BJT Transistor

Bipolar Junction Transistor is a three layer semiconductor device, which has a wide range of application in the semiconductor industries. Three layers Emitter, Base and Collector of a BJT transistor are formed by sandwiching alternative P and N layers. Emitter is a heavily doped region of the BJT transistor which provides majority carriers into the base region. Base region is a thin, lightly doped region and is sandwiched between emitter and collector. Majority carriers from the emitter pass through the base region and its flow can be externally controlled. Collector region is moderately doped. Majority carriers from emitter are finally collected at the collector region of the BJT transistor.

A BJT transistor can be constructed in two ways. In one method, N layer is sandwiched between two P layers called PNP transistor and in the other one, the P layer is sandwiched between two N layers called NPN transistor.



Structure and Symbol of Bipolar Junction Transistor - NPN Transistor, PNP Transistor

Structure and circuit symbol of PNP transistor and NPN transistor are shown in the above figures. In NPN transistor, electrons are the majority carriers and in PNP transistor, holes are the majority carriers. Mobility of electrons is higher than that of holes and therefore, NPN transistor are more preferred in electronic circuits. All other properties of NPN transistor and PNP transistor are same.

Working of BJT Transistor

Bipolar Junction Transistor can be defined as a three terminal current controlled device. Working of a BJT transistor is analogous to a water tap. Quantity of water flowing through the pipe can be controlled by moving the knob of the tap. Similarly, majority carrier flow from emitter to collector can be controlled by base current. The controlled power can be higher than that of the input controlling power. Therefore, a BJT transistor can be used as an amplifier.

Working of BJT

                                                                                                                                                                                 Working of BJT Transistor - NPN Transistor

The figure shows the most common configuration of NPN transistor. Here, Emitter-Base junction is forward biased and Collector-Base junction is reverse biased. That is, for a NPN transistor, Base is more positive with respect to Emitter and Collector is more positive with respect to Base. When the circuit is turned on, higher potential at the base region of the NPN transistor pulls the electrons from the emitter of NPN transistor towards it. The electron flow that reaches the base region of the NPN transistor is again pulled by the more positive voltage of the collector region of the NPN transistor. Only very little current flows out though the base terminal of  NPN transistor , since it is very thin and lightly doped, so the downward movement of electrons is restricted by its higher resistance (for downward movement, large length and small area is required).

Current Components of BJT Transistor

When BJT transistor is not biased, that is, there is no voltage drop across its junctions and thus, no current flows through it. If Emitter-Base junction is forward biased and Collector-Base junction is reverse biased, the voltage across the device causes electrons from emitter to flow to collector. In this, electrons pass through P type lightly doped base region and some of the electrons recombine with holes. Therefore, collector current is less than that of emitter current. Emitter current, Base current and Collector current can be related by.  

Emitter current = Base current + Collector current

Mainly three parameters are used to define BJT transistor performance. Current Amplification factor, Base Transport Factor, Emitter Injection Efficiency parameters shows the performance of NPN transistor and PNP transistor.

(a). Current Amplification Factor

Current amplification factor in a BJT transistor is defined as the ratio of output current to its input current. In a common base configuration, current amplification factor is the ratio of collector current to the emitter current.

α = Ic / Ie

(b). Base Transport Factor

It is defined as the factor of base current required to transfer emitter current to collector of the BJT transistor. Base transport factor is the ratio of collector current to base current of a BJT transistor. That is, it is the ratio of output current to input current in common emitter configuration.

β = Ic / Ib

(c). Emitter Injection Efficiency

Emitter injection efficiency in a BJT transistor defines the efficiency of majority carrier injection from emitter. It is the ratio of current due to emitter majority carriers to the total emitter current. It defines the injection capability of an emitter. Heavily doped region will have high injection factor.   


Modes of operation of BJT Transistor

BJT Transistor can be modeled as two PN junctions connected back to back. Depending on the application, each junction can be forward or reverse biased independently. Thus there are four different biasing methods.

(a). Forward-active Mode

In forward active mode of BJT transistor Emitter-Base junction is forward biased and collector base junction is reverse biased. When transistor operates in this mode, collector current increases linearly with the increase in base current. Therefore when BJT transistor is used as an amplifier it is biased to operate in active mode.

(b). Reverse-active Mode

Reverse active mode is also called inverse active because its biasing conditions are just opposite to that of forward active mode. That is emitter-Base junction is reverse biased and Collector-Base junction is forward biased.

BJT Transistor is symmetrical and so if bias conditions are inversed the emitter and collector of a transistor gets interchanged and current flows in the opposite direction. Because of the difference in the doping concentration and size of the collector and emitter region, current gain of the transistor is two to three times less than that in the forward active mode.

(c). Saturation Mode

In saturation mode both Emitter-Base and Collector-Base junctions are forward biased. Maximum current flows through the transistor because in both junctions depletion width is very narrow. BJT Transistor behaves like a closed switch in this mode.

(d). Cut- off Mode

Both Emitter-Base and Collector-Base junctions are reverse biased in cut-off mode. In this mode transistor is inactive that is no current flows from emitter to collector. Transistor behaves like open switch in cut-off mode.

The three transistor configuration modes are analogous to the movement of a car. Forward active mode is when the car is moving at an average speed and its speed can by controlled by accelerator liver. Similarly current flow in the transistor in forward active mode is controlled by the base current.

Cut off mode is when the engine to the car is turned off and even if accelerator is pressed to its maximum nothing happens. Similarly for a transistor in cut off mode collector current is close to zero, increase in base current has no effect on it.

Saturation in BJT transistor is analogous to a car moving down a steep hill such that it has already achieved its maximum speed. No further increase in the speed is possible for a car in this state. For a transistor in saturation region maximum current flows through the device. Increase in base current has no effect on the collector current. 

Modes of Operation of Bipolar Junction Transistor

Modes of Operation of Bipolar Junction Transistor (BJT Transistor)

A  BJT transistor is configured to operate in saturation and cut off mode for applications when its used as a switch. BJT Transistor in cut-off mode behaves like an open switch and transistor in saturation mode behaves like a closed switch.

For applications like amplifiers, NPN transistor  and PNP transistor is biased to operate in active mode. BJT Transistor amplifies the magnitude of the signal given at the base terminal without affecting any other parameters.

Configuration of BJT Transistor 

In electronic circuits depending upon the applications, NPN transistor and PNP transistor can be configured as common base, common collector or common emitter. The term 'common' means that the terminal is common to both input and output. Best method to identify the configuration of NPN transistor and PNP transistor in a complicated electronic circuit is by checking terminals to which input and output is connected. Then we can conclude that third terminal is the common terminal.

(a). Common Emitter Configuration

It is the most frequently used BJT transistor configuration. In this, input voltage is given across Base-Emitter junction and output is taken across Collector-Emitter junction. Voltage gain, current gain and power gain of a common emitter configuration is high when compared to other transistor configurations. As input is given across forward biased junction, input resistance of a common emitter amplifier is low. Output resistance is high as the output is taken across reverse biased junction. Another important property of common emitter configuration is that, its output is phase shifted by 180 degree.

Common Emitter Configuration of NPN Transistor

Common Emitter Configuration of NPN Transistor

(b). Common Base Configuration

In common base configuration of a BJT transistor, input is given to emitter terminal and output is taken across collector terminal. It is used in applications, where, low input impedance and high output impedance are required. Unlike common emitter, in common base configuration, input and output are in same phase. Following figure shows a PNP transistor connected in common base configuration.

Common Base Configuration of PNP Transistor

Common Base Configuration of PNP Transistor

(c). Common Collector configuration

Common collector configuration of a BJT transistor  is also called voltage follower or emitter follower. In this, collector is common to both input and output. As in the figure shown below, input is connected across reverse biased base-collector junction and output is taken across forward biased emitter-collector junction. Therefore, input resistance is high and output resistance is low and common collector configuration is used in applications for impedance matching. It gives high current gain but voltage gain of common collector configuration is less than unity.

Common Collector Configuration of NPN Transistor

Common Collector Configuration of NPN Transistor

Coding Formats used For Transistor

1. JIS System

JIS is an acronym for Japanese Industrial Standard, used in Japan. The format used is

                                                          JIS System Format

  • Digit – 2 is the digit used for transistors.
  • Letters:

         JIS System

  • Sequential Number – This number can range from 10-9999.
  • Suffix – It is optional. It indicates the type approved by the Japan organizations.

Examples: The transistors that use JIS coding format are:

  • 2SC733 – This is a transistor. SC represents that they come under NPN HF transistor. 733 is the sequential number used. No suffix is used here.
  • 2SA1187, 2SB646 are other transistor examples that come under JIS coding format.

2. JEDEC System

JEDEC is an acronym for Joint Electron Device Engineering Council. This system takes the format given below:

                                                             JEDEC System

  • Digit – The digit used for transistors is 2.
  • Letter – ‘N’ is the letter used always.
  • Sequential Number – It can range from 100-9999. It is used to show when the transistor was firstly introduced.
  • Suffix – This is optional.  If it is included in the numbering format, then suffix means the gain of the device.

A – Represents Low gain, B – Represents Medium gain, C – Represents High Gain.

Examples: 2N3906 is a PNP transistor which comes under the JEDEC System.

3. Pro – Electron System

                                                        Pro-Electron System

  • Two letters – The first letter indicates the material used by the transistor and the second letter indicates the application of the transistor.
  • First letter can be anyone of them as given below:
  • Second letter can be anyone of them as given in the table. 
  • [letter] – This is optional.  It indicates the whether the transistor is used for commercial or industrial applications. Usually W, X, Y, Z letters are used to represent this.
  • Sequential Number – Can be any number from 100-9999.
  • Suffix – This is optional.

Examples: BC107, BD139, AD140 etc.

Tips and Tricks – How to read a Transistor Code?

By following the simple tips and tricks given below, you can easily identify your transistor.

  • First understand and learn the JIS, JEDEC and Pro- Electron Coding format of the transistor.

                               Learn coding system

  • Now read the numbers printed on the transistor electronic component.

                               Read the transistor code

  • Identify the coding system used, i.e., check whether it belongs to JIS, JEDEC, and Pro-electron coding system.
  • If the transistor code starts with ‘2N’ then follow JEDEC system, '2 followed by two letters’ then use JIS format, ‘Two letters’ then use Pro-Electron system.

                              Identify coding system

  • After identifying the coding format (JIS/ JEDEC/ Pro-Electron), follow the rules and steps described in ‘Coding Formats for transistors’ above for the particular coding system.
  • Now look the transistor catalog to get more information about the electronic component working, specifications, characteristics, applications etc. Also make sure whether the transistor selected is good for the DIY project/ application selected by you…

Usually, the Pro- Electron coding system is used commonly than the JEDEC system. But both are used by many device manufacturers to code the transistors. The coding system is really helpful and helps in easy identification.

Note:  Other than the JIS, JEDEC and Pro- Electron Coding system some of the electronic component manufactures bring out their own type for the commercial purposes. Some examples for this are ZTX302, TIS43 etc. Here the characters represent the particular manufacturer. Some of them are given in the table below:

Other form of transistor naming

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