Materials are found in three primary forms: amorphous, polycrystalline, and crystalline.
An amorphous material is a totally disordered or random material that shows no short-range order. In polycrystalline material, large numbers of small crystallites can be identified. A crystalline material exhibits a highly regular bonding structure among the atoms over the entire macroscopic crystal.
Electronic materials can be separated into three classifications based on their electrical resistivity. Insulators have resistivities above 105 Ω • cm, whereas conductors have resistivities below 10-3 Ω • cm. Between these two extremes lie semiconductor materials.
Today's most important semiconductor is silicon (Si), which is used for fabrication of very-large-scale-integrated (VLSI) circuits. Two compound semiconductor materials, gallium arsenide (GaAs) and indium phosphide (InP), are the most important materials for optoelectronic applications including light-emitting diodes (LEDs), lasers, and photo-detectors.
The highly useful properties of semiconductors arise from the periodic nature of crystalline material, and two conceptual models for these semiconductors were introduced: the covalent bond model and the energy band model.
At very low temperatures approaching 0 K, all the covalent bonds in a semiconductor crystal will be intact and the material will actually be an insulator. As temperature is raised, the added thermal energy causes a small number of covalent bonds to break. The amount of energy required to break a covalent bond is equal to the band gap energy EG.
When a covalent bond is broken, two charge carriers are produced: an electron, with charge -q, which is free to move about the conduction band; and a hole, with charge +q, which is free to move through the valence band.
Pure material is referred to as intrinsic material, and the electron density n and hole density p in an intrinsic material are both equal to the intrinsic carrier density ni , which is approximately equal to 1010 carriers/cm3 in silicon at room temperature. In a material in thermal equilibrium, the product of the electron and hole concentrations is a constant: pn = n (2/i).
The hole and electron concentrations can be significantly altered by replacing small numbers of atoms in the original crystal with impurity atoms. Silicon, a column IV element, has four electrons in its outer shell and forms covalent bonds with its four nearest neighbors in the crystal. In contrast, the impurity elements (from columns III and V of the periodic table) have either three or five electrons in their outer shells.
In silicon, column V elements such as phosphorus, arsenic, and antimony, with an extra electron in the outer shell, act as donors and add electrons directly to the conduction band. A column III element such as boron has only three outer shell electrons and creates a free hole in the valence band.
The donor and acceptor impurity densities are usually represented by ND and NA, respectively.
If n exceeds p, the semiconductor is referred to as n-type material, and electrons are the majority carriers and holes are the minority carriers. If p exceeds n, the semiconductor is referred to as p-type material, and holes become the majority carriers and electrons, the minority carriers.
Electron and hole currents each have two components: a drift current and a diffusion current.
Drift current is the result of carrier motion caused by an applied electric field. Drift currents are proportional to the electron and hole mobilities (μn and μp, respectively).
Diffusion currents arise from gradients in the electron or hole concentrations. The magnitudes of the diffusion currents are proportional to the electron and hole diffusivities (Dn and Dp, respectively).
Diffusivity and mobility are related by the Einstein relationship: D/μ = kT/q. The expression kT/q has units of voltage and is often referred to as the thermal voltage VT. Doping the semiconductor disrupts the periodicity of the crystal lattice, and the mobility--and hence diffusivity--both decrease monotonically as the impurity doping concentration is increased.
The ability to add impurities to change the conductivity type and to control hole and electron concentrations is at the heart of our ability to fabricate high-performance, solid-state devices and high-density integrated circuits. In the next several chapters, we see how this capability is used to form diodes, field-effect transistors (FETs), and bipolar junction transistors (BJTs).
Complex solid-state devices and circuits are fabricated through the repeated application of a number of basic IC processing steps, including oxidation, photolithography, etching,ion implantation, diffusion, evaporation, sputtering, chemical vapor deposition (CVD), and epitaxial growth.
To build integrated circuits, localized n- and p-type regions must be formed selectively in the silicon surface. Silicon dioxide, silicon nitride, polysilicon, photoresist, and other materials can all be used to block out areas of the wafer surface to prevent penetration of impurity atoms during implantation and/or diffusion. Masks containing window patterns to be opened in the protective layers are produced using a combination of computer-aided design systems and photographic reduction techniques. The patterns are transferred from the mask to the wafer surface through the use of high-resolution photolithography.
