Semiconductors: Value of Multiple Electrical Personalities
Electrical property understanding in the 1800s led to ideas of “resistance, rectification, light sensitivity,” and more being introduced – with the decreased resistance of silver sulfide with heating contradictory to copper, as noted by Faraday in 1833; voltage being created post-trigger by light, between solid and liquid (the “photovoltaic effect”); the photovoltaic effect on selenium; and the subsequent of an actual particle named the electron (by J. J. Thomson in 1897) and the idea by Koenigsberger that materials may be metals, insulators, and “variable conductors” in 1914 and description of the lattice structure (by Bloch). After some discussion of positive vs. negative charges (by Baedeker vs. others) existing, the “Hall effect” influencing electric charge by an applied magnetic field, and Gudden’s theory of “impurities” causing conductivity all contributed, along with the “band theory and gaps” of Wilson, to the modern “p-n” junction model used today.
Alexander Graham Bell’s use of selenium’s light sensitivity to transmit sound (1880), and the light emitting diode (LED) by H.J. Round, noting light emission with electricity through silicon carbide crystals, and power rectifiers using copper oxide and selenium replacing vacuum tubes rectifiers, all led to eventual miniaturization with semiconductors, transistors, and the integrated circuit over time.
Hampered by the ability to suitably amplify such a small signal, the field continued research till important discoveries of a p-n junction in silicon actually observed by Ohl in 1941 with a light-sensitive material with p-n boundary and development of voltage. Almost simultaneously in France and the U.S., these ideas leading to the transistor would be discovered.
Present-day Semi-Conductor Concepts
One of the greatest recent inventions, which has led to the development of integrated circuit technology (and consequently allowed man to go to the moon, among other accomplishments), is the semiconductor discovery that led to transistor development and use. A semiconductor, (SC) so named because of its intermediate status between a conductor and an insulator. Two types of semiconductors exist – elemental and compound materials. A crystal lattice is used by physics to model, using silicon and germanium typically. Increased in conductivity by heat, sometimes better for unidirectional vs. bidirectional current flow, and with variable behavior with light or heat including variable resistance, such properties may be used in amplifiers, switches, and converters. In the basic physics model of an “n” type it contains electrons, and “p” type it contains holes – doping with impure atoms to a semiconducting material results in increased conductivity. Within a single semiconductor crystal, numerous p-n junctions result in electrical behavior. While poor conductors in their resting state, doping or gating (adding or removing electrons to create an imbalance of charge, hence promoting flow) can be used to increase conductivity. Heterojunctions, where two different doped materials join (e.g. p-doped or fewer electron with n-doped or excess electron) and meet till equilibrium is reached, while in the process generating ions with an electrical field. The process of generating and filling electrons and holes is known as generation or recombination, respectively. Found in Group XIV of the periodic table of elements, Si and Ge (with 4 valence electrons in outermost shell) allow same number of electrons to be gained or lost together. Other binary compounds, such as Groups III and V such as gallium arsenide, II and VI, IV and VI, and certain oxides, alloys, and mixtures of Arsenic, Se, Te, and others may be used (along with organic semiconductors made with organic compounds).
Preparation of a semiconductor must be done in a precise fashion – with thermal oxidation forming silicon dioxide on surface (used as a gate insulator and field oxide), photomasks and photolithography (with creation of the circuitry patterns on the integrated circuit). UV light, used with photoresist layer, is used to create such circuitry patterns. The non-etched silicon (uncovered by photoresist) is plasma-etched using gas (chlorofluorocarbon, or CFC, a.k.a. Freon) – a high-RF voltage between + and – is used – the silicon wafer on cathode (hit by positively charged ion released from plasma) results in anisotropically-etched silicon. Doping, the last process which is known as “diffusion” is done using 1100C heating and diffusion of impure atoms.
The Semi-Conductor Future
Various opinions have been put forth about the use of semiconductors in the future. At the 60th Anniversary International Solid State Circuits Conference (ISSCC) hosted by IEEE, one of the main issues was the energy problem as addressed by Rambus founder Mark Horowitz, who noted that 3 steps could help:
- reduce the proportion of memory and I/O-system energy (which now occupies half a system’s power) – could storage be done elsewhere, e.g. a cloud, or another method as noted by Horowitz in (2)?
- Use of ASICs, more efficient (3-fold (!)) than processors;
- Tailoring applications that best fit ASICs with short-integer and local storage, with more complex processors for dispersed storage.
In short, a concerted effort to use semiconductors from the standpoint of application developers could result in significant energy savings in the industry.
In addition to conservation of energy, the efficiency of battery and speed has been cited by some among the venture capitalist community, as being adequate to support the power needs. Hence, some such as John Doerr (KPCB) at an invited IEEE-covered event, noted that in the absence of such innovation, VC money was perhaps more likely to be invested in small smartphone apps, rather then infrastructural components as semiconductor research.
Fabrication issues such doping, effects of temperature, and manufacturing itself, have all contributed significantly since the days of Nobel Prize winner Wiliam Shockley and the industry spawned from discovery of semiconductors. In a separate section, we consider the utility of semiconductors as a part of transistors, and the great innovations spawned thereafter.