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3.2 Semiconductor physics

Solar energy is one of the world's most abundant renewable energy sources.Current photovoltaic and photocatalytic systems are almost entirely based on semiconductor materials. (Source : http://pubs.rsc.org/en/content/articlelanding/2011/ee/c0ee00162g#!divAbstract (Links to an external site.))

Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electrical properties somewhere in the middle, between those of a “conductor” and an “insulator”. They are not good conductors nor good insulators. They have very few “free electrons” because their atoms are closely grouped together in a crystalline pattern called a “crystal lattice” but electrons are still able to flow, but only under special conditions.

The ability of semiconductors to conduct electricity can be greatly improved by replacing or adding certain donor or acceptor atoms to this crystalline structure thereby, producing more free electrons than holes or vice versa. That is by adding a small percentage of another element to the base material, either silicon or germanium.

silicon atom

 

The diagram above shows the structure and lattice of a ‘normal’ pure crystal of Silicon.

antimony atom

The diagram above shows the structure and lattice of the donor impurity atom Antimony (group 5A). The semiconductor material get more free electrons and becomes N-type.

boron atom

The diagram above shows the structure and lattice of the acceptor impurity atom Boron (group 3A). The semiconductor material get more holes and becomes P-type.
(Resource : http://www.electronics-tutorials.ws/diode/diode_1.html Links to an external site.)

Furthermore, one of the basis of semiconductor, p-n junction diodes, is not only the basic of solar cells, but of many other electronic devices such as LEDs, lasers, photodiodes and bipolar junction transistors (BJTs). A p-n junction aggregates the recombination, generation, diffusion and drift effects into a single device. (Source : http://pveducation.org/pvcdrom/pn-junction/pn-junction-diodes (Links to an external site.))

So, how does the p-n junction diodes work?

 More theories of p-n junction can be found in : PV EDUCATION - P-N Junction (Links to an external site.)

 

Recommended courses :

  • MIT OpenCourseWare - Semiconductors (Links to an external site.)
    "After completing this session, you should be able to:
    - Describe the mechanisms for forming charge carriers in a semiconductor, and how they behave in the presence and absence of an applied voltage.
    - Calculate how many charge carriers exist at a given temperature for intrinsic and extrinsic semiconductors, and how much dopant must be added to produce a desired band gap or charge carrier density.
    - Sketch the Maxwell-Boltzmann distribution at several different temperatures, and explain how it applies to electrons in semiconductors.
    - Explain why donor levels don't form a continuous band structure in doped semiconductors."
    • Lecture from the MIT course: Introduction to Solid State Chemistry 
    • Undergraduate level

 

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