Gate Current in MOSFET

[Rohit Chatterjee]

The gate current in a MOSFET is zero....but why is this so???is it because of the insulator between the metal and the semi-conductor....but in that case there would have been at alternating gate current when ac voltage source was applied.....

 

[MikeMl]

Look up the gate input capacitance specs for some real NFETS. The gate is a very large capacitor; so much so that it is difficult to charge and discharge the gate capacitance when trying to minimize power dissipation during turn-on and turn-off.

 

[peterbrown]

When you thinking about MOSFET, it is a voltage control rather than current control.

 

[Rohit Chatterjee]

True....but when MOSFET circuits r analysed the gate current is assumed to be zero....isnt this erroneous????
the insulator is nothing but a capacitor,n quite an appreciable amount of alternating current may flow through it.....

 

[mneary]

The net DC current will be (near) zero.

When you analyze a MOSFET circuit, the model should include the gate capacitance. If not, it is a bad model.

 

[Rohit Chatterjee]

Agreed...n when we r analyzing simple models....like when the operation of common source amplifier circuit is explained in books,they never include the gate capacitance...just assume gate current to be zero evn thought the gate voltage is ac...doesnt this cause a problem???

 

[peterbrown]

It all depends on what is switching frequency. remember every amplifter has it own working frequency. amplifiter has differnet model in different frequency band.

 

[MikeMl]

....doesnt this cause a problem???

Sure it does: look at this very poor example of how to drive the gate of an NFET used as a switch. The red trace is the gate voltage. Note how the input square wave is modified by the series resistance and the gate capacitance. There are two basic effects which you can see; one is the RC time constant, the other is the effect of "miller capacitance".

 

[nigelwright7557]

In reality the gate current is not zero as you are charging and discharging the gate capacitance.

 

[mneary]

The books are probably simulating at low frequency when the gate capacitance is not drawing much current.

A good engineer knows what factors are significant, and when.

 

[RCinFLA]

Gate drive current consumption is CVF, where C is effective input capacitance (more then just Cgs, there is Miller capacitance from gate to drain), V is gate voltage swing, and F is switching frequency.

This is one of the trade offs in designing switching power supplies. The faster the switcher the more gate power consumed (the gate driver).

 

[Rohit Chatterjee]

Thnx for all the response guys....

 

[Claude Abraham]

Yes, a MOSFET is a voltage controlled device, but it still needs current. All electrical devices need both. A bjt needs both, and is classified as current controlled. Whether a device is VC or CC, it still needs both in order to work.

One of the quantities gets *directly* driven, and the other indirect. A device labeled as VC simply means that the voltage is the directly controlled quantity, and current becomes indirect or incidental. Any voltage source connected across the gate to source terminals of a FET will provide the current needed to charge the gate.

The voltage controlled FET needs current. The current controlled bjt needs voltage. The control quantity only specifies which gets directly driven. It is understood that the other is indirect, but just as necessary.

Make sense?

 

[Sceadwian]

That's not entirley acurate Claude. Mind you were drawing a VERY fine line here but FET's don't need current to be active, they do need a current flow to switch but once charged the gate itself if properly insulated will not draw current the leakages discussed are a separate matter. It seems like a small quibble but it's technically correct to say FETS are charge controlled devices. BJT's are almost always referred to as current controlled devices, however this is also technically inaccurate as even though current has to flow for the devices to operate; on an atomic scale it's actually the voltage fields that develop at the PN junctions that cause the amplification effect to occur, current however has to flow for those voltage fields to exist. So it's definitely a co-dependancy thing, I just think it's an important distinction to say voltage controlled fets need charge, not current. This is of course only correct for ideal FET's, the real world is much more complicated and some gate current is unavoidable in real world operation.

 

[Rohit Chatterjee]

Make sense?

Ya it does make a lot of sense actually....without the forward bias across the B-E jn. in a BJT there would be no current at all n so no amplification.....

FET's don't need current to be active, they do need a current flow to switch but once charged the gate itself if properly insulated will not draw current the leakages discussed are a separate matter.

FETs may not NEED current to be active....n in the case of a JFET the gate current may be zero (or equal to the reverse saturation current) as the gate current has to flow through a reverse biased p-n jn....
But wht about the MOSFET???when an ac voltage is applied across the gate (as in amplifier circuits),shouldnt there be an alternating current flowing through the gate,as the gate capacitor cannot block the alternating current???now this may be conduction current,but current nevertheless....

 

[MrAl]

Hi there,


The input capacitance is partly due to the gate capacitance itself, but usually there is another spec that is used to
determine a good way to drive the mosfet gate and that is called the "gate charge" and is often in units of nC.
The effect of the capacitance as others have pointed out is that it draws current when the input drive voltage tries to
change. In the typical use, there is a pulse applied and that means the gate must be charged at the leading edge
and must be discharged near the falling edge. Both of these actions require significant current, much much more
than when the gate voltage does not change. The current pulse can be as high as 1 amp or higher, and there are
special ic's made just for the sole purpose of driving the gates of mosfet devices. Also, with some high efficiency
power converters the gate power consumption is figured in with the total power wasted in order to get a more
accurate estimation of the expected efficiency of the circuit.
Capacitors always draw current when driven with a sine wave, so of course if the gate is driven with a sine wave
then there will be some input current all the time, but of course it depends on the frequency and also the
current conduction state of the mosfet. The simplified model looks like the gate has a cap from the gate terminal to
ground, and another cap from the drain terminal to the gate terminal, and both of these capacitances have to be
considered in the calculation for the input current.

 

[Rohit Chatterjee]

That was quite enlightening Mr.Al....thnx for ur tym n effort....

 

[MrAl]

That was quite enlightening Mr.Al....thnx for ur tym n effort....

u r wlcm

 

[MikeMl]

Hi there,


The input capacitance is partly due to the gate capacitance itself, but usually there is another spec that is used to
determine a good way to drive the mosfet gate and that is called the "gate charge" and is often in units of nC.
The effect of the capacitance as others have pointed out is that it draws current when the input drive voltage tries to
change. In the typical use, there is a pulse applied and that means the gate must be charged at the leading edge
and must be discharged near the falling edge. Both of these actions require significant current, much much more
than when the gate voltage does not change...

Which is what the SIM I posted to this thread shows

 

[Claude Abraham]

That's not entirley acurate Claude. Mind you were drawing a VERY fine line here but FET's don't need current to be active, they do need a current flow to switch but once charged the gate itself if properly insulated will not draw current the leakages discussed are a separate matter. It seems like a small quibble but it's technically correct to say FETS are charge controlled devices. BJT's are almost always referred to as current controlled devices, however this is also technically inaccurate as even though current has to flow for the devices to operate; on an atomic scale it's actually the voltage fields that develop at the PN junctions that cause the amplification effect to occur, current however has to flow for those voltage fields to exist. So it's definitely a co-dependancy thing, I just think it's an important distinction to say voltage controlled fets need charge, not current. This is of course only correct for ideal FET's, the real world is much more complicated and some gate current is unavoidable in real world operation.

I'm afraid it is technically accurate. First highlight in your quote, as far as FETs being *charge controlled* devices, I've been saying that forever! Check my posting history and that will be affirmed on this forum or anywhere else I've posted. A FET is ultimately charge controlled. But at the mAcro black-box viewpoint, in order to charge up the gate and channels, should we use a constant voltage source, or constant current? A CVS works better for FETs, and a CCS better for bjt's. Hence the terms current & voltage controlled.

Second bold highlight, you keep referring to "voltage fields", and I ask you where that comes from. In all fields texts in EE & physics, there are E, D, B, & H fields. What on earth is a voltage field? In a bjt, the amplification takes place with both current and voltage, just as a FET. The transport of electrons from emitter to collector is what we call transistor action, since the base is so thin, carriers go right through and reach the collector. Otherwise it is just 2 back to back diodes. The current gain is due to transport as I just mentioned. The voltage gain is due to the small b-e voltage drop needed since a forward p-n junction exhibits large current swings with only small voltage swings. Hence the bjt has a large transconductance, higher than a FET. Ultimately a bjt is charge controlled as well. I've been saying this for years.

I've always stated that bjt & FET are charge controlled at the mIcro level. The CC & VC models are only black box 1st order models, not meant to explain atomic interaction.

Third bold statement in your quote - sure voltage controlled FETs need charge, but to say "not current" is incorrect. How does the charge reach the device if we don't transport it there? We have to move charges into and out of the device. That charge motion time derivative is current. When you say "we need *charge*, *not current*, you are contradicting yourself. You treat current as if you wish it didn't exist, acknowledging it but downplaying its role entirely. Current is just dq/dt where q is charge. Do you know the related math? Have you had calculus, differential equations, fields, etc.?

All electrical devices in the universe need I & V both. One is just as important as the other, but not more so. So we appear to agree that at the mIcro level, the charge control model is best for both FET & bjt. At the mAcro level we regard the voltage control FET model & the current control bjt model to be 1st order approximations based on black box behavior, NOT on semi physics. No point in arguing further.