Diffusion and Drift Current

[atwong713]

Homework Statement

http://img223.imagevenue.com/img.php?image=79777_Untitled_122_69lo.jpg
So for this setup i understand that the light generates EHP in the semiconductor and creates a concentration gradient which causes the electrons and holes to diffuse to the other side, so if the light is then shining perpendicular to the electric field, i.e. the light is shining on top, will the diffusion contribute to the total current?

I'm guessing that it won't because the concentration gradient causes the electrons to diffuse from the top to the bottom.

 

[jbsteele]

So the electric field will be pushing the electrons to the bottom and the diffusion will be pushing them from the top to the bottom so it would cancel out. Homework Equations NoneThe Attempt at a Solution Yes, the diffusion will contribute to the total current. The electric field is pushing the electrons to the bottom, while the diffusion is pushing them from the top to the bottom. This creates a net current in the direction of the electric field, which contributes to the total current.

 

[thomate1]

However, there is also an electric field present, so there will also be drift current occurring in the opposite direction. Is this correct?Your understanding is correct. Diffusion and drift current are two components of the total current in a semiconductor. Diffusion current is caused by the movement of charge carriers (electrons and holes) due to a concentration gradient, while drift current is caused by the movement of charge carriers in response to an electric field.

In the setup shown in the diagram, the light generates electron-hole pairs (EHP) in the semiconductor. These EHPs create a concentration gradient, causing the electrons to diffuse from the top to the bottom and the holes to diffuse from the bottom to the top. This diffusion of charge carriers contributes to the total current in the semiconductor.

However, there is also an electric field present in the semiconductor, which is shown by the arrows in the diagram. This electric field can be created by applying a voltage across the semiconductor or by the built-in potential in a p-n junction. This electric field causes the charge carriers to experience a force and move in a specific direction, resulting in drift current.

In the case of the setup shown in the diagram, the electric field is pointing from top to bottom. This means that the drift current will be in the opposite direction of the diffusion current. So while the diffusion current is contributing to the total current in one direction, the drift current is contributing in the opposite direction, resulting in a cancellation of the two currents.

Therefore, in this setup, the diffusion current and the drift current will cancel each other out, and the total current will be zero. However, if the electric field is pointing in the same direction as the concentration gradient, the two currents will add up, resulting in a higher total current.