conduction band is about 1/20th the size of the band gap for the silicon valence electrons.
In sum, an n-type semiconductor has a set of electrons ready to move, while in contrast a p-type semiconductor has a place ready for valence
electrons to go, freeing up holes that can then move. When an external electric field is applied to a doped semiconductor, current will flow
much more readily than in a pure semiconductor.
36.14 - p-n junction
p-n junction: p-type and n-type semiconducting
materials placed adjacent to one another. This
type of junction is the basis of devices like
diodes and transistors.
Diode: A component that readily allows the
flow of current in one direction, and is highly
resistant to current in the other.
In this section, we discuss what happens when p- and n-type materials are placed in
contact with one another. One result is a useful device, the diode.
Consider the p-n junction shown in Concept 1. A junction refers to the region or a
device where the two types of semiconducting material are touching. Remember that
the n section has excess electrons that can flow fairly readily, while the p section has
excess holes that could accept those electrons.
When these two types of material are placed next to one another, some holes flow from
the p to the n material, and some electrons flow from the n to the p material.
This is called a diffusion current. The electrons diffuse from the n-type material, where
there is a higher concentration of them, to the p-type, where they are relatively scarce.
The holes move in the opposite direction, from the p-region where they are abundant to
the n-region where they are scarce. They diffuse, just as perfume molecules diffuse
across a room.
The p-n junction is the essential element of a diode. When it is connected in a circuit in
the fashion shown in Concept 2, the net flow of current is relatively great at low
voltages. This is called a forward-bias connection.
Why does the current flow easily? Electrons flow from the n-region of the semiconductor
rather readily to fill holes on the p side. The negative terminal of the battery acts to
replenish the supply of electrons in the n-region and the positive terminal replenishes
the holes in the p-region.
Now we reverse the orientation of the battery, as shown in Concept 3. The diode will
block nearly all the current for low applied voltage: It acts as a resistor of great
resistance. This is called a back-bias connection.
Why does this orientation prove so resistant to current? As Concept 3 shows, the
battery causes a significant depletion zone. Holes in the p-region are moved away from
the junction as they move toward the negative terminal of the battery, and free electrons
in the n-region move away from the junction toward the positive terminal. The region
around the junction on both sides loses its mobile charge carriers; it becomes depleted.
The battery can “pull” harder and harder, but in effect, all it does is expand the depletion
zone, instead of causing a continuing current.
The two graphs on the right show current versus voltage curves for forward bias and
reverse bias connections. In the forward bias case, the current increases with potential
difference. The diode acts roughly like a resistor. In the reverse bias case, the diode acts almost like a break in the circuit, and even relatively
large potential differences cause negligible currents.
p-n junction
p-type material has excess holes
n-type material has excess electronsDiode
Allows current to flow in one direction
Battery reversed
Creates depletion zone
Prevents flow of current