Power MOSFETs are well known for superior switching speed, and they require very little gate drive power because of the insulated gate. In these respects, power MOSFETs approach the characteristics of an "ideal switch". The main drawback is on-resistance RDS(on) and its strong positive temperature coefficient. This application note explains these and other main features of high voltage N-channel power MOSFETs, and provides useful information for device selection and application. Microsemi's Advanced Power Technology MOSFET datasheet information is also explained.
Power MOSFET structure
Figure 1 shows a cross section of an APT N-channel power MOSFET structure. (Only N-channel MOSFETs are discussed here.) A positive voltage applied from the source to gate terminals causes electrons to be drawn toward the gate terminal in the body region. If the gate-source voltage is at or above what is called the threshold voltage, enough electrons accumulate under the gate to cause an inversion n-type layer; forming a conductive channel across the body region (the MOSFET is enhanced). Electrons can flow in either direction through the channel. Positive (or forward) drain current flows into the drain as electrons move from the source toward the drain. Forward drain current is blocked once the channel is turned off, and drain-source voltage is supported by the reverse biased body-drain p-n junction. In N-channel MOSFETs, only electrons flow during forward conduction — there are no minority carriers. Switching speed is only limited by the rate that charge is supplied to or removed from capacitances in the MOSFET. Therefore switching can be very fast, resulting in low switching losses. This is what makes power MOSFETs so efficient at high switching frequency.
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