Power Diode Structure

Power Diode Structure:

The structure of the power diode is little different from the small signal diodes.

Power_Diode_Structure

  • As shown in the figure, there is heavily doped n+ substrate with doping level of 1019/cm3. This substrate forms a cathode of the power diode.
  • On n+ substrate, lightly doped n- epitaxial layer is grown. This layer is also known as drift region. The doping level of n- layer is about 1014/cm3.
  • The the PN junction is formed by diffusing a heavily doped p+ region. This p+ region forms anode of the diode. The doping level of p+ region is about 1019/cm3.
  • The thickness of p+ region is 10µm. The thickness of n+ substrate is 250µm.
  • The thickness of n- drift region depends upon the breakdown voltage of the diode.
  • The drift region  determines the reverse breakdown voltage of the diode.
  • Its function is to absorb the depletion layer of the reverse biased p+n- junction.
  • As it is lightly doped, it will add significant ohmic resistance to the diode when it is forward biased. 
  • For higher breakdown voltages, the drift region is wide.
  • The n- drift region is absent in low power signal diodes. 

Conductivity modulation of drift layer:

  • When the power diode is forward biased (anode is made positive with respect to cathode), the holes will be injected from the p+ region into the drift region.
  • Some of the holes combine with the electrons in the drift region. Since injected holes are large, they attract electrons from the n+ layer.
  • Thus holes and electrons are injected in the drift region simultaneously.
  • Hence resistance of the drift region reduces significantly.
  • Thus diode current goes on increasing, but drift region resistance remains constant.
  • So on-state losses in the diode are reduced. This phenomenon is called as Conductivity modulation of drift region.

Read More:
Power Electronics System: Introduction,Advantages, Disadvantages
Applications of Power Electronics in Various Fields

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