1.1 What is Semiconductor

“The English physicist Cavendish has proved experimentally that water conducts electricity 400 million times worse than metals；nevertheless it is not a very bad conductor of electricity. Bodies which take the intermediate position between conductors and nonconductors are usually called SEMICONDUCTORS. ”

Ivan Dvigubsky

“Fundamentals of Experimental Physics”，

1826

There are several ways of defining a semiconductor. Historically，the term semiconductor has been used to denote materials having conductivities between those of metals and insulators. Today，there are two more types of conductors：superconductors and semimetals. Typical conductivities of superconductors，metals，semimetals，semiconductors and insulators are listed in Tab.1.1.

Tab.1.1 Typical conductivities of superconductors，metals，semimetals，semiconductors，and insulators at room temperature

This definition is not complete. What really distinguishes metals from semiconductors is the temperature dependence of the conductivity. While metals（except for superconductors）and semimetals retain their metallic conductivity even at low temperatures，semiconductors are transformed into insulators at very low temperatures. In this sense semiconductors and insulators are actually one class of materials，which differs from metals and semimetals. This classification is directly connected to the existence of a gap between occupiedand empty states，i. e.，an energy gap，in semiconductors and insulators. In Tab.1.2 the classification according to the energy gap is summarized.

Tab.1.2 Classification of solids according to their energy gap and carrier density at room temperature

The border line between semiconductors and insulators is rather arbitrary. In particular，the value of the energy gap separating the semiconducting materials from insulating one is not well-defined. For example，diamond（C）was considered for a long time an insulator，but today it is possible to prepare it in such a way that it has semiconducting properties even at room temperature. The important distinction between these two systems originates historically from their different conductivities at room temperature. However，an insulator at room temperature can become a semiconductor at higher temperatures. Therefore，wide energy gap materials are currently under investigation for high temperature electronics.

Another possibility of defining a semiconductor，which is related to the energy gap，is through the free carrier concentrationat room temperature. While metals and semimetals have a rather large carrier density，semiconductors exhibit a moderate carrier density at room temperature，while insulators have a negligible carrier density. Typical carrier densities for these different types of solids are compiled in Tab.1.2. The listed densities are intrinsicvalues，i. e. for pure materials. However，real semiconductors always contain some impurities，which can act as dopantsleading to larger values for the carrier densities than the intrinsic ones.

In solid state electronic deviceand integrated circuitapplications，a semiconductor is required which must be crystalline and must contain a carefully controlled concentration（or volume density）of specific impurities. Semiconductor containing at least two specific impurities is needed for the following two fundamental device reasons：

（1）to provide a wide range of conductivity in one semiconductor by controlling its impurity concentration profile（density versus distance or space location）since each group-V impurity atom（P，As，Sb and Bi in Si）gives one negatively charged conduction electron，and each group-III impurity atom（B Al，Ga，In in Si）gives one positively charged conduction hole，and

（2）to provide two types of charge carriers（electrons and holes）to carry the electrical current or to provide two conductivity types，the n-type（conduction by electrons）and p-type（conduction by holes）.

The wide range of electrical conductivity makes it possible to control and modulate the magnitude of the conductivity by applying a time-dependent voltage，current，light，temperature，or mechanical force. In contrast，metal has so many conduction electrons that it is difficult to change its conductivity by modulating its electron concentration. In the other extreme，insulator has so few electrons that its conductivity cannot be modulated significantly at all.

To summarize，a semiconductor is a solid with a finite energy gap below 4 eV，which results in a moderate conductivity and carrier density at room temperature. By doping the semiconductor in a controlled fashion，the conductivity and carrier density can be varied over several orders of magnitude. Due to the existence of the energy gap，semiconductors are transparent for energies below the gap，i. e.，in the far-to near-infrared region depending on the value of the energy gap. However，they strongly absorb light for energies above the energy gap，typically in the near-infrared to visible regime. In the absorptive region，the conductivity of semiconductors increases，when they are irradiated.