How to calculate MOSFET internal gate resistance?

Question

I was trying to figure out how to calculate the internal gate resistance of the MOSFET. For the purpose of analysis, Let's take the MOSFET as - BUK7Y3R5-40E

From most of the documents I read, best way to get the internal gate resistance is to measure it out.

With my current circumstances, I would like to go with a mathematical calculation but I could not find anything as reference.

Can someone suggest how to do calculate the MOSFET internal gate resistance. If it is difficult to calculate the value accurately, any approximation will be sufficient.

Answer 1

You can extract some typical values from the SPICE model for the MOSFET you have in mind. For example, for the one you mention in the question the model has the following lines:

LG GATE 1 7.81292696075128e-10

RLG GATE 1 1.96360271543439

So the gate acts like it has about 2 ohms in series with 0.8nH

Answer 2

A simplified way to model/look at the gate of an NMOS can be this:

^{simulate this circuit – Schematic created using CircuitLab}

There will be a series resistance Rs

There will be a parallel resistance Rp

There will be the gate capacitance Cgate

Although not so clear from your question I think you are asking about Rs, the series resistance.

Asking about Rp would not make much sense anyway as it is often too high to even measure. Some manufacturers specify a gate input current and then Rp should be a current source in the model or even better (more accurate), a diode with a leakage current. This diode is sometimes present for ESD protection but it often leaks more than the gate itself!

But back to Rs, this Rs depends on the shape and material used to make the gate structure inside the MOSFET. There is no way for you as an end user to know what size, shape and material (and it's resistance) the gate is made off. Semiconductor companies regard this as their "trade secret".

Luckily, for almost all of the normal applications of these MOSFETS, you generally do not need to know the value of Rs as it will almost always be low enough. My guess is that for most power MOSFETs the value of Rs will be a few ohms at most.

The only applications I see where the value of Rs does matter is for extremely fast switching. Fortunately the fastest switching speed or largest delay is often mentioned in the datasheet.

I have seen other cases where the gate resistance was indeed relevant but that was for an on-chip CMOS filter. There the value of Rs was calculated by the MOSFET models so I could optimize my MOSFETs to minimize Rs. But that's on-chip low power design, this has little to do with power MOSFETs for switching applications.

Answer 3

MOSFET gate resistance is a very high value (much higher than megohms) and so should be of next to zero concern when working out how to drive the gate. On the other hand the gate of a MOSFET will have a capacitance. On a power MOSFET this capacitance can be a very respectable amount and the data sheet for the part will provide specifications that characterize this capacitance.

It is the gate capacitance that you need to be concerned with when you are selecting the gate driver. A large gate capacitance that needs to have the gate switch very quickly (to permit efficient switching behavior of the MOSFET) may require a driver that has to dynamically switch multiple Amps of current. The reason you often see a small valued resistor between a gate driver and the MOSFET gate is to limit the maximum dynamic current flow to a level that the driver can handle. Of course slowing down the maximum dynamic current flow has some other advantages as well including reducing voltage rail and GND switching noise.

Answer 4

Depends upon the material used in making the gate electrode.

1) metal gate process was used in early 15 volt Motorola logic devices, and is still used in some integrated circuits, because of the very low cost (5 to 7 masks); a square of "gate" would be 0.02 ohms, for example.

2) "more modern" processes use silicon-gate, specifically polysilicon; polysilicon resistance can vary at least 100X in its resistance, say from 10 ohms (low voltage FETs) to 1,000 ohms (high voltage FETs)

All this is from memory. No guarantees.