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my formula and and values

Hi, everyone I was wondering if I anyone could help me out I am trying to work out the reverse saturation current for the a diode. The attached photo shows how I have rearranged the formula in terms of reverse saturation current. I dont know what I am doing wrong I keep getting really small answers such as ^-19 small and ^-85 are answers I have had. P.S the VD value is 5V as thats the supply to the circuit. Any help would be really appreciated. Thanks in advance.

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    \$\begingroup\$ Please say what diode you're using. 42mA at 5V isn't consistent with any diode that I know of on the market, so please give us a schematic of your circuit. If you can't do that -- a drawing or a photo, please. \$\endgroup\$
    – TimWescott
    Commented Apr 25, 2021 at 18:04
  • \$\begingroup\$ Hint: what happens to the exponential term when \$V_D\$ is very large and negative? \$\endgroup\$
    – The Photon
    Commented Apr 25, 2021 at 18:33
  • \$\begingroup\$ Vd is not the supply voltage, it's the voltage drop across the diode. See the Wikipedia article. But, yes, Is can be very small, typically 1 pA ... 1 nA (more or less, depending on the diode). \$\endgroup\$
    – a concerned citizen
    Commented Apr 25, 2021 at 19:59
  • \$\begingroup\$ Dean, See here. The saturation current is the \$y\$-axis intercept when you extrapolate the line formed by the diode's ln(current) vs voltage chart (when the bulk resistance can be ignored.) There's a BJT chart that kind of gets the idea across here. \$\endgroup\$
    – jonk
    Commented Apr 25, 2021 at 22:58

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