# Why is Diode reverse saturation current I_s still in the V-I equation when when a diode is forward biased?

• Muhammad Usman
Muhammad Usman
TL;DR Summary
why reverse current or dark saturation current is considered if the diode is forward biased ?
Hi all,

This is not homework help or something, it is my general query. I read that ideal diode equation is given as

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, thanksUsman

Last edited by a moderator:
You are misunderstanding. This parameter Is is the multiplier that determines the amount of current that flows when forward biased. It is very important in forward bias. Of course, this equation is for an ideal diode. For a real diode, there are other things that contribute to the reverse biased current. So in a real diode, one typically determines Is by measurements in the forward biased region, not in reverse bias.

DaveE and berkeman
Is can also be seen as a "scale current", that is dependent on the area of the diode. That is why it can be used for both modes of operation.

DaveE and berkeman

## Why is the reverse saturation current I_s present in the V-I equation for a forward biased diode?

The reverse saturation current I_s is a fundamental parameter in the Shockley diode equation, which describes the diode's current-voltage (V-I) characteristic. It represents the small leakage current that flows when the diode is reverse biased. Even when the diode is forward biased, I_s remains in the equation because it is part of the exponential relationship that defines the diode's behavior. The forward current is exponentially dependent on the applied voltage and I_s, serving as a scaling factor.

## Does the reverse saturation current I_s have a significant impact on the forward bias operation of a diode?

In forward bias, the current through the diode is primarily determined by the exponential term involving the applied voltage. While the reverse saturation current I_s is much smaller compared to the forward current, it still plays a crucial role in defining the diode's overall current. However, its impact is relatively minor because the forward current is typically several orders of magnitude larger than I_s.

## Can the reverse saturation current I_s be ignored in practical forward bias applications?

In most practical forward bias applications, the reverse saturation current I_s is very small compared to the forward current and can often be considered negligible for approximate calculations. However, for precise modeling and understanding of the diode's behavior, I_s should not be ignored because it is an intrinsic part of the diode's V-I equation.

## How does temperature affect the reverse saturation current I_s in a forward biased diode?

The reverse saturation current I_s is highly sensitive to temperature. As temperature increases, I_s increases exponentially due to the enhanced generation of charge carriers. This means that at higher temperatures, the forward current for a given forward voltage will be slightly higher because I_s has increased. Therefore, temperature variations can affect the diode's performance and must be considered in precise applications.

## Is the reverse saturation current I_s the same for all diodes?

No, the reverse saturation current I_s varies between different types of diodes and even between individual diodes of the same type. Factors such as the material properties, doping levels, and manufacturing processes all influence I_s. For instance, silicon diodes typically have a lower I_s compared to germanium diodes due to differences in their material properties.

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