BUFFALO, N.Y. State University of New York (SUNY) researchers who were studying the problem of bacteria that sabotage the yield on semiconductor lines may have found a way to use them to create "biotransistors," harnessing a particular bacterium's photosensitivity to create an optoelectronic switching element.
Engineers will benefit not only from the deeper understanding they will gain of how bacteria elude even the harshest clean-room procedures, but also from the next-generation preview of living transistors inside the biochips of the future.
The researchers recently discovered that errant bacteria survive in the cleanest of clean rooms by inducing the top semiconductor layer on chips to grow over them, thereby embedding themselves inside the chip.
When SUNY researcher Robert Baier started to investigate the role bacteria play in the yield problem, he knew they were avoiding even the most stringent attempts to eradicate them the bugs just wouldn't die. How could bacteria survive where no other living thing can? Baier, funded by the National Science Foundation, found the answer, but it wasn't what he expected.
"When we started this study, we were just trying to find the source of bacteria in the fab, and how they could remain alive after all the heroic measures to eradicate them with ultraviolet light, ozone and everything else including a dollar a gallon to purify the water," said Baier, who is director of the Center for Biosurfaces at SUNY.
Other scientists and engineers participated in the research at the Center for Microcontamination Control at the University of Arizona, the Rensselaer Polytechnic Institute in New York and the Center for Environmentally Benign Semiconductor Manufacturing at the University of Arizona.
The problem wasn't people in the fab getting sick those kinds of bacteria were easy to kill. Rather, it concerned some clever bugs that just wouldn't die, no matter what bacteria that can survive in the vacuum of space, or inside a volcanic vent at the bottom of the sea. They can hibernate indefinitely and only need the slightest bit of light to wake up and thrive anew.
"We found that these extremely hard-to-kill bacteria were coming in with the ultrapure water, and the way they survived our calculated assault was to capture a tiny bit of semiconductor that had dissolved in the ultrapure water and start growing," said Baier.
Once the bacterium sticks a molecule of semiconductor to itself, other dissolved crystals spontaneously attach themselves to the formation, growing islands atop silicon wafers during a subsequent vapor deposition step.
In short order, the bacteria have encased themselves inside armored shells of semiconductor, making them impervious to all the attempts by clean-room personnel to kill them. "These bacteria can cause a lot of problems in the clean room, like shorting out adjacent lines on chips, and inside these armored shells they are almost impossible to kill," said Baier. "Now we are turning a problem into a feature. A plant is basically a single-electron photonic device converting light into electricity. If we embed a photosensitive bacteria inside a chip, we have the beginnings of a biotransistor."
Baier's goal of harnessing bacteria as the active element in a transistor may not be as far-fetched as it sounds at least his theory sounds convincing. He points out that copper is a conductor because it has one free electron per atom to contribute to current flow. Semiconductors are called "semi" because they have only about one free electron per thousand atoms, depending on doping levels. Current flow in those semiconductors, unlike copper, can be precisely controlled by parameters that match the parameters of bacteria, according to Baier, enabling regular transistors to switch from an insulator into a conductor by changing state.
These small charge transfers, Baier contends, are just what happens in common biological processes like respiration and photosynthesis. In fact, he believes that the current flowing in a semiconductor can be controlled by the chlorophyll in a single cell. For instance, when light shines on a photosensitive bacterium, it yields up an electron that could be used to switch a primitive biotransistor. "This is a new class of biochips, which I believe can be adapted to many uses, but at present it's at a primitive stage, like the crude crystal detectors that preceded today's radios," said Baier.
The theory is that doping semiconductors is always done to disrupt the perfect lattice, making free electrons or "holes" available in "n-" and "p-" type semiconductors, respectively. Likewise, if a biological atom, say phosphorus with five electrons from a bacterium's cell, is doped into a silicon crystal that only needs four, it then makes the fifth electron available, enabling biological cells to serve as metabolic "sources" and "sinks" for electrons and "holes" in biotransistors.
"Biological membranes have been known to develop potentials of a million volts per centimeter. They are perfect for semiconductors, and biochips using them could be made as small as five microns on a side," said Baier.
According to Baier, most of the steps used to build chips today will continue to be used in manufacturing his biochips. For instance, masks will be used on the semiconductor doped with bacteria, as will diffusion, sputtering and other common deposition techniques. All the other common devices like resistors and capacitors will be built into circuits connected to the biotransistors.
Biotransistors will also simplify optical communications by amplifying optical beams the way a normal transistor amplifies electrical current. And Baier envisions the construction of light-sensitive heterojunctions where lattices of different energy gaps are "biodoped" so that their crystalline lattices mesh imperfectly, creating atomic-scale defects and strains with useful photonic and electrical side effects to drive circuitry. Such heterojunctions would harness complicated biophysical reactions to provide a tunable variable for design problems.
For now, Baier will be satisfied if he can build a "crystal" radio with his biochip architecture. His approach will be to follow the recipe of Bardeen and Brattain when they invented the world's first transistor. As with the original, Baier plans to attach two fine wires only micrometers apart, but this time in a biodoped germanium crystal that has been bonded to a metal disk. The assembly will be housed inside a metal cylinder electrically grounded to the disk. Bardeen and Brattain's "cat's whisker" will be connected to a meter to register success or failure.