The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter. This trigger voltage is a fraction (n) of interbase voltage, Vbb. The UJT circuit symbol, junction schematic, and characteristic curve are shown below:

Unijunction_Transistor_UJT_circuit symbol_junction_schematic_characteristic_curveThe emitter terminal does not inject current into the base region until its voltage reaches Vp. Once Vp is reached the base circuit conducts and a postive pulse appears at the B1 terminal and a negative pulse at B2. The UJT incorporates a negative resistance region, a low emitter current, and a high output pulse current at terminals B1 and B2, making it an ideal pulse trigger. A simple RC timer circuit using a UJT is shown below:

The very basic specifications of a UJT are:

(a) Vbb(max) – The maximum interbase voltage that can be applied to the UJT
(b) Rbb-the interbase resistance of the UJT
(c) n – The intrinsic standoff ratio which defines Vp.
(d) Ip – The peakpoint emitter current

Here are a few of our most requested UJTs

Part No.

RBBO Interbase Resistance @VBB=3V  IE=0(KOhms)

h

Intrinsic Standoff Ratio @VBB=10V

Iv

Valley Current Min. (mA)

Ip

Peak Point Emitter Current     Max. (渙)

VOB1

Base One Peak Pulse Voltage  Min. (V)

Package Style
  2N489   4.7 – 6.8   .51 – .62        8        12         —

TO-5
  2N490   6.2 – 9.1   .51 – .62        8        12         —
 2N491   4.7 – 6.8   .56 – .68        8        12         —
  2N1671   4.7 – 9.1   .47 – .62        8        25         —
  2N2160   4.0 – 12.0   .47 – .80        8        25         3
  2N2646   4.7 – 9.1   .56 – .75        4        5         3

TO-18
  2N2647   4.7 – 9.1   .68 – .82        8        2         6

 

Programmable Unijunction Transistors (PUT) supplied by American Microsemiconductor

  Part No.

Gate to Anode Reverse Voltage Max. (V)

DC Anode Current Max. (mA)

IGAO

Leakage Current @40V Max. (nA)

IV

Valley Current         Min. @RG =10k  (渙)

Vo

Output Voltage Min.   (V)
2N6027 40 150 10 70 6
2N6028 40 150 10 25 6

 

 

American Microsemiconductor produces a very broad line of standard unijunction transistors.
Many of the basic applications include:

oscillators, timers, sawtooth generators, SCR triggers, frequency dividers, stable voltage sensing,