Jay did a teardown of a Curtis 1231C-8601 500A PWM DC motor controller:
The Curtis 1231c-8601 power board is relatively simple. It uses 18 MOSFETS in parallel to switch current from the motor- terminal back to the battery- terminal (the motor+ lead is already connected to battery+). The MOSFETs it uses are IXYS IXTH50N20 SP9536 chips. The center lead is bent up over the chip and soldered to a ring terminal, such that the screw that attaches the chip to the heat sink also electrically connects that pin to the heat sink. Most chips that attach to a heatsink have a metal back, but the IXTH50N20’s used here do not, so it appears that they had to take extra assembly steps to electrically couple it to the heatsink.
More details at Jay’s blog.