The Amazing Story of Quantum Mechanics (33 page)

It may seem surprising that one could construct a complex literature using an alphabet consisting of only two letters. But if there is no constraint on the length of a given word, then there is indeed enough flexibility to perform even the most sophisticated mathematical operations, such as adding two numbers. A full discussion of how diodes and transistors are combined to perform a variety of logic functions, and the Boolean mathematics that underlies their calculations, warrants its own book, and would take us too far afield for a discussion of the applications of quantum mechanics. Nevertheless, I do want to conclude this chapter with a discussion of one modification of the transistor structure that is already changing our everyday life.

Long-term information storage in a computer is done via the magnetic hard drive. A disc contains a record of “ones” and “zeros” in the form of magnetic domains, with a magnetic field pointing in one direction counting as a “one” and a field pointing in the opposite direction as a “zero.”
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An externally applied magnetic field can polarize regions (“bits”) on the drive, and write the sequence of ones and zeros that encode information. A smaller magnetic sensor, essentially a layered metallic structure whose resistance is very sensitive to the external magnetic field, is brought near the disc. If the magnetic field of a bit points in one direction, the resistance of the sensor will have one value; it will have another if the bit’s magnetic field is in the opposite direction. The disc itself spins like a DVD or CD at high speeds of more than five thousand revolutions per minute, and the sensor rides just above the hard drive, with a spacing that is equivalent to one-hundredth the diameter of a human hair. To store more information, one makes the platter larger and the bits smaller. That this magnetic device is capable of storing information without an external power supply (once the bits are magnetized, they stay in the same orientation), and with a relatively low failure rate (despite fears of “hard-drive crashes,” the medium is extremely reliable given the use it endures), is a testament to the skill of engineers.

Transistors are also able to store information. For the device configuration we have discussed, the conductance of the semiconductor is low if there is no voltage to the gate metal (representing a “zero”), and high (standing in for a “one”) when a positive voltage is applied. However, once the external voltage is turned off, then all transistors in a circuit default back to their low conductance state.

How can I store and preserve the high-conductance channel of a transistor, that is, keep the “ones” from turning into “zeros” after the voltage is turned off? Flash memory devices add a very small wrinkle on the field-effect structure we have described. To the standard field-effect transistor configuration, the flash memory adds a second metal electrode in the insulating layer, a very small distance above the semiconductor. So the device has a metal gate, a thin layer of insulator, another thin metal electrode, and then a very thin layer of the insulator atop the semiconductor.

What’s the point of the second metal layer? If the two electrodes that used to pass the current through the semiconductor are shorted, and a large voltage is applied to the gate metal, then electrical charges can quantum mechanically tunnel to this interior electrode. This electrode is not connected to any outside wires and is termed the “floating gate.” The floating gate can be a thin metal film, or it could be a layer of silicon nanocrystals, separated from one another so that these charges remain on the silicon particles and do not leak away. The charged floating gate generates an electric field in the semiconductor, influencing the current-carrying channel and maintaining the device in either a high- or low-conductance state (that is, recorded as a “one” or a “zero”) even after the voltage is removed from the gate metal. Until a voltage of opposite polarity is applied, the transistor will store this state of the transistor, even when the transistor is unplugged from any power supply (such a memory is termed “nonvolatile”). The story goes that a colleague of Fujio Masuoka, the inventor of this type of transistor memory, when describing how quickly the stored information could be erased, said that it reminded him of a camera’s flash, whence the nickname for the device derives. At the time of this writing, flash memory devices capable of storing 256 gigabytes of information (large enough to store more than ten thousand copies of this book as Word documents) are being manufactured.

Nonvolatile memories have also revolutionized photography. In conventional, nondigital cameras, a light photon induces a chemical change in a photographic film. The information as to where the photon was absorbed by the molecule in the film is stored, and then a series of wet chemistry steps transfers this information to a photographic print. The graininess of individual molecules in a conventional film is now replaced with a pixilated grid. When photons strike the photodetectors in a given pixel, they will, if absorbed, create mobile charges. Using different semiconductors, the energy separation between the filled and empty bands of states can be changed, enabling photodetectors that can image in the infrared, visible, or ultraviolet portion of the spectrum. The charges up in the balcony can be converted to voltages, and then stored on flash memories. The location of each pixel is known, so a digital record of the number of photons that struck the array of photodetectors is obtained.

Once an image is digitally captured, the ability to display it on a flat panel screen, as opposed to the bulky cathode ray tubes that were a feature of televisions up until fairly recently, also makes use of semiconductor transistor technology. The bits of information in this case are the pixels on the display screen. In each pixel is a small amount of a “liquid crystal,” consisting of long chain organic molecules (that is, carbon atoms bonded in a line, with various other elements and chemical groups protruding from the carbon chain). Geometric constraints and electrostatic charges along the carbon line will lead certain long chain molecules to pack together in different arrangements, from a loose, random collection to a herringbone pattern not unlike a professor’s tweed coat to a more ordered phase similar to matches tightly stacked in a box. Just as the matches can be easily poured out of the matchbox regardless of their packing, these long chain molecules retain the ability to fill a container and flow as a fluid.

Certain liquid crystal molecules will make a transition from one ordered configuration to another when the temperature is changed—or if an external electric field is applied to the molecules. The optical properties of water change dramatically when ice undergoes a phase transition and melts—similarly, when certain liquid crystals change from one packing state to another under an external voltage, there can be an associated change in their optical properties, such as whether the material reflects light and is shiny or absorbs light and appears dark. Early “liquid crystal watches” had metal electrodes in “broken eight” pattern, and depending on which metal plate had an applied voltage, different regions of the liquid crystal would appear dark, and thus form different numerals depending on the time of day. These liquid crystal displays (LCDs) are still employed in certain clocks and timers. For more sophisticated image displays, a capacitor and a thin film transistor (sometimes referred to as a TFT) are placed behind each liquid crystal pixel. Color filters can convert a grayscale image to a color one, and by changing the timing of when each pixel is turned on and off, one can view a moving image, similar to the television screen shown on the cover of the December 1936
Amazing Stories
science fiction pulp (seen in Figure 45).

The ability to instantly display the stored image (or video) and the convenience of data transfer and large storage capacity, coupled with the incorporation of these cameras into other devices (such as cell phones or computer screens), has exceeded the expectations of science fiction pulp magazines—well, with one exception. As illustrated in Figure 46, the notion that a device capable of wireless video reception and broadcasting small enough that it would fit on a person’s wrist was indeed anticipated in 1964 by the comic strip creator Chester Gould. Wrist phones that are capable of video transmission are now becoming available, another example of fiction becoming reality through quantum mechanics. Now, if we could only figure out how to construct personal “garbage cans” (Chapter 4, Figure 8) that fly by means of magnetism!

 

 

 

One of the most surprising
discoveries made by physicists probing the inner workings of the atom was that electrons—subatomic particles that are the basic carriers of negative charge—also are little bar magnets, like those shown in Figure 10 in chapter 4. This intrinsic magnetic field is associated with a property called “spin,” though this term is a misnomer—while it does relate to intrinsic angular momentum, the magnetic field associated with the electron doesn’t really come from its spinning like a top. Nevertheless, when physicists refer to the internal magnetic field possessed by electrons (or protons or neutrons), they inevitably speak of the particle’s spin.

A transistor modulates the current flowing through a semiconductor by the application of an electric field to an insulating slab on top of the conducting material. In this way the current flowing through the semiconductor is regulated through the electron’s negative charge. The magnetic field that the electron exhibits has been, in most electronics up till now, completely ignored. As one might imagine, this situation changes in devices characterized as “spintronic,” a shorthand expression for “spin transport electronics.” Here the electron’s magnetic field is a crucial component of the signal being detected or manipulated. One form of spintronics has been employed in computer hard drives, while the next generation of such devices (discussed in Section 6) may make hard drives unnecessary.

As described in Chapter 15, a DVD encodes information in the form of ones and zeros as smooth or pitted regions on a shiny disc. A laser reflected from the surface of the disc does so either specularly, that is, smoothly onto a photodetector if the surface is smooth, or diffusely, away from the detector if it strikes a jagged pit. Similarly, the hard disc drive in a computer is a magnetic material with regions magnetized in particular patterns; the smallest elements of the pattern are termed “bits.” The drive stores information in the form of ones and zeros as magnetized regions, with north poles pointing in one orientation representing a “one,” and in the other direction standing for a “zero.” Each bit (in current disc drives) is written by moving a magnet over the region, which orients the magnetic pattern. To create the opposite pattern, a magnetic field in the reverse direction is applied. To erase the bit, a depolarizing magnetic field is applied. To read the “one” or “zero” stored on the disc, hard drives employ sensors such as “giant magnetoresistance” devices or “magnetic tunnel junctions.”

All solids have bands of allowed states in which the electrons may reside, separated by energy gaps where there are no allowed quantum states. The difference between an insulator and a metal is that for an insulator (or a semiconductor), the last filled band, the orchestra in our auditorium analogy, is completely filled, with every possible energy state being occupied by an electron. In contrast, in metals, the lower orchestra level is only half filled, as shown in Figure 34b in Chapter 14. If a voltage is applied to a metal, the electrons feel a force. This force in turn accelerates the electrons, causing them to speed up and increase their kinetic energy. Recall the water-hose analogy of metal wires—the voltage is like the water pressure, and the electrical current is the resulting flow of water through the hose. As there are always some unoccupied seats in the lower orchestra level of a metal, electrons in the upper, filled rows can always move to higher energy states, and the material is able to conduct an electrical current.