Unijunction transistor

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A Unijunction transistor (UJT) is an electronic semiconductor device.

There are two types of unijunction transistors:

• The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. The 2N2646 is an example of such a device.

• The programmable unijunction transistor, or PUT, is a close cousin to the thyristor. Like the thyristor it consist of four P-N layers and has an Anode and a Cathode connected to the first and the last layer, and a Gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function. The 2N6027 is an example of such a device.

For both types, their main use is as a trigger device for thyristors and as the active device in relaxation oscillators. The graph of emitter voltage against emitter current of a unijunction transistor shows an area of negative resistance; this is what makes the UJT useful. The Uni Junction Transistor has only one junction, hence the name is 'UNI'Junction transistor. The UJT has three terminals. The terminals are emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when emitter is open is called interbase resistance.

[edit] External links

• Diagrams and description of unijunction transistors from American Microsemicondutor