Why do we want to have an ohmic contact on a substrate terminal of mosfet? What would happen if we used schottky contact instead?
FETs are often used as Ideal Diodes when RdsOn is << lower than the load R unlike Schottky diode which have a low threshold but higher Rs than Silicon. So adding a diode in series would degrade the typical performance for losses as a switch.
We can compare cost, size, power, speed , voltage drop, max current to compare a power Schottky power diode with a power FET. There is no advantage with a Schottky contact.
Low RdsOn FET 1.8mOhm @ 29A = 52mV , max cont. = 185A (/w heatsink) < $1 in volume
High current Schottky FET 630mV @ 30A max cont. current < $0.50 in volume
- but its "contact" ...
- making it much smaller increases diode series Rs=ΔV/ΔI based on smaller power capacity. Rs[Ω] ~0.5/Pd [W]max for most diodes +/-50%
- whereas RdsOn above is Rs=0.0018Ω and Pd max= 108W +Hsink
Regardless of the polarity of charge movement, the substrate needs to be controlled. A rectifying-contact blocks that control in at least one polarity.