Power Diode Structure:
Power Diode is the two terminal(namely anode and cathode) two layer(P-N) device which is used in most of the power electronics circuits. The power semiconductor diode is similar to low power PN junction diode (signal diode). In fact, power diode is more complex in structure and in operation than their low power counterparts. This complexity happens because low power device must be modified to make them suitable for high power applications.
When the anode terminal is positive with respect to cathode, it is known as forward biased. When the anode terminal is negative with respect to the cathode, it is known as reverse biased.
The power diode plays the vital role in the power electronics circuits. The major and most important applications of power diode in converter circuits are working as a rectifier ( remember that the rectification operation is uncontrollable), freewheeling diode or flyback diode, reverse voltage protection, voltage regulation circuits etc.
When the anode is positive with respect to the cathode terminal the diode starts conducting. Then it act as an uncontrolled switch. ie we no need to provide any gate/base voltage to make it conduct the current. It operation as switch cannot be controlled by applying controlling voltage/current.
The structure of the power diode is little different from the small signal diodes. In this post we will discuss about the power diode structure in detail. The fundamentals of power diode is discussed separately. Please click here to know about basics of Power Diode.
- 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.
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