I'm looking for PMOS with BJT to drive a load (motor,) that is controlled by a microcontroller (IO_FET):

enter image description here

The motor needs three amperes of current. I looked at the manufacturer's pages of the PMOS (datasheet) and I see the following parameter:

enter image description here

Shpuldn't the input resistance of the PMOS be a couple of MEGA/GIGA ohms? What am I missing here?

  • \$\begingroup\$ Related question: electronics.stackexchange.com/questions/384385/… \$\endgroup\$
    – nanash1
    Sep 5 at 9:03
  • \$\begingroup\$ @nanash1 I don't ask how to calculate the gate resistance, I'm asking how it makes sense that the gate resistance of mosfet is so low instead it should be so high, That was my question, your related question justifies my question, it 2 ohm in related question! \$\endgroup\$
    – Knowledge
    Sep 5 at 9:06

1 Answer 1


The gate resistance in this datasheet refers to the series resistance between the gate (pin) and the actual gate on the substrate.
This resistance is made up by the pin, bond wire, the substrate itself etc. Ideally this resistance should be 0 Ω, but in reality it's not.

Here is a simple drawing of a typical PMOS transistor model inside an actual package with bond wires and pins.
The gate resistance includes all it takes to charge up gate capacitance on the die. MOSFET drawing

Gate resistance does not refer to the resistance from gate to source or drain, which should of course be close to infinite.

  • \$\begingroup\$ can you add to your answer the cross section of PMOS to understand your meaning between the gate pin and the actual gate on the substrate? \$\endgroup\$
    – Knowledge
    Sep 5 at 10:13
  • \$\begingroup\$ The pin itself has some length of thin wire/conductor which has some resistance, and then the internal interconnects from the gate pin to the actual gate electrode "plate" add their own resistance to this, which end up being fairly low. Basically, this resistance is taken from the gate pin to the gate "plate" in the MOSFET. \$\endgroup\$ Sep 5 at 11:04
  • 1
    \$\begingroup\$ @Knowledge Drew up a simple picture for you. \$\endgroup\$
    – Klas-Kenny
    Sep 5 at 11:08
  • 1
    \$\begingroup\$ The general concept is correct, but discrete MOSFETs often use a different geometry to allow source and drain connections on opposite sides of the substrate. \$\endgroup\$
    – supercat
    Sep 5 at 21:25
  • \$\begingroup\$ @supercat You are right, but when drawing the picture I chose to use the most commonly known text book geometry for easy understanding, since the question was specifically regarding gate resistance and not overall MOSFET design. \$\endgroup\$
    – Klas-Kenny
    Sep 6 at 5:45

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