The ebook FEEE - Fundamentals of Electrical Engineering and Electronics is based on material originally written by T.R. Kuphaldt and various co-authors. For more information please read the copyright pages.

The Unijunction Transistor

The unijunction transistor, or UJT, is a three-terminal device with a single pn-junction. It is basically a switching device with a negative resistance region in it characteristic diagram. Typical applications of UJTs include oscillators, voltage comparators, trigger circuits for thyristors, timers.

There are three types of unijunction transistors, the UJT, the CUJT, and the PUT:

The original unijunction transistor, or UJT, 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 fuction. 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.

Unijunction Transistors (UJT)

Part No. RBBO Interbase Resistance
@VBB=3V IE=0[kΩ]
ηIntrinsic StandoffRatio
Valley Current Min. [mA]
Peak Point Emitter Current Max. [ľA]
Base One Peak Pulse Voltage Min. [V]
2N489 4.7 - 6.8 .51 - .62 8 12--TO-5
2N490 6.2 - 9.1 .51 - .62 8 12--TO-5
2N491 4.7 - 6.8 .56 - .68 8 12--TO-5
2N1671 4.7 - 9.1 .47 - .62 8 25--TO-5
2N2160 4.0 - 12.0 .47 - .80 8 253TO-5
2N2646 4.7 - 9.1 .56 - .75 4 53TO-18
2N2647 4.7 - 9.1 .68 - .82 8 26TO-18

Programmable Unijunction Transistors (PUT)

Part No. Gate to Anode Reverse Voltage Max. [V] DC Anode Current Max. [mA] IGAO
Leakage Current
@40V Max. [nA]
Valley Current Min.
@RG =10k [ľA]
Output Voltage Min. [V]

Last Update: 2010-12-01