# Contact on substrate of a MOSFET

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

is as big as the FET.

• So the implicit assumptions are that the bulk and source are tied together and the on resistance of the shottky diode added to RdsOn will violate the R << Rload condition? – Bran Tran Jun 19 at 4:12
• Thank you for the clarification. – Bran Tran Jun 19 at 5:01

Regardless of the polarity of charge movement, the substrate needs to be controlled. A rectifying-contact blocks that control in at least one polarity.

• So it isn't a problem when the substrate is tied to a constant voltage? – Bran Tran Jun 19 at 4:13
• The source-bulk diodes need to be respected. Typically source-substrate re shorted, to get the predictable Vthreshold. Even if the source and drain are implanted directly onto the substrate, the source-substrate tend to be shorted RIGHT AT THE FET, to minimize time-delays in building or in erasing the channel (which impacts time-jitter, etc). – analogsystemsrf Jun 19 at 4:29
• Thank you for your answer! But if the bulk and source aren't connected, does it mean that the schottky diode isn't a problem? – Bran Tran Jun 19 at 4:35