Electronic – Resistance of a MOSFET in saturation mode

mosfetresistancesaturation

I have been studying MOSFETs and have a question regarding saturation mode. To my understanding, the drain current is constant in saturation mode. Does this mean that the drain-source resistance increases with drain-source voltage (to keep drain current constant)? If so, does this mean that the voltage drop across the transistor increases with drain-source voltage? For instance, if I were applying a high voltage (say 20V, and the transistor was saturated at 10V) across the drain-source and had a load in series with the transistor, would I get a voltage drop much less than 20V across the load (because of the high resistance of the transistor)? I am not sure if my interpretation is too "ohmic". Any help/resources would be appreciated. Thank you!

Best Answer

enter image description here

For a given gate-source voltage and above a certain drain-source voltage, the MOSFET behaves like a constant current sink. Say you have 3 volts applied to the gate with respect to the source - if if the drain-source voltage rises to 2 volts, the drain current reaches a plateau of about 5 milliamps. If you increase the drain-source voltage (whilst keeping gate voltage at 3 volts), the drain current remains largely at 5 mA.

This is broadly referred to as "constant current" mode of operation.

Is the current drawn determined by the transistor or the load itself?

The current is determined by the transistor mainly but, because there is a slight slope there is a small change in current with a big change in drain load resistance.

for example, at saturation is this the max current it can draw or the amount of current it will draw?

This is the typical current it will draw for a given gate-source voltage (3 volts in my example).

Related Topic