In the last chapter we saw how negative charges can screen the electric field generated by positive charges and vice versa. We assumed we can put these charges whenever we want. Let's see how these charges appear and distribute in metals and semiconductors and how the screening effect works in these materials. In this chapter we assume the dielectric constant is the same everywhere.
The density of electrons in metals is extremely high. However, for
every free electron there is a positive charge within the metal and
the material is initially neutral. Apply a voltage to move charges
between the two metallic electrodes and see what happens. You can
also change the amount of transferred charge.
Note: the number of electrons and positive charges shown here are
representative, and in reality there are many many orders of
magnitude more charges involved.
The two sheets of positive and negative charges will generate an electric field. Can you explain why the electric field exists only between the two electrodes?
Apply a charge and then drag over the box on the right to measure
the charge inside the shaded volume to get a sense of where the
charge is concentrated.
What happens when we have a doped semiconductor on one side? For simplicity, for now, let’s consider only majority carriers and ignore minority carriers and generation/recombination. Add positive and negative charges to the semiconductor side and see how they distribute.
Let’s see how the screening effect works when you add positive or negative charge to the semiconductor.
Try changing the charge amount and observe what changes. What is the difference when you change the polarity of the added charges?
Apply charge and then drag over the box on the right to measure the charge inside the shaded volume and get a sense of how the charge is distributed.
The dopant amount in the semiconductor also changes what happens when we apply a charge. Try changing the charge and doping concentration and see what relationships you notice.