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

enter image description here is as big as the FET.

  • 1
    \$\begingroup\$ 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? \$\endgroup\$ – Bran Tran Jun 19 at 4:12
  • \$\begingroup\$ Thank you for the clarification. \$\endgroup\$ – 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.

  • \$\begingroup\$ So it isn't a problem when the substrate is tied to a constant voltage? \$\endgroup\$ – Bran Tran Jun 19 at 4:13
  • \$\begingroup\$ 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). \$\endgroup\$ – analogsystemsrf Jun 19 at 4:29
  • \$\begingroup\$ 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? \$\endgroup\$ – Bran Tran Jun 19 at 4:35

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