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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.

Dean
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    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. – TimWescott Apr 25 '21 at 18:04
  • Hint: what happens to the exponential term when \$V_D\$ is very large and negative? – The Photon Apr 25 '21 at 18:33
  • `Vd` is not the supply voltage, it's the voltage drop across the diode. See the [Wikipedia article](https://en.wikipedia.org/wiki/Diode#Shockley_diode_equation). But, yes, `Is` can be very small, typically `1 pA ... 1 nA` (more or less, depending on the diode). – a concerned citizen Apr 25 '21 at 19:59
  • Dean, See [here](https://electronics.stackexchange.com/a/559679/38098). 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](https://electronics.stackexchange.com/a/305720/38098). – jonk Apr 25 '21 at 22:58

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