Both Normal diode and Power diodes are uncontrolled devices because their On/Off situation depends upon the external circuit used. Normal diodes are used in small signal applications whereas power diodes are used for high voltage and current applications such as in inverter.
The power semiconductor devices are not same as signal level semiconductor devices. In general, symbol of the power devices and the respective signal level devices are one and same. But their construction (structure) will differ entirely.
- Cut in voltage of normal diode = 0.7 V (For Si) and Cut in voltage for power diode = 1.5 V (for SCR).
- In normal diode, Heat sink is not required but in power diode conduction is more hence area or size required to dissipate more heat (Heat sink is required).
- In case of power diodes, semiconductor losses during switching are more as comparison to normal diode.
- Power handling capability of Power diodes are very high.
- Structure of Power diode and normal diode is little different (Drift region is absent in normal small signal diodes).
Substrate – cathode of the diode (Heavily doped and thickness is 250 μm)
Anode – Heavily doped p+ region, forms p-n junction (thickness is 10 μm)
Drift region – Lightly doped n- region (Thickness depends upon the breakdown voltage of the diode, for higher breakdown voltage – the drift region is wide, Its function is to absorb the depletion layer of the reverse biased p+n- junction)
Conductivity modulation of Drift Region
- 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.