TUNNEL DIODE

  • The  tunnel  diode  is  a  thin  junction  diode;  under  forward  bias  condition  itexhibits negative resistance. This makes the tunnel diode Useful foroscillations or amplification.
  • In conventional PN diode is doped to have impurity atoms in theconcentration of 1 part in 108. Then the width of the depletion layer is of the order of a micron i.e., 5 micron.
  • But in tunnel diode the impurity concentration is greatly increased to 1 part in103   then   the   depletion   layer   width   reduces   to   about   100A   thus   device characteristics get completely changed.
  • This  diode  utilizes  the  phenomenon  called  tunneling  and  hence  the  diode  isreferred as tunnel diode.
Tunnelling Phenomenon:

  • The  width  of  the  junction  barrier  varies  inversely  as  the  square  root  of  theimpurity  concentration  (i.e)  if  the  concentration.  of impurity  atom  is  greatly increased, the barrier width ‘W’ reduces.
  • A  particle  must  have  an  energy  atleast  equal  to  the  height  of  the  potentialbarrier in order to cross over the junction. However if the barrier is extremely     thin,  then  instead  of  crossing  over  the  junction  barrier  the  electron  may penetrate through the barrier.
  • This  behavior  exhibited  by  the  electron  to  the  applied  potential  is  called“tunnelling” and hence the diode is called as tunnel diode.
  • Thus  a  tunnel  diode  is  a  high  conductivity  two  terminal  PN  Junction  diodedoped heavily about 1000 times higher than a conventional junction diode.

VI Characteristics:

  • The  heavily  doped  tunnel  diode  results  in  a  thin  depletion  layer  so  as  topermit tunnelling to occur. The VI characteristic of a typical germanium tunnel diode is shown in the figure 19.
  • It is seen at first the current rise sharply as voltage is applied and reaches thepeak current Ip point (A).
  • As  the  forward  bias  is  increased  above  this  point  (A)  the  forward  currentdrops  and  continues  to  drop  until!  a  point  B  is  reached.  This  point  is  the Valley voltage or valley current (Iv)
  • After  the  valley  point  is  reached  further  increase  in  input  voltage  increasesthe current very rapidly as PN junction diode.
  • The  tunnel  diode  exhibits  negative  resistance  characteristics  seen  betweenthe peak current IP and minimum value IV and it is called valley current.

  • The peak current IP depend on the impurity. Tunnel diodes with 1 to 100 mA,are commonly used in computer applications.

Equivalent circuit of Tunnel diode 





where,

c = junction capacitance = 1 to 10 PF -RN = negative resistance of diode

Ls = inductance due to terminal leads.

Advantage 

1. Low Cost

2. Simplicity of construction

3. High Speed Operation

4. Low Temperature senstivity, low noise and low power

5. Environmental immunity

Disadvantage


1. Low output voltage swing

2. Since it is two terminal device there is no isolation between input and output.

Applications

1.   Tunnel   diode   amplifiers   may   be   used   throughout   the   micro-wave   range   as moderate to low-noise pre amplifiers in all kinds of receivers.

2. Tunnel diode are used as mixers

3.  Being  high-speed  devices,  tunnel  diode  also  lend  them  selves  to  high  speed switching and logic operations as flip flop and gate.

4. Finally they are used as low power oscillator upto about l000Hz,because of their simplicity, frequency stability and immunity to radiation.

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