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I'd like to know what's the characteristic curve of Idss in N-Channel MOSFET transistors over different Vds values at high temperature. As I've checked datasheets (like Fairchild's 2N7000 or FDC637BNZ), the Idss parameter is normally specified for Vds voltages very close to BVdss (breakdown voltage). It is clearly stated that current will increase considerably at higher temperatures, but the effect of Vds is not mentioned. I've also read some application notes (like NXP's AN211A or Fairchild's AN-9010) and they do not provide much information about it either.

I have this design where I need to keep Idss within the range of the low µA @ 100°C (the lower, the better), and I've thought I could get away with a transistor that has a much larger BVdss than the working voltage (e.g., BVdss=100V, working voltage=3.3V), but it's unclear if that would be of any effect.

The question is: is the Idss parameter solely dependent of temperature, or the Vds/BVdss ratio would play a role with it?

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  • \$\begingroup\$ I think with Vds Idss would increase, I would expect even non-linearly (Double the Vds => Idss would go higher than double). If you need to keep Idss low, maybe to try with negative Vgs? \$\endgroup\$ – Darko Mar 22 '16 at 12:59
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The leakage current and the breakdown voltage Vds_max are not so related in my opinion.

Leakage current can be lowered by an increased gate doping which in turn increases the MOSFET's threshold voltage. The switching MOSFETs you're looking at are designed to have a low threshold voltage and are thus more "leaky".

The breakdown voltage Vds_max is related to the doping profile of the drain, it has no direct relation to leakage as far as I know. A high breakdown voltage means a more lightly doped drain and this could result in a higher Rds_on.

You could search for a MOSFET with a high threshold voltage, maybe it will also have lower leakage.

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IDSS for a MOSFET with VGS=0 does not change much with VDS.

In theory, once VDS exceeds about 100 mV (from exp(VDS/kT-1) ), there is no further increase in ID. However depletion effects (analogous to Early voltage in BJTs), and some interactions in the device do cause small increases. As the VDS approaches BVDSS (say within 20 % of actual BVDSS), the current does increase further due to avalanche multiplication.

A FET with given RDSON will generally have higher IDSS for a higher BVDSS device (mostly because it will be a larger device with lighter doped junctions).

IDSS also depends on the gate voltage (e.g. applying a negative voltage to the gate will further decrease the current), and more specifically the difference between gate voltage and threshold voltage -- i.e. how much lower than VT the gate voltage is. For a power MOSET, IDSS will decrease by a factor of approximately 10 for each 200 mV decrease in VGS when the device is biased with VGS lower than VT. Thus if you could arrange to bias VGS = -200 mV, you would have very significantly decreased currents -- but generally this is complex to achieve.

Note that the 200 mV parameter depends on the construction of the device, including the gate oxide thickness. Thinner GOX devices will have a lower value.

In summary, a FET without excessive BVDSS ratings, with high threshold voltage (but a lower max VGS rating which means thinner GOX) will be a better choice.

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EDIT: This answer might not be relevant after all; see jp314's comment and answer for details.

With further investigation, I've found the answer. The Idss value does change with Vds, but the change is not very significant after Vp (i.e., within the saturation region). I've found this in AN73-7 from Siliconix.

enter image description here

I've even found that NTA7002N from ON Semiconductor has 1µA leakage @ 85°C, which might work for my design.

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    \$\begingroup\$ That AN is referring to a JFET which is quite different. \$\endgroup\$ – jp314 Mar 22 '16 at 14:07
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I just did a quick check of a 2N7000 using two mulitmeters; one measuring ohms from drain to source, and the other measuring voltage, with the gate shorted to the source and positive ohms lead to drain. I got 270 mV and 24 Mohms, which comes way under the spec'd 1 uA figure down near zero volts; closer to 10nA which happens to be the gate leakage spec. I'm playing around with some high impedance circuits (low power) that cause me to worry about drain leakage current. Checks experimentally down in this region are problematic. Ohm meter impedance could cause bad data by changing the circuit bias.Could be my test here suffers this issue since one meter influences the other, though volts should be least intrusive.

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