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UJT UniJunction Transistor Tutorial

UJT – Introduction:
The UniJunction Transistor is a three-terminal, two-layer, semiconductor device. The terminals are Emitter(E), Base-one(B1) and Base-two(B2). As it exhibits a negative resistance region, it is used as an oscillator and triggering device.

Symbol:
UJT_SYMBOLThe symbol shows that Base-1 and Base-2 terminals at right angles to indicate that are non-rectifying ohmic contacts. But the emitter is represented by an arrow since it is a rectifying p-n junction. The arrow is pointing from the p-type emitter to the n-type base.

Equivalent Circuit:

UJT_Equivalent_Circuit
The RB1 and RB2 are the internal resistance from bases B1 and B2 to point A.
When a voltage VBB is applied across B1 and B2, voltage across point A and B1 is VAB1 = VBB * RB1/RB1 +RB2 = η.VBB
VAB1 = η.VBB
Where η = intrinsic stand-off ratio = RB1/RB1 +RB2  .

Construction:

UJT_Construction
The UJT consists of an n-type silicon bar with two ohmic contacts for the base-1 and base-2 terminals. The Emitter terminal is a p-type aluminium wire which is alloyed onto the silicon bar to form a p-n junction.

Working & V-I Characteristics:
UJT_Characteristics

  • When VBB is applied emitter diode D is reverse biased due to voltage drop across RB1(ie,  VAB1 = η.VBB).
  • Therefore only a small leakage current will flow as shown in the device V-I characteristic.
  • At  this condition the emitter junction being reverse biased and the inter base resistance being in the region of 5-10 kΩ.
  • Now when emitter voltage VE is applied between E and B1, the diode remains reverse biased till
    VE < ηVBB + VD      (VD = Voltage drop across diode)
  • So emitter current is negative.
  • When VE = ηVBB + VD, the E-B1 junction is forward biased and it begins to conduct. So IE is positive.
  • This value of Emitter voltage which causes the internal diode to conduct (or)  UJT to conduct is called Peak point voltage.
    ie, VP = ηVBB + VD.
  • When UJT turns-On, the current IE increases and voltage across the device (VE) decreases.
  • The  voltage decreases till it reaches the point called as Valley voltage(VV).
  • After Valley voltage the device is in saturation region. When VE reaches VV, the device turns-off.

Datasheet Parameters:
The datasheet of the device provides several parameters to design the UJT based circuits.

 

  • The interbase resistance RBB is measured between the two base terminals with the emitter open, and its value varies slightly with applied voltage and with temperature.
  • The UJT data sheets normally provides a curve showing the variation of RBB with temperature. 
  • The peak and valley point currents and voltages are also given in datasheets and these values define the negative resistance region.
  • The valley point location is affected by the temperature and the value of the interbase voltage. 
  • The peak point location is depends on interbase voltage and intrinsic stand-off ratio.
  • The intrinsic stand-off ratio is the very important parameter in the design of UJT circuits which is given in datasheet.
  • Even though it varies from one UJT to another, it remains relatively constant for a device even with variations in supply voltage and temperature.
  • The Emitter-Base2 leakage current, when this junction is reverse biased with Base-1 open, is also given in the datasheets. 
  • It is similar in value to that of leakage through a diode, and it affects the charging current of any capacitors used in timing circuits. So it needs to be taken into account in the circuit design. 

Read More:
Triac – Basics, Working, Characteristics


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