The Environmental Engineering Hub

In the previous post, we explained that semiconductors do not conduct electricity very well. One way to manipulate electrical conductivity in semiconductors is to manipulate the concentration of electrically charged carriers. We can do this by using doping. In this post, we will first introduce what doping is. Then we will discuss how doping changes important material properties in semiconductors. The concentration of charge carriers in a semiconductor can be manipulated by doping the material. Doping means that we add impurities in a controlled way to the material. Let’s take the example of silicon. Silicon has four valence electrons. In a silicon lattice, each atom is bonded, covalently, to four other silicon atoms. We can take that silicon lattice and substitute a small amount of silicon atoms with different atoms. This is commonly done with atoms of two different elements: Boron and Phosphorus. Boron atom has three valence electrons, while phosphorus atom has five val

In this post we continue our discussion of the charge carrier concentrations. In the previouspost you have been introduced to the density of states function, the Fermi-Diracdistribution and the fermi level. We will now use these parameters to determine the chargecarrier concentrations in thermal equilibrium.Let’s recap. First we discussed the band diagram, with its well-defined bands for the valenceelectron energies, conduction electron energies and band gap, where there are no energystates that an electron can occupy. Then we defined the density of states function. This function is material dependent. Thedensity of states function therefore tells you the number of states available for an electronto occupy at a certain energy level, for a given material. In highpurity intrinsic semiconductor materials, there are no energy states available for an electronto occupy in the bandgap.In the next step we looked at the occupation of the energy states. For this, we introducedthe F

In the previous post, we have introduced band diagrams and you learned about the conduction and valence energy band. Now we will start taking a deeper look at the charge carriers that can occupy energy states in those bands. The occupation of energy states in the valence and conduction bands of semiconductors determines the concentrations of charge carriers. Concentration of charge carriers is a very important property for understanding performance of solar cells. Let’s start with the equilibrium condition. We define equilibrium of a system as a state in which the system is unperturbed. Therefore, no external forces are applied on this system. These forces could be: external voltage, magnetic field, illumination or mechanical stress. We can define the thermal equilibrium of as ystem as a condition in which its parameters do not change with time. We use the word thermal because the equilibrium conditions will change depending on the temperature. Several steps are necessary

In this post we will look at the energy of electrons in a material with periodic atomic structure. Most semiconductor materials, such as silicon, have periodic atomic structure. We call it a crystalline structure. By following a simplified quantum-mechanical approach, we will show that the energy of electrons in semiconductors are grouped in bands. This post aims to understand the origins and the importance of electron energy-bands for semiconductors. Specifically, we demonstrate how we visualize these energy levels in so-called energy band diagrams. Understanding band diagrams is of utmost importance for solar cell engineers. In the previous post it was described how atoms bond together to form a solid material. Now we will take a look on the energy of electrons in a regular atomic structure. This first part of the post will essentially serve as a derivation for energy-band and dispersion diagrams. Since the quantum-mechanical approach to determine the energy