UNIJUNCTION TRANSISTOR



As the name implies, it has only one PN junction but it has a three terminal silicon  diode.  It  differs  from  an  ordinary  diode in  the  sense  that  it  has  three  leads and also its difference from BJT and FET is that it has no ability to amplify. It has the    ability  to  control large  ac  power  with  a  small  input  signal.  It  also  has  the  negative resistance characteristics which makes it useful as an oscillator.

Construction

  • It is a three terminal device, having two layers. It consists of a slab of lightly doped n type silicon material. The two base contacts are attached to both the ends of this n type surface. These are denoted as B1 and B2 respectively.

  • A  p  type  material  is  used  to  form  a  p-n  junction  at  the  boundary  of  the aluminum rod and n type silicon slab. The third terminal called emitter (E) is  taken out from this p-type material.

  • The n-type is lightly doped while p type is heavily doped.

  • As  n type  is  lightly  doped,  it  provides  high  resistivity  and  p  type  as heavily doped; the symbolic representation of UJT is shown in the Fig.

  • The  emitter  is  shown  by  an  arrow  which  is  at  an  angle  to  the  vertical  line representation n type material.

  • This  arrow  indicates  the  direction  of  flow  of  conventional  current  when  the UJT is forward biased.

EQUIVALENT CIRCUIT OF UJT




The  internal  resistances  of  the  two  bases  are  represented  as  RB1  and  RB2  In  the actual  construction,  the  terminal  E  is  closer  to  B2  as  compared  to  B1.  Hence resistance RB1 is more than the resistance RB2. The p-n junction is represented by a normal diode with VD as the drop across it. When the emitter diode is not conducting     then the resistance between the two bases B1 and B2 is called interbase resistance   denoted as RBB.

Its value ranges between 4k and 12K.

Intrinsic Stand off Ratio:

Consider UJT as shown in the to which supply VBB is connected with IE=0  i.e. emitter diode is not conducting,

Then the voltage drop across RB1 can be obtained by using potential divider rule.



The typical range of   is from 0.5 to 0.8. The voltage VRB1 is called intrinsic stand off voltage  because  it  keeps  the  emitter  diode  reversed  biased  for  all  the  emitter voltages less than VRB1

PRINCIPLE OF OPERATION:

While operating an UJT, the supply VBB is applied between B1 and B2 whilethe   variable   emitter   voltage   VE   is   applied   across   the   emitter   terminals.   This arrangement is shown in the Fig


Let us see the effect of change in VE. The potential of A is decided by   and is equal to  VBB.

Case1: VE

As  long  as  VE  is  less  than  VA,  the  p-n  junction  is  reverse  biased.  Hence  emitter current IE will not flow. Thus UJT is said to be OFF.

Case 2 : VE> Vp

The diode drop VD is generally between 0.3 to 0.7 V. Hence we can write,

Vp = VA+VD=  VBB+ VD

When  V  becomes  equal  to  or  greater  than  Vp  the  p-n  junction  becomes  forward biased and current IE flows. The UJT is said to be ON.

UJT CHARACTERISTICS

The  graph  of  emitter  current  against  emitter  voltage  plotted  for a  particularvalue  of  VBB  is  called  the  characteristics  of  UJT.  For  a  particular  fixed  value  of  VBB such characteristics is shown in the Fig.


The characteristics can be divided into three main regions which are,

1. Cut off region:  The  emitter  voltage  VE  is  less than  Vp  and  the  p-n  junction  is reverse  biased.  A  small  amount  of  reverse  saturation  current  flows  through  the device,  which  is  negligibly  small  of  the  order of  µA.  This  condition  remains  till  the peak point.

2. Negative resistance region: When the emitter voltage VE becomes equal to Vp the  p-n  junction  becomes  forward  biased  and  IE  starts  flowing.  The  voltage  across the device decreases in this region, though the current through the device increases.   Hence the region is called negative resistance region. This decreases time resistance RB1.  This  region  is  stable  and  used  in  many  applications.  This region  continues  fill valley point.

3. Saturation region: Increase in IE further valley point current IV drives the device
in  the  saturation  region.  The  voltage  corresponding  to  valley  point  is  called  valley point voltage denoted as Vv. In this region, further decrease in voltage does not take   place.  The  characteristic  is  similar  to  that  of  a semiconductor  diode,  in  this  region. The  active  region  i.e.  negative  resistance  regions,  the  holes  which  are  large  in number on p-side, get injected into n-side. This causes increase in free electrons in the n-type slab. This increases the conductivity i.e. decreases the resistivity. Hence the  resistance  R  decreases  in  this  region.  As  the  VBB  increases,  the  potential  VP corresponding to peak point will increase.

Applications

  • The UJT is mainly used in the triggering of other devices such as SCR.

  • It is also used in the sawtooth wave generators and some timing circuits.

  • The  most  popular  application  of  UJT  is  as  a  relaxation  oscillator  to  obtain short pulses for triggering of SCRs.

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