Mosfet Gate resistance

Why gate resistance of a mosfet need to be low even it doesn't carry any current?

It doesn't carry steady-state current, but there is a large gate capacitance that has to be charged and discharged when the MOSFET is used as a switch, and that requires significant gate current if the switching is to be done rapidly.

Hi,

the intrinsic gate resitor forms a RC circuit with the gate-source capacitance of the MOST. Thus a large resistance value leads to a longer charging and discharging time. If the gate source voltage changes “slowly“ the MOST is operated in linear mode (where the MOST aczdts as a varible resuisto) for a longer time, which leads to power losses due to the drain current and the MOST on-resistance. So especially for switching applications like DCDC converters this would lead to higher losses and thus to a lower efficiency. Have a look at [1] for further information.

[1]https://www.st.com/resource/en/appl...impact-on-applications-stmicroelectronics.pdf

BR

The gate does not conduct DC current.
But it controls the FET by its voltage (gate voltage).
To change the gate voltage, you need to charge gate capacitance (gate to source/drain/channel).
This charging is done through gate resistance / metallization.
Too high gate resistance leads to slow switching, which is bad for many applications.
It also causes many other bad things - like large gate (thermal) noise, low Q factor in MOS capacitors, etc.

Also, gate resistance is just one number, and does not reflect well the distributed properties of the gate network - which may be causing non-uniform gate delay and gate switching, hot spots, and other parasitics effects.

In particular, too large gate resistance will cause a dynamic gate opening due to drain dV/dt effect.

Note that sometimes, too low gate resistance is bad too - it may cause too large charging current, that can burn the gate network interconnects.

Regards, Dana.