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I want to usea 1uF capacitor to create an electric field between Gate and Source (VGS applied to dielectric) of a transitor.

schematic

simulate this circuit – Schematic created using CircuitLab

Since no current is flowing can i assume the eletrci field persist for very long time (years)?

If not, what do i need to calulate the discharge time? I'm guessing the dielectric leakage?

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    \$\begingroup\$ What's your actual goal here? Things that really leverage something along these lines (EPROM, DRAM, etc) for massive density tend to use microstructures built just for that purpose. There'd be little reason to try to do it with discrete parts since there are such better ways of accomplishing macro-scale goals. \$\endgroup\$
    – Chris Stratton
    Commented Apr 24, 2020 at 20:40
  • \$\begingroup\$ I am using the transitor as a sensor, I need to apply a specific VGS to get the desired IDS resposnse. But since the sensor is to be used over a year or so, i need the electric field to remain constant over VGS \$\endgroup\$
    – Leo
    Commented Apr 25, 2020 at 21:15
  • \$\begingroup\$ I need the capacitor or whatver manages to keep the voltage constant to do it without using energy, as the sensor is passive \$\endgroup\$
    – Leo
    Commented Apr 25, 2020 at 21:17
  • \$\begingroup\$ How high voltage do you need to have there? \$\endgroup\$
    – quetzalcoatl
    Commented Apr 25, 2020 at 23:39
  • \$\begingroup\$ @quetzalcoatl within a range (0-10 V) for most appllications. I shift use the applied Voltage to shift Vth so that the transitor goes in the On state at different quatities of measured stimulus. \$\endgroup\$
    – Leo
    Commented Apr 26, 2020 at 5:18

2 Answers 2

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+1 to using a commercial memory product, but if you want to know the answer...

Yes, there will be some gate leakage. There are various physical mechanisms that contribute to this, but it will never be 0 so long as there is a voltage difference between the gate and either the source or drain.

If you are using a discrete MOSFET, check the data sheet and see if it specs a maximum gate leakage. For example, here's OnSemi's 2n7000: https://www.onsemi.com/pub/Collateral/2N7000-D.PDF

Under Off Characteristics, we have "Gate-Body Leakage Current, Forward", with a max of 10 nA @ Vgs = 15 V, Vds = 0 V.

You can take that current and use it to calculate how fast the capacitor will discharge. For a 1uF cap, 10 nA of current will discharge it at a rate of 10e-9/1e-6 = 0.01 V/s.

Then, for this transistor, the max Vgs(th) is 3.0 V, so if you start at a Vgs of 10.0 V, it will take (10.0 V - 3.0 V)/(0.01 V/s) = 7/0.01 = 700 seconds for the gate voltage to fall beneath the threshold voltage. This is a roughly worst case analysis, because as Vgs falls there will be less gate leakage current, etc. Edit: As someone pointed out, you will also need to account for the self-discharge of the capacitor.

700 seconds may be long enough for your application, or may be way too short. Note also that the transistor characteristics will not stay constant over this time period. You are reducing the Vgs, so the Rds(on) will rise and Ids will fall. (See figure 1)

Note that this discharging process is exactly why DRAM needs to be refreshed, though in that case the capacitor is connected to the drain of a MOSFET, not the gate, but same idea applies.

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  • \$\begingroup\$ Ok, for my appication i need it to stay contant for a period measure in years, so since i am using the VGS to keep RDS constant, i need it to really not leak. So i will need a transitor with leakage in the picoA range or even lower. is it possible to have low capacitor discharge for a period of years, ot should i not even attempt this? \$\endgroup\$
    – Leo
    Commented Apr 25, 2020 at 21:25
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    \$\begingroup\$ Not possible, do not attempt this. You need some kind of battery or energy harvesting solution (e.g. solar cell) to power it when not receiving power from the RFID reader. Also, unless you are doing this for academic reasons, it would probably be smarter to use an commercially available, integrated temp-sensor IC, with a digital comm interface that lets you read out the current temperature. There are many variety of temp logger products available that can track temp over time too, but these will require a battery or power source of some kind. \$\endgroup\$
    – Evan Cox
    Commented May 4, 2020 at 22:07
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I think you will find that the leakage through the capacitor may dwarf the leakage through the MOSFET gate. The actual amount of the leakage depends a great deal on what kind of capacitor you select...electrolytic, ceramic, etc.

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  • \$\begingroup\$ What kind of capacitor do you recoomend? i need it to also be very small as i am making a rather small, lightweight sensor. \$\endgroup\$
    – Leo
    Commented Apr 25, 2020 at 21:19
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    \$\begingroup\$ I don't know of any capacitor that will hold a charge for a year, particularly a small one. \$\endgroup\$
    – Elliot Alderson
    Commented Apr 25, 2020 at 22:05
  • \$\begingroup\$ ok, so do there isn't any solution to hold VGS at a preset voltage for long perdiods of time without input energy? \$\endgroup\$
    – Leo
    Commented Apr 26, 2020 at 5:20
  • \$\begingroup\$ maybe i should open a new question on how to keep VGS fixed in an RFID (since my application used an RFID for passive readout), perhaps there is a way of using the harvested RF energy to harvest VGS \$\endgroup\$
    – Leo
    Commented Apr 26, 2020 at 5:22
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    \$\begingroup\$ You cannot do this on a continuous basis for anything but a large change such as switch closure. To actually measure an impedance with any precision, you'd have to poll. You're whole approach here is unworkable no matter how magic an FET you think you could find. \$\endgroup\$
    – Chris Stratton
    Commented Apr 26, 2020 at 19:05

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