An electron’s best friend
A hole is essentially a lack of an electron. Holes are quasiparticles that describe the behavior of positive charges in semiconductors. Semiconductors are poor conductors of electricity unless they are either doped or heated. Doping refers to adding elements into a semiconductor to give it extra charge carriers. For example, doping a block of silicon with phosphorus will increase its conductivity by adding extra electrons. Electrons are negative charge carrying particles; holes, on the other hand, have a positive charge and are thus positive charge carriers. It is possible to dope semiconductors with elements that will add positive charge carriers. What the positive dopants do is “take away” an electron from the semiconductor. A positive dopant will need an electron to fill its valence shell and will thus take it away from the semiconductor. The same number of electrons are still there, but it leaves an opening. This opening is now positively charged, since the negative charge is taken away. This is how a hole is made.
While doping will increase conductivity regardless of temperature, the primary mechanism for conduction in semiconductors is thermal activation. Natural atomic oscillations have enough energy to break some chemical bonds, thus freeing electrons. Well, we know that when electrons are freed, they leave behind a positive charge. This charge is a hole and can be modeled as its own particle. Holes move around as electrons do. Hole motion is due to electrons from other bonds being attracted to the positive charge and moving there instead. When one electron breaks a bond and fills a hole, another hole is consequently formed, and so on. Thermal activation generates electron-hole pairs (EHPs), also known as excitons.
Doping is used for adding charge carriers while thermal activation naturally generates EHPs. Photogeneration is yet another mechanism by which to generate EHPs. When light of sufficient energy (equal or greater to the band gap energy) hits a semiconductor, electrons break from their bonds and EHPs are generated. This is the operating principle behind solar cells. Light emitting diodes (LEDs) do the same thing but backwards. Solar cells take light, make EHPs, and produce power; LEDs take power, make EHPs, and produce light.
While holes are quasiparticles, they have an effective mass, since their motion is hindered by inertia just like an electron’s motion (electrons have an “effective” mass as well that differs from their rest mass). It goes to follow that just like electrons, holes can generate a current and follow the same diffusion rules. If one side has less holes than the other, holes will naturally diffuse into the other side to balance out the concentrations. Mobility is a property for holes and electrons defining how easily they can travel through the bulk material. Diffusivity defines how easily they can diffuse. Despite the fact that holes are quasiparticles, they are defined with the same properties as electrons.