What is UJT?

  • A Unijunction transistor is a three terminal semiconductor device having only one p-n junction like diode but has three terminals.
  • This device has a unique characteristics that when it is triggered, the emitter current increases regeneratively until is limited by emitter power supply.
  • The unijunction transistor can be employed in a variety of applications like switching pulse generator, saw tooth generator etc. The basic construction of UJT and its symbol is shown in below figure.

CONSTRUCTION:

    • It consists of an N type silicon bar with an electrical connection on each end and the leads to these connections are called base leads, Base 1 (B1), Base 2 (B2).
    • The small p-type region is doped at one side of the bar near to B2 terminal and the lead taken from this p-type region is known as emitter.
    • Thus a p-n junction is formed between the emitter and base region. The emitter region is heavily doped, while the base region is lightly doped.
    • So resistivity of base material is very high. The resistance ratio is an important characteristic of UJT which is known as intrinsic stand-off ratio which is denoted by η which is given by the equation:


CHARACTERISTICS:

  • The static emitter char­acteristic (a curve showing the relation between emitter voltage VEand emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure.
  • From figure it is noted that for emitter potentials to the left of peak point, emitter current IEnever exceeds IEo . The current IEo corresponds very closely to the reverse leakage current ICo of the conventional BJT. This region, as shown in the figure, is called the cut-off region.
  • Once con­duction is established at V= VPthe emitter po­tential VE starts decreasing with the increase in emitter current IE. This Corresponds exactly with the decrease in resistance RB for increasing cur­rent IE. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current IE places the device in the saturation region, as shown in the figure. Three other important parameters for the UJT are IP, VV and IV and are defined below:
  1. Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It repre­sents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage VBB.
  2. Valley Point Voltage VVThe valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage VBB.
  3. Valley Point Current IVThe valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.

OPERATION:

  • Here emitter diode is reverse biased by voltage drop across resistance RB1 and its own barrier potential VD. So total reverse bias is VD + VB = VD + η VBB (for silicon VD = 0.7 Volts ).
  • As long as applied emitter voltage is below the total reverse bias voltage across the diode, it remain reverse biased. And there is no emitter current. The value of emitter voltage which causes the diode conduction is called peak point voltage VP. i.e. VP = ηVBB + VD.
  • Then the UJT is turned on and emitter current begins to flow. Under this condition the UJT is said to be fired or triggered or turned on. Thus as VE along with IE increases, RB1, η and VA This produce further decrease in RB1, η and VA. This process is regenerative.
  • VA as well as VE decreases as IE Due to this the UJT has negative resistance region in its VI characteristics. The curve between Emitter voltage VE and emitter current IE of a UJT at a given voltage VBB between the bases this is known as emitter characteristics of UJT Initially in the cut off region as VE increases from zero, slight leakage current flows from terminal B2 to the emitter the current is due to the minority carriers in the reverse biased diode.
  • Above a certain value of VE forward IE begins to flow, increasing until the peak voltage VP and current Ip are reached at point P. After the peak point P an attempt to increase VE is followed by sudden increases in emitter current IE with decrease in VE is a negative resistance portion of the curve.
  • The negative portion of the curve lasts until the valley point V is reached with valley point voltage Vv. and valley point current Iv after the valley point the device is driven to saturation the difference Vp-Vv is a measure of a switching efficiency of UJT fall of VBB

Special Features of UJT.
        The special features of a UJT are :

  • A stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.
  • A very low value of triggering current.
  • A high pulse current capability.
  • A negative resistance characteristic.
  • Low cost.

ADVANTAGES OF UJT

  • It is a Low cost device
  • It has excellent characteristics
  • It is a low-power absorbing device under normal operating conditions

APPLICATIONS OF UJT
It can be used as trigger device for SCR’s and TRIAC’s.

  • To generate non sinusoidal oscillations.
  • To generate saw tooth waveforms.
  • In timing circuits it is used as switch
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