Semiconductor Doping#
Definition: Doping
Doping is the process of introducing impurities in the structure of a semiconductor.
Doping is primarily done to change the properties of semiconductors.
P-Type Semiconductors#
Definition: P-Type Semiconductor
A p-type semiconductor is a semiconductor which has been doped with an element which has fewer valence electrons than the semiconductor itself.
The dopant is known as an acceptor in this case. In practice, the difference in valence electrons is rarely greater than one. If the host has \(N\) valence electrons and the dopant has \(N - 1\) valence electrons, then the dopant forms a bond with \(N - 1\) of its neighboring hosts. However the dopant cannot form a bond with the \(N\)-th neighboring host because it lacks the necessary valence electrons. This means that the charge density between the dopant and this neighboring host is more positive than between it and its \(N-1\) neighboring hosts. It turns out that these regions of more positive charge density behave exactly like electrons but with a positive charge and are thus called electron holes.
However, this configuration is unstable and small energy perturbations can cause an electron from one of the bonds between two neighboring hosts to move so as to form a bond between the dopant and its \(N\)-th neighbor, which moves the electron hole. The dopant becomes a fixed negative ion. This process continues further between the hosts many times as the electron hole moves randomly around the lattice.
Notation: Doping Concentration
We use different notations depending on the amount of doping:
Most commonly, silicon is doped with boron.
N-Type Semiconductors#
Definition: N-Type Semiconductor
An n-type semiconductor is a semiconductor which has been doped with an element which has more valence electrons than the semiconductor itself.
The dopant is known as a donor in this case. In practice, the difference in valence electrons is rarely greater than one. If the host has \(N\) valence electrons and the dopant has \(N + 1\) valence electrons, then the dopant forms a bond with all \(N\) of its neighboring hosts. However, the dopant has an extra valence electron left over which cannot form a bond. This extra electron moves freely around the lattice, thus making the dopant a positive ion.
Notation: Doping Concentration
We use different notations depending on the amount of doping:
Most commonly, silicon is doped with phosphorus.
P-N Junctions#
A p-type semiconductor or n-type semiconductor can be additionally doped with a dopant of the opposite type, thus forming a p-type region and an n-type region which are in direct contact with one another. At the boundary between these two regions, free electrons from the n-type region flow into the p-type region to fill in holes. However, the positive and negative ions are stuck in place. Therefore, a third region known as a depletion region or p-n junction forms at the boundary, where there are no free charge carries but there is a difference in charge density due to the ions. This difference creates an electric field throughout the depletion region which points from the n-type region to the p-type region. In other words, voltage forms across the p-n junction.
The amount of doping determines the properties of the p-n junction. In particular, the p-n junction extends further into the region with less doping than into the region with more doping because it must remain electrically neutral as a whole and a larger region of lower dopant concentration is necessary to accumulate the same electric charge as a smaller region with a higher dopant concentration.