-GTR and Power MOSFETs. It has good properties and has 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.

  • 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.

  • 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. hfe and β are closely related but also different. The two parameters are close at low frequency and have some differences at high frequency.

  • 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.Characteristic Frequency (fT)

When the operating frequency of 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.

  • 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.

  • 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.

  • 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.

Reverse current between collector and base electrode

> 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.

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

Relevant information about "What is transistor as well as its function and characteristic"

About the article " What is transistor as well as its function and characteristic", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.


    • Cezar on 2018-3-26 14:15:38

    Good day~sir,thank you for you excellent page. I want to do a set of testing devices that require a stable phototransistor to receive light signals. But the temperature change of our environment will change from room temperature to -60 C, so is there any stable photoelectric transistors? The luminescent body is also affected by the change of temperature. Could you recommend it! ? Esteem it a favor。

      • author on 2018-3-26 14:23:36


      The phototransistors can be connected to the external environment by optical fiber in the device, so it's not necessary to directly withstand low temperature devices.

    • Cezar on 2018-3-26 14:25:02

    @author what do you mean? sir Cezar

      • author on 2018-3-26 14:26:08


      Try to give a thermostat to the phototube, for example, at a constant temperature of 50. It can be coupled through a medium such as a passive way.

    • Mehran on 2018-3-26 14:28:48

    The article is clear. I am confusing about that CPU is a collection of switches. These switches are transistors. If one of them is broken, can CPU work properly? by Mehran

      • author on 2018-3-26 14:36:18


      CPU is composed of hundreds of millions of transistors, each transistor will have its function, and each tube where the unit will affect each execution outcomes such as an arithmetic logic unit ALU transistor error, CPU performs arithmetic and logical operations are the inevitable result is wrong, even if other parts like registers and the normal controller, feedback to the memory or IO are incorrect results. The pipe you say is bad if it is understood that the triode is not guided and knocked out of two kinds. No guidance may also feed back to you a bit of wrong information. But this situation is almost nonexistent. On the IC, the triode is mostly broken down, which may be a large chain of breakdown, and the tube can't work directly.

    • chant on 2018-3-26 14:34:37

    @Mehran The transistor with the electronic tube is not the same, the situation is not exsit that just a bad transistor, even if there will be a single transistor broken, I believe there is no mature design of the CPU in any one transistor is not essential, in other words, the emergence of a transistor broken case this chip is useless.

    • mayye on 2018-3-26 14:38:58

    There are three operating states of a transistor 1. Normal amplification of transistors 2.Transistor cut-off distortion 3. Transistor saturation distortion

    • asgs on 2018-4-9 15:53:20

    vivid !

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