Computer Circuitry: Semiconductors

Computer Engineering

Semiconductor materials are the principal component of computer technology, and are primarily based on silicon and germanium. Pure silicon does not have enough ‘free’ valence electrons to conduct electrical current adequately, and small percentages of other elements are blended into the silicon to enhance the conductivity of the material. Current flows in semiconductors as electrons moving through the material driven by a voltage difference, and as ‘holes’ equivalent to positive charge. Production of silicon-based semiconductor material and the subsequent manufacture of integrated circuit chips is a complex, multistep process requiring stringent quality control measures.

The ability of any material to conduct electricity is a function of the distribution of electrons and energy levels in the atoms and molecules of that material. Materials in which electrons can easily enter the conduction band of the material across a small band gap between the normal ground state of the electrons and the conduction band generally are good conductors of electrical current. Materials that have a large band gap between the normal ground state of the electrons and the conduction band are generally poor conductors of electrical current. Materials that have a band gap too large to be good conductors, and too small to be non-conductors, are called semiconductors. Good conductors have little energy difference between the occupied valence shell of electrons and the vacant orbitals in the next highest electron shell, and the valence electrons are able to move between them and from atom to atom through them relatively easily. In poor conductors and non-conductors, this energy difference is much greater, making it more difficult for electrons to move through the material. The essential semiconductor materials used in computer technology are silicon and germanium. The electron distribution in the atoms of these materials is very stable and not easily disrupted.

The principal material for the production of semiconductor-based devices is silicon. Pure silicon, however, does not function well as a semiconducting material for the production of transistor structures used for all kinds of integrated circuit (IC) chips, and especially for central processing unit (CPU) chips and solid-state storage devices. To improve the desired characteristics of the material, a small percentage of another element can be added as a “dopant” to either increase or decrease the number of available electrons in the resulting alloy. This silicon blend is prepared by melting a quantity of pure silicon together with the proper amount of dopant. A single, large cylindrical crystal is then drawn slowly from the molten mass and then cut into thin wafers for use in the manufacture of chips.

The thin wafers obtained from slicing up the large silicon crystal are polished to produce a highly uniform surface. Millions of transistor structures are then etched onto the surface of each wafer in a multistep process. The pattern and order of etching can constitute a proprietary or patented segment of the final product, known as a semiconductor intellectual property (SIP) block within the overall design of the integrated circuit. The overall process requires several individual steps that are carried out with stringent quality control at all stages of production. The number of transistor structures that can be etched onto the surface of a silicon chip, and hence the computing capabilities of computers, has followed the empirical observation known as Moore's Law quite well for several decades. Moore's Law states that the number of possible transistor structures and the corresponding\ computing power doubles approximately every eighteen months. Logically, this means that there is a finite limit to both, determined by the physical minimum size of the structures themselves, and when that limit is reached no further advance can be made. However,current research with novel materials such as graphene and nanotubes, and toward the successful development of quantum computers may eventually end dependency on traditional semiconductor materials entirely.

—Richard M. Renneboog M.Sc.