Let me use this post to replace The #14 post, that Claude responded to with the #20 post with the following single sentence. Claude, I believe you should say instead that MOSFETs are charge controlled devices typically driven by voltage sources.Yes MrAl, that's why I said calm down to Claude, cause I didn't mean to argue anything at all =P Claude didn't however use the word charge controlled device in his post. I know he's argued for them being charge controlled devices in other threads because I was part of one of them that turned into a big deal (Much like yours and my arguement in the Ohm's law thread MrAl)
However I still side with description of the semi-conductor effect as being defined by the electric field, in many devices current is required to be moving for those fields to exist, however it is the field itself which causes the change in the conduction state of the material, these fields are VERY closely bound to atomic structure so it's difficult to describe them without the currents, and in the real world the currents are actually the important part of what's going on, they are the initiator of the switching action (please read switching action as change in conductance), but they are not the cause of it. That's where me and Claude disagree. Because as modeled (and believe me the models are damn accurate) a transistor can be completely described by the currents flowing through the junctions and their discrete properties. However without the electric fields that are produced the actual PN junctions, those currents can't flow.
In slightly different words a difference of potential can not CHANGE without the flow of a current, however the electric field itself changes the possible electron configurations in the atoms near it to allow current to flow in the first place. The manipulation of those boundaries is the entire foundation of nearly all modern commercial semi conductor devices.
A field can exist without a current, the field causes the state in a semi conductor material. To change states the field has to change which means a current needs to flow. It's a symbiotic relationship for switching, it is however the field that determine the actual state.
1st bold highlight. "MOSFETs are charge controlled devices best driven from a *constant voltage* source"! But I've stated the same since forever. Remember the OP topic, "gate current in MOSFET". I was explaining its significance. Sure a FET needs current to charge the gate & establish the E field. I've argued that for years. Once the FET is in a given state, no gate current is needed to hold that state. I agree with Mr. Al that FETs need current in the dynamic mode, but not in the static mode. We have universal agreement there.
2nd highlight. Sure, fields are all important. But fields exist due to the presence of charges. A charged particle is surrounded by an E field. I know about time changing magnetic fields inducing E fields, but that is not how a bjt or FET operates. Any E field is due to charges. The quantity & location/distribution of said charges ultimately controls device behavior. Voltage is defined as work per unit charge along a specific path. But what is the work done? It is the line integral of force over distance. But what defines said force? Answer, Coulomb force, an inherent property of charges. First, charges exerting forces against one another over distance give meaning to potential or voltage. E fields are due to charge, and voltage is a derived quantity. Thus the bjt & the FET are ultimately charge controlled when viewing them at the mIcroscopic semiconductor physics level.
3rd highlight. "A difference of potential cannot change without the flow of current"!!! Those are my words exactly for the last 30 years of my EE career. We agree. How about that!
As far as a field existing w/o a current goes, it depends. For a FET in the static mode, charge flow from drain to source does not expend energy from the gate to source E field. The E field from gate to source requires no gate current to sustain it, only to change it. But in a bjt, the E field from base to emiter would be neutralized by charge flowing from base to emitter, holes, and emitter to base recombination, electrons, ref npn. Thus base & emitter current are needed to sustain the E field, even in static conditions. So a bjt needs current just to stay in the same state. Likewise for diodes, LEDs, SCRs, etc. The E field requires I & V to switch, and in the case of non-insulated semiconductors, to sustain a static state as well. You earlier stated that I & V are basically co-dependent. I've never said otherwise.
"The field determines the state" is just a roundabout way of saying that bjt & FET are *charge controlled*. I & V are important & necessary, but charge is the most basic quantity, and devices are said to be charge controlled.
In a nutshell, what is the significance of gate current in a FET per the OP original question? Answer, in the analog/amplifying mode, the gate needs current to amplify an analog signal. At low frequencies, said gate current is so small, one can inadvertently ignore it and not even know its significance. But although it is small, it's still important. At high frequencies, gate current is too large to ignore. In switching mode, gate current is needed to switch states, but not to sustain a state.