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Power MOSFET Information


power mosfet selection guidePower MOSFETs are specialized metal oxide semiconductor field-effect transistors (MOSFET) designed to handle significant power levels.


Although early power MOSFET manufacturing used several different designs, the Vertical Diffused MOS (VDMOS) — also known as the Double-diffused MOS (DMOS) — emerged as the predominate design. The terms "vertical" and "diffusion" are adequate to describe the construction of a power MOSFET. "Vertical" refers to the fact that, unlike traditional (or "lateral") MOSFETs, a power MOSFET's source terminal is above its drain terminal, resulting in a primarily vertical current. By contrast, a lateral MOSFET's source and drain terminals are on opposite sides of the device, with the gate situated on the n-doped channel. "Diffusion" refers to the device's manufacturing process: the p-type and n-type wells that sit on the n-type substrate are created using a double-diffusion process.

power mosfet selection guide

A VDMOS diagram, showing its vertical construction and N+ and P+ wells.

Image credit: Wikipedia


Despite the predominance of VDMOS, some lateral power MOSFETs are still used in specialized applications.power mosfet selection guide


Power MOSFET operation is slightly different than that of a lateral e-MOSFET. In lateral MOSFETs, planar construction results in current and breakdown voltage values which are functions of the channel dimensions, rendering the device relatively inefficient. In contrast, the vertical construction of power MOSFETs results in voltage ratings which are functions of the doping and thickness of the n-type well, while current rating remains a function of channel width. Power MOSFETs are therefore able to sustain high voltages and high currents without increasing the size of the device.


The video below provides an excellent introduction to transistors, semiconductors, doping, and, towards the end of the video, power MOSFET.



Video credit: DesignWorldVideo/ CC BY 3.0



Power MOSFETs are used in many applications necessitating power semiconductors. Compared to insulated-gate bipolar transistors (IGBT) and thyristors, power MOSFETs feature very high switching speeds and improved efficiency at low voltages. Power MOSFETs are the most common low-voltage (less than 200 V) switch in use, and can be found in power supplies, DC-DC converters, and controllers.


Operating Principles and Specifications


power mosfet selection guideLike all MOSFETs, power MOSFETs switch and regulate a current that flows between the source (S) and drain (contacts) by varying voltage at the gate (G) terminal. By applying a voltage to the gate, a channel is created between the gate and the source, allowing current to flow.


By increasing the gate-source voltage (VGS), the channel becomes larger and the drain current (ID) increases. The relationship between the gate voltage and drain voltage is dependent upon the following equation:





ID = drain current

K = device constant

VGS = gate voltage

VT = threshold voltage


Threshold voltage (VT) refers to the minimum voltage necessary to create a channel. By visually plotting a device's drain current against its various gate-source voltages we can determine the device's transfer characteristics. A graphical representation of a typical n-channel power MOSFET's transfer characteristics is shown below.


power mosfet selection guide

Image credit: Circuitstoday


Other Specifications


When selecting power MOSFET products, two important specifications to consider include drain saturation current and gate-source cutoff voltage.


Drain saturation current (IDSS) is a measure of drain current saturation, which occurs when the drain-source voltage equals the gate-source voltage. When a MOSFET's drain current reaches a maximum value it remains there despite any increases in the drain-source voltage; this extra voltage is accommodated by a depletion layer located at the drain end of the gate. This condition is known as drain current saturation, and is represented by IDSS as a maximum current value.


Gate-source cutoff voltage (VGS(Off)) is the value of the gate-source voltage (VGS) which results in a drain current (ID) value of close to zero.




Power MOSFETs can be manufactured and tested using standards developed by a diverse range of societies and associations. Examples include:




Fairchild Semiconductor - Introduction to Power MOSFET (pdf)


Georgia State University - MOSFETs


University of Colorado - MOS Field-Effect Transistors


Image credit: Advanced Power Electronics Corp. | RS Components | Mouser Electronics



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