4 October 2017

Structure, Equivalent Circuit, Working and Characteristic and Application of the UJT

  • The full form of the UJT is uni junction transistor. 
  • It is a single PN junction semiconductor switching device. 
  • It has three terminals : base 1 ( B1 ), base 2 ( B2 ) and emitter. The structure and symbol of the UJT is shown in the Figure A.
structure and symbol of the ujt

Structure

  • The UJT consists of lightly doped N type silicon bar with terminal ends with base 1 ( B1 ) and base 2 ( B2 ). 
  • A small heavily doped P type material is alloyed to its one side for producing single PN junction. 
  • The terminal brought out from P type material is called as emitter ( E ). 
  • The P type material ( emitter E ) is placed closer to base 2 rather than base 1. 
  • The arrow in the emitter shows the direction of the forward current through the junction and it is inclined towards base 1 terminal.

Equivalent circuit of the UJT
  • The equivalent circuit of the UJT is shown in the Figure B. The resistance between terminal base 1 and base 2 with emitter open is called as inter - base resistance      ( RBB ). Therefore RBB = RB1 + RB2
        Where
            RBB = Inter base resistance
            RB1 = Resistance between terminal base B1 and emitter
            RB2 = Resistance between terminal base B2 and emitter
  • The value of inter base resistance lies is in the range of 4.7 kilo ohm to 10 kilo ohms. 
  • The value of RB1 and RB2 depends upon where the P type material is located along the n type bar material. 
  • As the emitter ( E ) is located closer to base 2 terminal , the resistance RB2 is greater than the RB1.
equivalent circuit of the ujt

Instrinsic Ratio ( η )

  • The battery VBB is connected between terminal base 1 and base 2 as shown in the Figure B. 
  • The point A acts as voltage divider point for the resistance RBB
  • Let the voltage drop across resistance RB1 is VA. According to voltage divider rule
        VA = [ RB1 / ( RB1 + RB2 ) ] VBB
        VA = ηVBB
  • The η is called as intrinsic standoff ratio and its value lies in the range of 0.5 to 0.85. Therefore the intrinsic standoff ratio is given by
        η = [ RB1 / ( RB1 + RB2 ) ]
           = VA / VBB
Operation of the UJT
  • The basic circuit for the operation of the UJT is shown in the Figure C. 
  • The DC voltage source VEE is kept variable and DC voltage source VEE is generally kept fixed.  
  • We consider following two cases for the operation of the UJT.


operation of the ujt


When NO voltage is applied to the emitter
  • The voltage between point A and base 1 reverse biased the PN junction in this condition therefore the emitter is cut off. 
  • Therefore small amount of leakage current flows from the base 2 to the emitter E due to minority charge carriers.
When a positive voltage is applied at the emitter
  • The voltage drop across RB1 is given by
         VA = ηVBB
  • If the thresh – hold voltage of the diode is given by voltage VD , the reverse bias voltage
           = VA + VD
           = ηVBB + VD
  • The value of VD = 0.7 voltage for the silicon diode
  • The voltage VEE is applied to the emitter increases progressively. 
  • The diode becomes forward biased when the voltage VEE exceeds total reverse biased voltage. 
  • The value of the voltage is called is peak point voltage and it is given by
           VP = ηVBB + VD
  • The emitter current start to flow and holes of the emitter are injected into N type bar. 
  • These holes are repelled to base 2 and attracted by base 1 terminal resulting current IP start to flow through RB1
  • When the emitter to base 1 voltage VEE increases beyond peak point voltage VP, the emitter current decreases. 
  • The resistance RB1 decreases due to minority charge carriers therefore the voltage drop VA decreases.
  • This will result in emitter current increases regenerative until it is limited by external power supply. 
  • It should be noted that emitter E and base 1 are active terminals and base 2 is only used for applying triggering external voltage across base 1and base 2 terminals.
  •  The UJT can be turned off by applying negative trigger pulse to its emitter terminal E.

Characteristic of the UJT
  • The static ( voltage – current ) characteristic of the UJT is shown in the Figure D.
  •  The emitter junction becomes reverse biased when VEE < η VBB + VD resulting small leakage current flows through the device. 
  • When VEE < η VBB + VD, the emitter junction becomes forward biased and emitter current start to flow. 
  • The current corresponding to point P is called as Peak point current. 
  • Now the holes of the emitter region injects in to N type bar region. 
  • These holes are attracted by base 1 region. It means that it decrease the voltage drop between point A and base 2. 
  • Consequently it further increases the forward bias of the PN junction and further increases the current.
static v-i characteristic of the ujt
  • This process continues until the current increases up to valley point. 
  • The valley point current IV is so large that the conductivity between point A and base 1 does not increases further. 
  • The emitter current is limited by the external resistor RE
  • The slope of the UJT characteristic under conducting state ( VP – VV ) is very steep resulting very low resistance. 
  • Therefore the region between VP – VV is known as negative resistance region.
  • The UJT behaves as a conventional forward biased PN junction diode beyond valley point.  
  • If the emitter current IE decreases below valley current, the UJT turns OFF. 
  • Therefore the valley current ( similar to holding current ) is minimum emitter current to keep the UJT is in ON state.
Application
  • Pulse generation 
  • Saw tooth generation
  • Sinusoidal wave generation
  • Switching
  • SCR / TRIAC triggering circuit
  • Timing circuit and 
  • Oscillators
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1 comment:

  1. why it is called intrinsic stand off ratio. why the word intrinsic is used?

    ReplyDelete