There are two kinds of unijunction transistors. The programmable unijunction transistor 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. The 2N6027 is an example of such a device.
The original unijunction transistor, or UJT, is a simpler 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.
Unijunction transistors are very rarely used, as not many electronic engineers know about them.
Their main use is as a trigger device for thyristors and as the base for a relaxation oscillator.
Interestingly the the graph of emitter voltage against emitter current of a unijunction transistor shows an area of negative resistance 5.
More information can be found here: http://members.tripod.com/~baec/DEC90/uni_tran.htm
Common misspelling and questions (FAQ)
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Mine eyes forego their light, my false heart bleed? And extreme fear can neither fight nor fly, Or lain in ambush to betray my life, Might have excuse to work upon his wife; But as he is my kinsman, my dear friend, Hateful it is:-- there is no hate in loving; The worst is but denial and reproving: Who fears a sentence or an old man's saw 'Tween frozen conscience and hot-burning will, Urging the worser sense for vantage still; All pure effects, and doth so far proceed, And gaz'd for tidings in my eager eyes, Where her beloved Collatinus lies. First red as roses that on lawn we lay, Forc'd it to tremble with her loyal fear; Until her husband's welfare she did hear; That had Narcissus seen her as she stood, All orators are dumb when beauty pleadeth; Love thrives not in the heart that shadows dreadeth: And when his gaudy banner is display'd, Respect and reason wait on wrinkled age! Sad pause and deep regard beseem the sage; Desire my pilot is, beauty my prize; Is almost chok'd by unresisted lust. Full of foul hope, and full of fond mistrust; So cross him with their opposite persuasion, .