MOSFETs and floating gate potential

Click For Summary

Discussion Overview

The discussion revolves around the behavior of floating gate potentials in MOSFETs, particularly in the context of a DE2 board. Participants explore the implications of measuring a voltage of approximately 1.06V at floating inputs and its relation to threshold voltage, static charge, and the effects of high impedance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the measured voltage of ~1.06V at floating inputs is related to the threshold voltage of the MOSFET.
  • Another participant suggests that the voltage may be due to static charge or protection diodes, asserting that it likely does not relate to the threshold voltage.
  • Some participants note that all measured floating inputs on the DE2 board exhibited the same potential of ~1.06V.
  • There is mention of high impedance effects, with one participant sharing an anecdote about a faulty connection causing intermittent issues due to high impedance in MOSFETs.
  • A participant raises the question of whether a floating voltage of 1.06V could be considered "floating high," given the board's defined high and low voltages of 3.3V and 0V, respectively.
  • Another participant introduces the idea of parasitic capacitance affecting the gate's behavior and suggests that measurement tools may influence the readings.
  • There is a discussion about the potential for the meter's internal capacitance to affect the reading, leading to speculation about the accuracy of the voltage measurement.
  • One participant emphasizes the need to refer to the manufacturer's data sheet for definitive information regarding threshold voltage.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the measured voltage and threshold voltage, with no consensus reached on whether the floating voltage can be classified as high or low. The discussion remains unresolved regarding the implications of the floating voltage and its causes.

Contextual Notes

Participants acknowledge the potential influence of measurement tools and the high impedance nature of MOSFETs, but do not resolve the implications of these factors on the readings or the relationship to threshold voltage.

sandy.bridge
Messages
797
Reaction score
1
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!
 
Engineering news on Phys.org
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.
 
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).
 
sandy.bridge said:
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.
 
Last edited:
Give a device and we'll check the data sheet for you.
 
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?
 
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!
 
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.
 
sandy.bridge said:
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.
 
  • #10
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?
 
  • #11
Consider the effect of a parasitic capacitance between gate and source (GND).
 
  • #12
sandy.bridge said:
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!
 
Last edited:

Similar threads

Earth referenced bias into floating circuit?
  • · Replies 2 ·
Replies
2
Views
1K
Extremely high current effects in semiconductors
  • · Replies 11 ·
Replies
11
Views
3K
What does negative voltage mean for MOSFET?
  • · Replies 18 ·
Replies
18
Views
9K
Why is there a resistor in a MOSFET-based inverter?
  • · Replies 9 ·
Replies
9
Views
2K
High power switching circuit problem.
  • · Replies 7 ·
Replies
7
Views
3K
DC-DC Converter for Gate Driver Circuitry (Electric Motorcycle)
  • · Replies 6 ·
Replies
6
Views
5K
DC Reverse polarity protection using P-MOSFET
  • · Replies 8 ·
Replies
8
Views
7K
What are the functional differences between MOSFETs and JFETs?
  • · Replies 2 ·
Replies
2
Views
7K
Does an SN754410NE H Brdige need a microcontroller?
  • · Replies 5 ·
Replies
5
Views
2K
How Do BJTs Help Create Logic Gates in Circuits?
  • · Replies 26 ·
Replies
26
Views
17K