Power MOSFET

The power MOSFET device is often used in circuits which produce current flow through the structure in the third quadrant of it’s ‘i – v’ characteristics. Two promi­nent examples of such circuits are the voltage regulator module (VRM) used to deliver power to microprocessors in computers and the H-bridge motor control circuits used to achieve adjustable speed drives. One of the advantages of the power MOSFET structure is an inherent reverse conducting diode within the structure which allows carrying current in the third quadrant of operation. Unfortunately, the switching speed of this diode is very slow in as-fabricated devices producing excessive power losses that limit the circuit operating frequency. This problem was first overcome by the use of electron irradiation to control the minority carrier lifetime to achieve improved reverse recovery characteristics for the body diode in the power D-MOSFET structure.

A circuit approach that has been utilized in VRMs to addressing the poor reverse recovery behavior of the integral diode in power MOSFET devices is the parallel connection of an external Schotlky diode. This approach has been found to be ineffective due to the inductance between the separately packaged power MOSFET and Schottky diode which results in some of the current flowing through the body diode of the power MOSFET structure. A preferred approach to improving the characteristics of the reverse conducting diode is by incorporation of a Schottky diode within the power MOSFET chip. The reverse leakage current from the integral Schottky diode can be minimized by utilizing the junction barrier control concept for the planar gate power MOSFET structure and the MOS barrier control concept for the trench gate power MOSFET structure.

In the case of high voltage power MOSFET devices, it is possible to utilize an integrated Schottky structure to create a Merged-PiN-Schottky rectifier which has superior reverse recovery characteristics when compared with the P-N body diode. With advent of the super-junction devices, the perfor­mance of the body diode has to be re-examined due to very large junction area introduced in these structures. The reverse recovery characteristics of the body diode in the super-junction power MOSFET structures can also be controlled by using electron irradiation.

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• What is power MOSFET? Explain.