NSF Media Tips - originally published at http://www.nsf.gov/od/lpa/news/tips/tipsrel.htm
July 10, 2000


Researchers supported by the National Science Foundation (NSF) have reported the discovery of organisms that form a protective armor of nearly perfect crystals from the atoms on the surface of a silicon or germanium semiconductor.  This characteristic could be exploited to make faster, more stable biochips for use in the next generation of information technology, the researchers believe.

"Instead of putting cells 'on' a chip, this research indicates they can be put 'in' a chip, potentially reducing the steps needed to manufacture and operate bio-based electronic components," said Robert Baier of the NSF Center for Biosurfaces at State University of New York (SUNY) at Buffalo.

Scientists and engineers at two other NSF research centers participated in the research: the Center for Microcontamination Control at the University of Arizona and Rensselaer Polytechnic Institute in New York and the Center for Environmentally Benign Semiconductor Manufacturing at the University of Arizona. The Queen's University of Belfast, Northern Ireland, also supported the research. [Amber Jones]

Invention of the "Biochip": True Semiconductor-to-Life Symbioses
R.E. Baier, 13Dec99

There is a class of materials known as semiconductors which furnish an occasional free electron for carrying current. Silicon and germanium are the most familiar examples; they have about one free electron for every thousand atoms (as contrasted with copper, which has one for every atom). These semiconductors, in modern integrated circuits, have long possessed a special interest for biomaterials researchers, in their promise as processors of signals from biological stimuli. The important fact about them is that the number of current-carrying electrons and "holes" in them can be controlled or modulated by the types of single-electron transfers common in biological processes such as respiration and photosynthesis. Semiconductors can be made to act as conductors under some conditions and as insulators under others. Indeed, they are so sensitive that the current flowing in a small crystal of germanium or silicon might be controlled by the chlorophyll in a single cell, as light shines on it in a region where a fine wire can carry off the amplified electron "signal". This new class of biomaterials ("biochips") can be adapted to many uses, but at present has the sophistication of only the rude crystal detector, as used in early radios and in an improved form in radar sets.

Mechanism of Action:
Each atom in a crystal of germanium (or silicon), has four electrons in its outer shell -- so called valence electrons that help keep the atoms together. Because the electrons are fully occupied in forming bonds between the atoms, they are not available for conventional electrical flow. If some impurity which has five valence electrons, say an atom of phosphorus from a biological reaction, gets into the crystal, four of these electrons can form bonds with adjacent germanium atoms, but the fifth electron will remain free to flow as electrical current. Intimate contact of a living cell with a semiconductor can be a method of controlling the movement and directions of electrons in the solid crystal, by providing metabolic "sources" and "sinks" for electrons and "holes" that are amplified in common transistor-type circuits.

Viability of the living cells is favored since the "biochip" transistor does not need to heat up, as a vacuum tube does, and it responds instantly. It can operate on a tiny amount of power -- about one tenth of that used by an ordinary flashlight bulb -- and be locally modulated by biomembrane potentials which can approach 1,000,000 volts/cm. And "biochips" can be made almost vanishingly small. The present experimental crystal adducts of germanium and bacteria produced by us are only about 5 micrometers on a side.



Reduction to Practice:
We are exploiting the superior conductivity of the surface layer of germanium, accounted for chiefly by the presence of "holes" (absence of electrons). These "holes" are produced by biologically contributed "impurities" and cell-modulated surface states, to be amplified by the current passing through the crystal.

Our present studies show a method for fabrication of the crystal/biology adducts and illustrate a biological method of controlling the electrons or "holes" in a crystal. Similar to the approach which led Bardeen and Brattain to the invention of the original transistor, the initial device we envision also consists of two fine conductive wires of which the tips, only a few micrometers apart, rest on a germanium + cell adduct crystal conductively bonded to a metal disk. These elements will be housed in a metal cylinder which is connected electrically to the metal disk and crystal, thus forming the ground terminal. The "cat's whisker" wires will be connected to pins that can be plugged into a socket.

So far, fundamental new knowledge has been gained about how biological cells may be incorporated into and modify the structure and energy states of solid matter and the electrical behavior of the surface atoms in a semiconductor. Basic study of these phenomena continues.

Prospects for Manufacturing:
Industry experts have declared many times that some physical limit exists below which miniaturization of integrated circuits could not go. An equal number of times they have been confounded by the facts of functioning smaller, less expensive devices. No limit can be discerned in the quantity of transistors that can be fabricated on germanium or silicon, which has increased through eight orders of magnitude in the 50 years since the transistor was invented.

It is anticipated that "biochip" semiconducting materials, based on germanium and silicon, can be made into functional transistors in an integrated process involving many of the steps now employed for integrated circuits. Templates, called masks, will be applied to the "bio-doped" germanium or silicon in order to expose desired areas. Next, various operations involving chemical diffusion, radiation, doping, sputtering or the deposition of metal can act on these areas, sometimes to construct device features, other times to erect scaffolding to be used in succeeding steps and then removed. Meanwhile, other devices -- resistors, capacitors and conductors -- can be built into the same circuit to connect the "biochip" transistors.

Extensions to Biophotonics:
One major difficulty that continues to limit the development of photonics, and especially nonlinear optical devices, is that the intensities of difficult-to-amplify optical beams replace the easily boosted currents and voltages of electrical circuits. The photonic operations depend on fine-tuning the system so that a small input will upset a delicate balance. Although such switches have been called "optical transistors", they do not share the amplifying principles of transistor action.

"Biochip" optical switches, in a biophotonics circuit, can overcome this fundamental problem. Although light hardly interacts with light, the interaction of light-induced signals is essential for photonic functions. With "biochips", optical signals can be converted by integral embedded cells into electrical ones in a semiconductor. The voltage thus produced will change the optical response of another material, thereby modulating a second beam of light.

Another use of the semiconductor-to-biology symbiotic combinations may be to produce light-sensitive "heterojunctions", in which crystalline lattices of different energy gaps meet. "Bio-doped" and conventional crystalline lattices will mesh imperfectly, creating atomic-scale defects, or elongate toward one another, creating elastic strain. Either defects or strain can produce useful biophotonic electrical side effects that can drive circuitry.

These combinations will involve complicated biophysics but at the same time will provide a tunable variable that will be useful in surmounting many design problems.

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