• PN junction diodes are used in a wide variety of applications, with each application taking advantage of a different set of diode's characteristics.
  
  A few of the applications along with the diode characteristics on which they are base are listed below:
  
  

  Application	Mode of Operation	Principle
  Switching circuits	Forward and Reverse bias with time varying signals	Rectification
  Photodetectors	Reverse bias with time varying signals	Sensitivity of reverse current to carrier generation
  Solar Cells	Forward bias with constant excitation	Sensitivity of diode current to carrier generation
  Mixers	Forward bias with time varying excitation	Nonlinear diode characteristics

  
  
  Let us consider applications where rectifying characteristics of the PN junction diodes is exploited. In these cases, the following characteristics are of interest:
  
  (i) Maximum forward current that can flow through the device beyond
  which the forward ON voltage begins to increase linearly rather than logarithmically with current.
  
  (ii) The reverse leakage currentat small reverse bias
  
  (iii) Breakdown voltage BV
  
  (iv) Reverse recovery time
  
  The desired diode characteristics have to be obtained at the least cost (C) possible
  Let us consider the tradeoffs among these diode characteristics for a wide base
  diode
  
  Th max. forward current limited by onset of high level injection is determined by the
  doping in the lightly doped region :
  
  
  
• This breakdown voltage can be expressed as
  
  
  
• The reverse leakage current is proportional to
  
  
  
• The reverse recovery is proportional to
  
  
  
• Finally, the cost of diode is proportional to silicon area used so that
  
  
  The equations listed above can be used to obtain the following expressions
  
  
• This expression describes a very important tradeoff among the max. current rating, breakdown voltage and cost of a diode
  
  
• It shows that an increase in forward current can only be obtained either at the expense of a lower breakdown voltage or increased cost due to use of larger Silicon area
  .
  
• The expression also shows that high voltage, high current diodes are likely to be the most expensive diodes.
  
  Another expression describing another important tradeoff can be written using the equations listed earlier:
  
  
  
• This expression shows that an improvement in switching performance can be obtained either at the expense of increased leakage current or a reduced cost. The reduced cost or smaller diode area implies either a lower forward current or a lower breakdown voltage according to Eq. (6)
  
  
• It is interesting to see how breakdown voltage can be traded with switching speed. A reduction in recombination lifetime through say addition of suitable impurities will also increase leakage current. This can be countered by decreasing diode area which however will lead to reduced forward current rating unless doping is increased. This will lead to a reduced breakdown voltage.
  
  
• The breakdown voltage and reverse recovery are also related together in more direct manner. Regions which have higher doping also have a lower recombination lifetime so that a lower breakdown voltage diode is likely to have lower lifetime and better switching speeds.
  
  The two tradeoff expressions can also be combined to obtain another expression shown below:
  
  
  
  
• The expression above shows why a single diode cannot meet the needs of diverse applications. Different applications put different demands on forward current, breakdown voltage etc which can only be obtained through separate designs.
  

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Example 6.1 Design a wide base diode with a breakdown voltage20 Volts and maximum current handling capacity 100mA.

Solution: We shall take diode and assume that and = 1µs

Taking the critical field we obtain
To be on the safe side , we take

The maximum current handling capability is determined by the onset of high level injection effects.
The maximum current density is

Again to be on the safe side, we take

To be considered as wide base diode, the width of N-region should be several times the diffusion length which is 28µm. so we take . There is no point in taking a larger value because it would unnecessarily add to the voltage drop in the neutral N-region. The final design is shown below :

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Example 6.2 Design a narrow base diode with similar specifications. Comment on the advantages of the narrow base over the wide base diode.

Solution : As before, we take At breakdown the junction depletion width should be less than the thickness WN of the N-region, otherwise punch though,discussed in chapter on BJT, would reduce the reverse blocking voltage.At breakdown, the depletion width W can be calculated using the expression

From this we obtain, W = 2.5µm. To be on the safe side, we take = 3µm.
The maximum current density now is

Again, to be on the safe side, we take .

Advantages: The area required is an order of magnitude better . Further the transit time of diode now is which is considerably smaller than the recombination lifetime limited transit time of 1µs for wide base diode.