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Difference between UJT and BJT
In electronics engineering, a number of semiconductor switching devices are there such as diode, transistor, SCR, GTO, IGBT, etc. The BJT and UJT both are the types of transistors used for switching application in different electronic circuits.
Since both UJT and BJT are the transistors, however, there are many differences between the two devices. One major difference between UJT and BJT is that the UJT is a unijunction transistor that can only be used as a switch in an electronic circuit, while the BJT is a bipolar junction transistor that can be used as a switch as well as an amplifier in the circuit.
This article will explain all the significant differences between BJT and UJT in detail. Also, a short description of BJT and UJT is added for the reference of the reader.
What is UJT?
The UJT stands for Uni-Junction Transistor. A UJT is a three terminal, one junction semiconductor device that is used to perform switching operation in electronic circuits. The three terminals of the UJT are: emitter (E), and two bases viz. B1 and B2. Since, the UJT consists of two base terminals, for this reason, it is sometimes known as double-base diode
The UJT is constructed by using a lightly doped n-type silicon bar that forms the base of the UJT and two ohmic contacts viz. B1 and B2 are connected to it. A heavily doped p-type bar is used to form the emitter of the UJT. The emitter terminal of the UJT is usually located near to the base terminal B2, which makes the device unsymmetrical.
The UJT (Unijunction Transistor) is widely used in free running oscillators, synchronized oscillators, pulse generating circuits, and triggering circuit of SCR, etc.
What is BJT?
The BJT stands for Bipolar Junction Transistor. A BJT is a three-terminal, three-layer and two junction semiconductor switching device which is used as an electronic switch and amplifier in various electronic circuits.
The three terminals of the BJT are: emitter (E), base (B) and collector (C). A bipolar junction transistor is constructed by sandwiching either a P-type semiconductor between two N-type semiconductors or an N-type semiconductor between two P-type semiconductors. Hence, based on the construction, the BJTs can be classified into two types viz. NPN and PNP.
The BJT or Bipolar Junction Transistor is extensively used in many electronic circuit such as oscillator, digital logic gates, music systems, computers, etc. as a switch or amplifier.
Difference between UJT and BJT
Since, both UJT and BJT are the types of transistors used in switching applications in the electronic circuits. However, there are many differences between UJT and BJT based on their construction, functioning, applications, etc. which are listed in the following table −
| Basis of Difference | UJT | BJT |
|---|---|---|
| Full form | UJT stands for Unijunction Transistor. | BJT stands for Bipolar Junction Transistor. |
| Definition | UJT is a three-terminal semiconductor switching device with only one junction. | BJT is a three-terminal three-layer semiconductor device which can work as a switch as well as amplifier. |
| Circuit symbol |  |  |
| Terminals | UJT has three terminal viz. Emitter (E), Base terminal 1 (B1) and Base terminal 2 (B2). | BJT has three terminals viz. Emitter (E), Base (B) and Collector (C). |
| Number of PN junction | There is only one PN junction present in UJT. | There are two PN junction in case of BJT. |
| Number of semiconductor layers | UJT has only two layers of semiconductor, one is P-type and the other is N-type. | BJT has three layers of semiconductor, one is of P-type and other two are of N-type (or one is N-type and other two are of P-type). |
| Alternate name | UJT is also called double-base diode, as it has two bases. | BJT simply known as transistor. |
| Types | There are three types of UJT viz. −
| Two types of BJT are there −
|
| Conduction | The conduction in UJT is based on the movement of majority charge carriers only. Thus, it is a unipolar device. | The conduction in a BJT is based on the movement of both majority and minority charge carriers. Thus, it is a bipolar device. |
| Function | UJT can only be used as a semiconductor switch in an electronic circuit. | BJT can be used as a semiconductor switch as well as an amplifier. |
| Type of device | UJT acts as a voltage controlled device. | BJT is a current controlled device. |
| Applications | The UJT is widely used in relaxation oscillators, synchronized oscillators, pulse generation circuits, triggering circuit of SCR, etc. | BJT is widely used in many electronic circuits such as amplifiers, high speed digital circuits, temperature sensors, avalanche pulse generator, logarithmic converters, etc. |
Conclusion
Since UJT and BJT both are the two types of transistors used as a switching device in electronic circuits. However, there are several differences between Unijunction Transistor and Bipolar Junction Transistor based on their construction, function, type of device, controlling quantity, and applications, etc. which are given in the above table.
The most significant difference between the two devices is that a UJT is one-junction device which conducts by the flow of majority charge carriers only, while a BJT is a two-junction device in which conduction is due to the movement of both majority and minority charge carriers.
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