Why reverse current in equation while diode is forward biased ?

Thread Starter


Joined Jan 22, 2017
Hi all,

I read that ideal diode equation and it is given as

View attachment 330290

in above equation Is is the reverse saturation current or dark saturation current (According to this website). And according to most of the study which I did this reverse saturation current doesn't exist while the diode is forward biased, I want to ask that why this parameter is still in equation if it does not exist in first place, thanks

Best regards,


Joined Mar 31, 2012
Is is due to the diffusion of minority carriers from the neutral region in a PN junction to the depletion region. To a very handwavy degree, it play the same role as resistance in a resistor, which relates the voltage and the current for both positive and negative voltages. The same holds for a diode -- put in any voltage into the Shockley equation, positive or negative, and you get the resulting current. That the current for negative voltages is essentially constant is why it makes sense to call this parameter the reverse saturation current. For those that want a name that is more descriptive from a practical sense and less tied to the physics, it is called the scale current.

Note that Is is NOT a constant -- while it varies little with voltage, it has a pretty strong temperature dependence.