Structure, Working and Characteristic 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

• 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      ( R_BB ). Therefore R_BB = R_B1 + R_B2

        Where

            R_BB = Inter base resistance

            R_B1 = Resistance between terminal base B₁ and emitter

            R_B2 = 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 R_B1 and R_B2 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 R_B2 is greater than the R_B1.

Instrinsic Ratio ( η )

• The battery V_BB 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 R_BB. 
• Let the voltage drop across resistance R_B1 is V_A. According to voltage divider rule

        V_A = [ R_B1 / ( R_B1 + R_B2 ) ] V_BB

        V_A = ηV_BB

• 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 = [ R_B1 / ( R_B1 + R_B2 ) ]

           = V_A / V_BB

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.

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 R_B1 is given by

         V_A = ηV_BB

• If the thresh – hold voltage of the diode is given by voltage V_D , the reverse bias voltage

           = V_A + V_D

           = ηV_BB + V_D

• The value of V_D = 0.7 voltage for the silicon diode
• The voltage V_EE is applied to the emitter increases progressively. 
• The diode becomes forward biased when the voltage V_EE exceeds total reverse biased voltage. 
• The value of the voltage is called is peak point voltage and it is given by

           V_P = ηV_BB + V_D

• 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 I_P start to flow through R_B1. 
• When the emitter to base 1 voltage V_EE increases beyond peak point voltage V_P, the emitter current decreases. 
• The resistance R_B1 decreases due to minority charge carriers therefore the voltage drop V_A 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 V_EE < η V_BB + V_D resulting small leakage current flows through the device. 
• When V_EE < η V_BB + V_D, 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.

• This process continues until the current increases up to valley point. 
• The valley point current I_V 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 R_E. 
• The slope of the UJT characteristic under conducting state ( V_P – V_V ) is very steep resulting very low resistance. 
• Therefore the region between V_P – V_V is known as negative resistance region.
• The UJT behaves as a conventional forward biased PN junction diode beyond valley point.  
• If the emitter current I_E 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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