Power MOSFET: Switch

-.the power MOSFET is widely used in power switching applications - several types of power MOSFET have been developed including the VMOS, UMOS, TrenchMOS & HEXFET.

MOSFET technology lends itself very well to power applications. With some adaptations, to the basic MOSFET format, power MOSFETs have been widely used.

Power MOSFETs are widely used in power switching applications. The low gate drive power needed, fast switching and low series resistance means that these devices are well suited to power switching applications.

Most power MOSFETs feature some form of vertical structure. To achieve this the source and drain are normally on opposite sides of the wafer. This enables the device to support higher current and voltage capabilities.

Power MOSFET types

Within the overall arena of power MOSFETs, there are a number of specific technologies that have been developed and addressed by different manufacturers. They use a number of different techniques that enable the power MOSFETs to carry the current and handle the power levels more efficiently. As already mentioned they often incorporate a form of vertical structure

The different types of power MOSFET have different attributes and therefore can be particularly suited for given applications.

  • Planar power MOSFET:   This is often thought of as the basic form of power MOSFET. This form of device is good for high voltage ratings because the ON resistance is dominated by the epi-layer resistance. This structure is generally used when a high cell density is not needed.

    Typical planar power MOSFET structure showing the vertical current flow through the device.
    Planar power MOSFET structure
  • VMOS:   VMOS power MOSFETs have been available for many years. The basic concept uses a V groove structure to enable a more vertical flow of the current, thereby providing lower ON resistance levels and better switching characteristics.   Read more about VMOS FETs
  • UMOS:   The UMOS version of the power MOSFET uses a grove similar to that the VMOS FET. However the grove has a flatter bottom to it and provides some different advantages.   Read more about UMOS FETs
  • TrenchMOS:   Again the TrenchMOS power MOSFET uses a similar basic grove or trench in the basic silicon to provide better handling capacity and characteristics. In particular, Trench power MOSFETs are mainly used for voltages above 200 volts because of their channel density and hence their lower ON resistance.   Read more about TrenchMOS
  • HEXFET:   This form of power MOSFET uses a hexagonal structure to provide the current capability.

Power MOSFET breakdown voltage

One key area of importance for any power MOSFET is the breakdown voltage. With these devices operating at might higher voltages that the more traditional small signal and logic FETs, maximum operating voltages are more of a key issue.

In most power MOSFETs the N+ source termination and the P body junction are shorted using source metallisation. This avoids the possibility of spurious turn on of the parasitic bipolar transistor within the structure.

In operation, when no bias is applied to the gate, then the device is able to provide a high drain voltage through the reverse biased P type body and N+ epitaxial layer junction (shown as P-silicon and N- on the planar power MOSFET diagram). When high voltages are present, most of the applied voltage appears across the lightly doped N- layer. If a higher operational voltage is required, then the N- layer can be more lightly doped and made thicker, but this also has the effect of increasing the ON resistance.

For lower voltage devices, the doping levels for the P silicon areas and the N- become comparable and the voltage is shared across these two layers. However if the P silicon area is not thick enough then it can be found that the depletion region can punch through to the N+ source region, giving rise to a lower breakdown voltage.

On the other hand, if the device is designed for too high a voltage, then the channel resistance and threshold voltage will increase. As a result careful optimisation of the device is needed. Also when choosing power MOSFET devices, it is necessary to opt for one that provides the correct combination of breakdown voltage and ON resistance.

Power MOSFET threshold voltage

The threshold voltage VGS(TH) is the minimum gate voltage that can form a conducting channel between the source and the drain.

For power MOSFETs the this threshold voltage is normally measured for a drain source current of 250µA.

The threshold voltage is determined by factors in the power MOSFET including te gate oxide thickness and the doping concentration in the channel.

Power MOSFET capacitance

The switching behaviour of the device is greatly affected by the levels of parasitic capacitance that occur within the MOSFET.

The main areas of capacitance that affect the switching performance are gate to source capacitance CGS; gate to drain capacitance, CGD; and the drain to source, CDS.

These capacitances are non-linear and they are dependent upon the device structure and the voltages present at any given time. Thy result from the bias dependent oxide capacitance and the bias dependent depletion layer capacitance. Typically as the voltages increase, so the depletion layers increase and the capacitance levels decrease.

Power MOSFET applications

Power MOSEFET technology is widely used in many switching applications, especially for applications like power supplies, DC to DC converters, and for low voltage motor controllers.

They are most widely used in applications where voltages are less than 200V but some are designed for use above the voltages.

The advantage of the power MOSFET is the low ON resistance that enables losses to be minimised when used in a switching configuration.

By Ian Poole


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