Intrinsic Semiconductors II

A semiconductor is a material whose electrical conductivity increases with increasing temperature. This broad definition distinguishes semiconductors from metals, whose electrical conductivity decreases as the temperature increases. In general, there are two basic classifications of semiconductors: intrinsic semiconductors and extrinsic semiconductors. Unlike extrinsic semiconductors, intrinsic semiconductors are naturally occurring elements within nature. The defining characteristic of these intrinsic semiconductor elements is their four valence electrons each occupying a different orbital.

Semiconductor Structure

The most well known electric conductors are metals. Metals are able to carry an electric flow because of their available free electrons that can pass from one atom to the next. This enables the electric flow (which is merely composed of electrons flowing across a substance) to travel through the length of the metal. Semiconductors work in a very different manner. All intrinsic semiconductors are group 14 elements, and thusly only have 4 valence P electrons available for bonding in their outermost shell. Because of these 4 electrons, group 14 elements like Silicon are able to form crystal lattice structures composed of countless atoms of that element. The image below depicts the bonding structure well:

Figure \(\PageIndex{1}\)

As shown, all of the electrons are used up in the covalent bonding structure, seemingly leaving none available to carry a current. Lacking any foreign doping agent that would introduce more foreign electrons, intrinsic semiconductors rely on a different mechanism for carrying a current.

Band Theory

Metals and semiconductors have a unique property that separates them from non-conductive insulators. In metals and insulators, the difference between energy orbitals are so small that electrons can be excited from lower energy orbitals to higher energy levels. These small orbital energy differences are referred to as bands. Unlike the discrete energy levels that you are taught about in beginning chemistry, these bands are so close together in energy that it takes a very small amount of energy to promote electrons to these higher band levels. The higher energy level that the valence electrons can be excited to is referred to as the conduction band, because the electrons in this band are free to carry an electric current.

As shown in the diagram above, the key difference between semiconductors and conductors is that conductors have a natural overlap between their valence electrons and the conduction band, while semiconductors have a noticeable gap between the two. The result is that the electrons of conductors are able to carry a current under any condition. Meanwhile, the conductive ability of semiconductors requires energy to be put into the semiconductor material. As the energy input into the semiconductor is increased, more electrons are promoted into the conduction band, increasing the number of available conducting electrons. This energy usually comes in the form of heat, which is why semiconductors are able to conduct more effectively as they rise in temperature.

Hole Conduction

As heat is added to the semiconductor, electrons promoted to the conduction band leave behind an empty spot in the lattice structure where they were previously situated. This concept of an electron hole is an important one in understanding the mechanism by which lattice structures are able recruit additional electrons for conductivity. By definition, an electron hole is the opposite of an electron. It represents the lack of an electron where an electron could potentially be situated or previously was situated in an otherwise full electron shell. In this case, electron holes result from electrons leaving their place in the lattice structure .

Electron holes have an important role in increasing the conductive abilities of semiconductors. Hole conduction first requires an electron to experience excitation in order to leave its valence band to the conduction band. Once the valence electron has left, neighboring electrons are pulled into the electron holes, filling existing electron holes while creating new neighboring ones. Once heat is put into the semiconductor and electrons begin continually crossing into the conduction band, hole conduction begins constantly moving electrons across the semiconductor, adding to the overall electron flow. This increases the conductive ability of the semiconductor, constantly allowing new electrons to cross into the conduction gap.

Intrinsic vs. Extrinsic

Semiconductors are categorized into one of 2 groups: intrinsic or extrinsic. Unlike intrinsic semiconductors, which are naturally occurring group 14 elements, extrinsic semiconductors are artificially doped with impurities that add additional electrons to the lattice, allowing increased conduction. The semiconductor industry heavily favors the use of extrinsic semiconductors because of the difficulty of obtaining large amounts of pure intrinsic semiconductors. Additionally, because of the added foreign electrons, extrinsic semiconductors are able to conduct more effectively than intrinsic semiconductors. However, intrinsic semiconductors still find many uses in today's world, especially in transistors and electronic product manufacturing.

References

1. Housecroft, Catherine. Inorganic Chemistry. 3rd ed. Essex, England: Pearson Education Lmited, 2008. Print.
2. Pierret, Robert F. Semiconductor Device Fundamentals. Reading, MA: Addison-Wesley, 1996. Print.
3. Turley, James L. The Essential Guide to Semiconductor Technology. Upper Saddle River, NJ: Prentice Hall PTR, 2003. Print.

Problems

1. Explain the fundamental differences between a metallic conductor and a semiconductor.
2. What are the two conduction mechanisms that allow semiconductors to effectively conduct electricity?
3. What disadvantages do intrinsic semiconductors have when compared to extrinsic semiconductors?

Answers

1. A metallic conductor relies on its available free electrons to carry an electric current. This is possible because the valence band and the conduction band have a large overlap in conductors, so the valence electrons are freely able to move to the conduction band. On the other hand, semiconductors have a larger gap between the valence and conduction bands, and thusly require some sort of energy input to be promoted to the conduction band. This is why semiconductors become more conductive as temperature increases.
2. Semiconductors rely on band conduction and hole conduction to create an electron flow. Band conduction involves the promotion of electrons to the conduction band away from the lattice valence band. Additionally, hole conduction replaces the lost electrons, allowing a continual flow of electrons into the conduction band.
3. For intrinsic semiconductors, you must have a large amount of the pure substance, while extrinsic semiconductors use their impurities to add electrons to the total conduction. It is often difficult to isolate enough pure substance to make an intrinsic semiconductors. Additionally, extrinsic semiconductors have more available electrons to conduct a current.