Semiconductor Fundamentals (Part 6)

High-Power MOSFETs George continues his article series looking at all aspects of the basic structures that make semiconductors work. In Part 6—his final article of the series—he builds on last month’s exploration of MOSFETs by this time examining MOSFETs designed specifically to handle high power. Last month’s article was dedicated to metal oxide semiconductor field-effect transistor or MOSFET designed to handle signals and power of relatively low level. Now let’s look at MOSFETs designed specifically to handle high power. Among the advantages of power MOSFETs are their high switching speed and good efficiency. At operating voltages less than 200V, the power MOSFET is the king. It is found in various applications from switching power supplies and motor controllers to analog amplifiers. Power MOSFETs come in three major categories: N-channel enhancement, P-channel enhancement and N-channel depletion—with the N-channel enhancement being the most popular and achieving the lowest RDSon. P-channel depletion type power MOSFETs theoretically exist but I haven’t found one commercially available. MOSFETs offer high input impedance and a forward voltage drop decreasing with temperature. That ensures even current distribution. As a result, power MOSFETs can be paralleled to achieve the desired current handling capability. Keep in mind that—like all other MOSFETs—power MOSFETs also contain the intrinsic bulk diode. Vertical Diffused MOS (VDMOS)—also known as Double-Diffused MOS (DMOS)—is the most prevalent power MOSFET structure today. Its cross-section is depicted in Figure 1. With the source electrode on the top and the drain electrode at the bottom of the device, the current flow through the device is vertical. The P wells and N+ regions are manufactured by a process called double diffusion—hence the “double-diffused” in its name. FIGURE 1 - Cross-section of VDMOS Unlike most signal devices that are planar, the structur