MOSFETs and floating gate potential

[sandy.bridge]

Hello all,
Did a lab pertaining to numerous gates (AND, Or, etc) and we decided to measure the input when nothing was connected. We found the voltage to be ~1.06V. Is this voltage we are reading have something to do with the threshold voltage? If not, can someone explain what is happening here? Thanks!

 

[yungman]

For MOSFET, the gate is really insulated from the device. You might be measuring the static charge potential or some device has diode type of protection and it might form a very high impedance voltage divider from leakage current or something. I don't believe that has anything to do with threshold voltage. The impedance of the gate is very very high and any charge can develop different voltage affect the output.

In fact MOS input is very funky, if you have a bad solder joint on an input pin but the pin is very close to the pad, the signal can couple through the small air gap and the the gate or IC will work most of the time and just fail once a while. We had a case long time ago with static RAM board in a computer. The computer fail once a week average and we went through it over and over and never managed to find the problem. It was not until we tried to upgrade the RAM chip with new ones and found out one of the address pin was bent under the socket and never got connected. It worked for over a year! There goes to show how high impedance are they.

 

[sandy.bridge]

Hmm, interesting. We had about 19 gates on a DE2 board, and every single input that was measured when floating had the same potential (~1.06V).

 

[yungman]

Hmm, interesting. We had about 19 gates on a DE2 board, and every single input that was measured when floating had the same potential (~1.06V).

As I said, it might have protection diode or something. But I don't think there is any bearing on the threshold. The metal oxide insulated gate is like a sheet of insulation. Whatever happen at the outside has nothing to do with the threshold.

 

[Antiphon]

Give a device and we'll check the data sheet for you.

 

[sandy.bridge]

It's a DE2 board.

Furthermore, there were a few LEDs on the board that were on when their input was high, as well as floating. The only time they were off was when the input was 0V. Is this due to the same effect described?

 

[yungman]

I think so as we were playing with JFET those days, we touch the gate and the light comes on and it stay on until we disconnect it. We did not wait for a long time though as it was just to prove a point that those FETs is so high impedance that once you charge the gate by touching it, the charge stays and hold the FET on!

 

[sandy.bridge]

Okay, but when you ground the gate, and then leave the input floating without touching it, the LED is on. It must be charging in the same manner that the floating inputs are; that is, protection diodes.

 

[yungman]

Okay, but when you ground the gate, and then leave the input floating without touching it, the LED is on. It must be charging in the same manner that the floating inputs are; that is, protection diodes.

I am confused, if you ground the gate, the input is not floating anymore. Am I missing something?

If you refer to say an OR gate, you ground one side, if the other side floats high, you still get a high output if that's what you are referring to.

 

[sandy.bridge]

No no, I'm the one that is confused. If the board's default high is 3.3V, and its low is 0V, is a floating voltage of 1.06V considered floating high? I was under the impression this voltage would neither be considered high, nor low. It could very will be in the 'noise margin', perhaps?

 

[Jony130]

Consider the effect of a parasitic capacitance between gate and source (GND).

 

[yungman]

No no, I'm the one that is confused. If the board's default high is 3.3V, and its low is 0V, is a floating voltage of 1.06V considered floating high? I was under the impression this voltage would neither be considered high, nor low. It could very will be in the 'noise margin', perhaps?

You cannot trust the reading. Your meter might have internal capacitance that is already charged up to about 1V. If you probe the gate, you might be the one that charge the gate to 1V because of your probe! It is hard to speculate your reading.

As for the threshold voltage, you should refer to the data sheet or application notes of the manufacturer. You should be able to find this information. I have no idea and I won't want to speculate. These are two link that I saw on quick search.

http://www.sparkfun.com/tutorials/225

http://www.nxp.com/documents/data_sheet/74LVT14.pdf

Sounds like the "1" threshold is like 2V, 1.06 is in the grey area.

FETs are strange animal!