How do MOSFET current and thermal limitations interact?

Power MOSFETs are popular switching elements in many modern power devices. As engineers try to squeeze ever more performance and efficiency from each design, then MOSFETs are pushed to their limit. However, there is obviously a limit to each device's capability, yet there is no single means to define this limitation as it depends on a number of factors that interact.

Generally speaking, MOSFETs with lower on-resistance (R_ON) are able to operate with higher currents. Devices exist with R_ON values lower than 1mW that can theoretically operate with currents up to 500A in a small 5mm x 6mm package. However, this is not realistic in practice as other parametric limits will inevitably be exceeded.

Toshiba MOSFET datasheets show the lowest (e.g. worst case) of several limitations, each of which is calculated or measured. Some of these datasheets show the silicon limitation as well as the lowest limitation, which always defines the limit for the device. For low resistance MOSFETs, the current limitation will mainly depend on the device package.

Any design incorporating MOSFETs is limited by temperature. Temperature depends on two factors; the ability to radiate heat and the on-resistance, which is proportional to heat generated. It does not depend on the drain current (I_D).

Other topics for consideration include the carrier density, which puts a limit on I_D to ensure that the MOSFET only operates in the linear region, and avalanche mode. Although avalanche mode does not apply to every MOSFET, for some the avalanche current can flow through an equivalent transistor formed by the diode-MOSFET combination. This current, if unchecked, can damage the MOSFET.

When comparing maximum I_D values between Toshiba and other suppliers, engineers must ensure a good and valid comparison to be able to select the most appropriate device. Only through understanding the operation of the MOSFET electrically and thermally and applying the appropriate limits can this comparison be made.

Toshiba has produced a white paper with example calculations on this subject. To download your copy, please click here: