ELECTRIC FIELD SCREENING EFFECT

Before we begin looking into how transistors work, it would be helpful to revisit the relationship between the distribution of charge and electric field.

1. An Infinite Plane Sheet of Charge

Let's assume we have an infinitely large two dimensional plane sheet of charge. Click on the plane to positively charge it.

In practice, the plane does not need to be infinite. If we only look at the points in space close to the plane, (i.e. x much smaller than the plane size in y and z directions) we can consider the plane infinite.

Note that charge is evenly distributed on the plane and the finite number of + signs is for representation purposes. In reality, the number of charged particles is many, many orders of magnitude larger.







2. Charge Density

What happens if we change the charge density?



3. Charge Polarity

What happens if you negatively charge the plane?

4. Two Plane Sheets of Opposite Charges

Let’s look at what happens when we put two sheets of opposite charges together.

This effect is called screening. Click on the button below to see how the electric fields generated by the two charges add or subtract in different regions:



You can click and drag to move the negatively charged sheet and use the buttons below to show or hide each component of the electric field.

Show electric field due to positive charges
Show electric field due to negative charges

5. Partial Screening

The charge density in the two plane sheets may not always match exactly. Let’s see what happens in that case.




Show electric field due to positive charges
Show electric field due to negative charges

6. Getting Back the Full Screening

What if we add an additional plane sheet of negative charge such that the total negative charges become equal to the positive charges?



You can hover over each sheet to see its charges more clearly.

Show electric field due to positive charges
Show electric field due to negative charges

7. Screening via Charge Distributed in a Volume

What if the negative charges are evenly distributed within a given volume? One can consider this as an extreme case of having a large number of plane sheets of negative charge. Here we are assuming the total negative charge is equal to total positive charge.

Use the slider to change the width of the volume.



Show electric field due to positive charges
Show electric field due to negative charges