The Unijunction Transistor is a two layer device, with but one P-N junction; it was once referred to as a double-base diode. It is a pulse generator, with the trigger signal applied at the emitter; a UJT will not conduct current until a peak voltage is reached on this trigger signal. This trigger voltage (Vp) is a fraction (eta, often called the intrinsic standoff ratio) of the interbase voltage (Vbb).

UJT symbolUJT cross-section

When Vp is reached, a positive pulse is generated at B1, and a negative pulse at B2; either can be used to trigger another device, such as an SCR. Note that Vp and eta are functions of the UJT's internal geometry. The area between B1 and B2 acts somewhat like a variable resistor, with the emitter acting as a sort of wiper arm; eta is then a function of the emitter's location between B1 and B2. Also note that strictly speaking, a UJT could be built either with a P-type region in an N-type substrate (an N-channel UJT, as shown above), or with an N-type region in a P-type substrate (a P-channel UJT). In practice, only N-channel UJTs seem to be manufactured.

UJTs are not often used in BEAM, and only rarely outside it either (they were very commonly-used components in the 1970's, but are mostly just used in oscillators nowadays). Coupled with SCRs, though, they can make a mean solar engine. In this case, the UJT is used as the voltage-level trigger for the solar engine.

See also TLA; compare to SCR and PUT. For more information, AMS has a good UJT tutorial page here.

For an example UJT's data sheet, see the 2N2646 in the BEAM Reference Library's Datasheet Collection.

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Page author: Eric Seale  
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