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The UniJunction Transistor(UJT) has three terminals like he Bipolar Junction Transistor, but has quite different characteristics. It is built up of a N-doped bar with a P-doped area on the middle:

           E
  B1  _____|_____  B2 
  ___|    \P/    |___
     |___________|

B1  Base 1
B2  Base 2
E   Emitter

The UJT is most used in pulse generating circuits:



     .------------*----- +
     |            |
    | |          | |
    | |R1        | |R3
     |            |
   B2`-| _E       |
       |/ `-------*----- Out
   B1,-|          |
     |  UJT       |
    | |          --- C1
    | |R2        ---
     |            |
     `------------*----- GND

     Output Waveform

       _    _    _  _Vp(peak voltage)
     .- | .- | .- |
    /   |/   |/   | _

The capacitor C1 will be charged through the resistor R3; the voltage between the emitter and B1 of the UJT rises and eventualy the PN junction starts conducting, and the capacitor is discharged through the UJT.

The UJT is used to trigger Silicon controlled rectifiers and used in timer, sawtouth and stable voltage sensing circuits.

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