Power Diode Structure:
The structure of the power diode is little different from the small signal diodes.
- 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.