The difference between insulators and metals is clear in this analogy. An insulator is a solid in which every single seat in the orchestra is filled, while a metal is a material for which only half of the seats in the lower level are occupied. In a metal, there are a large number of empty seats in the orchestra available to an electron, and the application of a voltage, whether big or small, can accelerate the electrons to higher energy states (which correspond to carrying an electrical current). Metals are good electrical conductors because their lowest occupied auditorium seats for electrons are only half-filled. For the insulator, every seat is occupied, and absent promotion to the balcony, no current will result when a voltage is set up across the material. If I raise the temperature of an insulator, providing external excess energy in the form of heat, some of the electrons can rise into the previously empty balcony. In the balcony there are many empty seats for the electron to carry a current, but this will last only as long as the temperature is elevated. If the temperature is lowered, the electrons in the balcony will descend and return to their low-energy seats in the orchestra.
If the insulator absorbs energy in the form of light, it can immediately promote an electron to the balcony. When the electron returns to its seat in the orchestra, it has to conserve energy and thus gives off the same amount of energy that it previously absorbed. It will either do this by giving off light of the same energy as initially absorbed, or the electron can induce atomic vibrations (heat). This is why shining light on an object warms it up—the electrons absorb the energy of the light, but then can return the absorbed energy in the form of heat. If the energy of the light is insufficient to promote an electron from the highest filled orchestra seat to the lowest empty balcony seat, the light is not absorbed. In this case the lower-energy light is ignored by the electrons in the solid, and passes right through it. Insulators such as window glass are transparent because the separation between the filled orchestra and empty balcony for this material is in the ultraviolet portion of the spectrum, so visible light with a lower energy passes right through. On the other hand, metals always have available empty seats to absorb light even in the half filled orchestra. No matter how small the light’s energy, an electron in a metal can absorb this energy and then return it upon going back to its lower-energy seat. This is why metals are shiny—and good reflectors. They always give off light energy equal to what is absorbed, and there is no lower limit to the energy of light they can take in.
A semiconductor is just an insulator with a relatively small energy gap (compared with the energy of visible light) separating the filled lower band from the next empty band. For such an energy separation, a certain fraction of electrons will have enough thermal energy at room temperature to be promoted to the balcony. When electrons are excited to the upper deck, the material now has two ways to conduct electricity. For every electron promoted to the higher energy band that is able to conduct electricity, an empty state is left behind. The empty chairs in the previously filled orchestra can be considered as “positive electrons” or “holes,” and can also carry electrical current. If an electron adjacent to an empty seat slips into this chair, then the empty spot has migrated one position over. In this way we can consider the hole to move in response to an external voltage, and also carry current. Of course, the original electrons will eventually fall back down into the orchestra, filling the empty seats they left behind (though not necessarily their original seats). When certain semiconductors absorb light, there are enough excited electrons in the upper band and holes in the lower band to convert the material from an insulator to a good electrical conductor. As soon as the light is turned off, the electrons and holes recombine, and the material becomes an insulator again. These semiconductors are called “photoconductors” and are used as light sensors, as their ability to carry an electrical current changes dramatically when exposed to light. Certain smoke detectors, television remote controls, and automatic door openers in supermarkets make use of photoconductors for their operation.
Semiconductor devices are typically constructed out of silicon because it has an energy gap conveniently just below the range of visible light. Furthermore, it is a plentiful element (most sand is composed of silicon dioxide) that is relatively easy to purify and manipulate. There are times when the physical constraints of the size of the energy gap in silicon limits a device’s performance, and in this case, other semiconducting materials can be used, such as germanium or gallium arsenide. Iron Man’s, and the military’s, night-vision capabilities make use of a semiconductor’s photoconducting properties and a small energy gap that is in the infrared portion of the electromagnetic spectrum.
All objects give off electromagnetic radiation due to the fact that they are at a certain temperature, so their atoms oscillate at a particular frequency that reflects their average kinetic energy. On a dark, moonless night, the temperature of most nonliving objects decreases (as they are not absorbing sunlight), so they emit less radiation, and what they do give off is at lower frequencies. Humans, on the other hand, have metabolic processes that maintain a uniform temperature of 98.6 degrees Fahrenheit. Consequently, we emit a fair amount of light (as much energy as a 100-Watt lightbulb) in the infrared portion of the spectrum. Our eyes are not sensitive to this part of the spectrum, but semiconductors can be chosen that have a large photoconductivity when exposed to infrared light. At night the infrared light given off by a warm-blooded person is much greater than his or her colder surroundings.
Certain night-vision goggles, such as Nite Owl II’s in Watchmen, use “thermal imaging” to detect this light by using semiconductors, which absorb the infrared radiation given off by an object at a temperature of roughly 100 degrees. The photocurrent in the semiconductor detector is then transported to an adjacent material, which is chemically constructed to give off a flash of visible light when the photoexcited electrons and holes recombine. In this way the infrared light that our eyes cannot usually detect is shifted to the visible portion of the electromagnetic spectrum, thereby enabling us to see in the dark. These goggles also detect visible light, as well as infrared light during the daytime. All objects give off roughly the same intensity of light if they are at the same temperature (recall our discussion of light-curves from Chapter 21). When the objects around a person are warmer (due to absorbed sunlight), the contrast between the infrared light from a person and his or her inanimate surroundings is diminished, as is the utility of the goggles.
WHAT COLOR ARE THE INVISIBLE WOMAN’S EYES?
An understanding of semiconductor photoconductivity also helps to resolve a question that has long perplexed comic-book fans: Why isn’t the Invisible Woman blind? When the Fantastic Four took their ill-fated rocket trip, Sue Storm (now Susan Richards) gained the ability to become completely transparent at will. How can she do this, and how can she see, if visible light passes right through her? The more basic question is: How do we see anything at all?
The molecules that make up the cells in our bodies absorb light in the visible portion of the electromagnetic spectrum. The addition of certain molecules, such as melanin, can increase this absorption, darkening the skin. As a result of her exposure to cosmic rays, the Invisible Woman gained the ability to increase the “energy gap” of all of the molecules in her body (this is presumably the nature of her “miracle exception”). If the separation between the filled lower orchestra and the empty upper balcony is increased such that it extends into the ultraviolet portion of the spectrum, then visible light will be ignored by the molecules in her body and pass right through her. This is not so far-fetched; after all, we all possess cells that are transparent to visible light. In fact, you’re using them right now, reading this text through the transparent lens of your eyes.
Sunlight contains a great deal of ultraviolet light, which has more energy than visible light. We typically don’t think about the ultraviolet portion of the solar spectrum until we sunburn on a bright summer day. When Sue becomes invisible, she still absorbs and re
flects light in the ultraviolet region of the spectrum. We can’t see her because the rods and cones in our eyes do not reso nantly absorb ultraviolet light. Special UV glasses (like the ones Doctor Doom installed in his armored mask) could shift the ultraviolet light reflected from Sue down into the visible portion of the spectrum, using a similar mechanism to the one used by night-vision goggles in shifting low-energy infrared light up into the visible portion of the spectrum.
This also explains how the Invisible Woman is able to see. The rods and cones in her eyes, when she is transparent, become sensitive to the scattered ultraviolet light that bounces off us, but that we cannot see. The world Sue sees while invisible will not have the normal coloring we experience, for the shift in wavelengths of the light she detects is not associated with the colors of the rainbow. Windows appear transparent to us because they transmit visible light and absorb ultraviolet light. We can’t see ultraviolet light, so we don’t notice its absorption. However, when Sue is invisible, a window will appear as a large dark space, while other objects will appear transparent to her. With a little practice, she would be able to maneuver just fine.
This mechanism to account for Sue’s ability to see while invisible was suggested in Fantastic Four # 62, Vol. 3 (Dec. 2002) that corresponded to the 491st issue in the numbering scheme that began in 1961. In this issue, written by Mark Waid and drawn by Mike Wieringo, we are told that while invisible, Sue sees by detecting the scattered cosmic rays that are all around us but cannot be detected by normal vision. Right idea—wrong illumination source. Cosmic rays from outer space are not light photons but are mostly high-velocity protons that, upon striking atoms in the atmosphere, generate a shower of electrons, gamma-ray photons, muons (elementary particles related to electrons), and other elementary particles. We usually don’t have to worry about radiation damage or gaining superpowers via cosmic-ray induced mutation, at least at sea level, as the particle flux is a million trillion times less than that of sunlight. If Sue depended on cosmic rays to see at street level, she would be constantly bumping into objects and people. It is more likely that her vision makes use of the same mechanism by which she could become transparent—namely, a shift in her molecular bonding into the ultraviolet portion of the spectrum.
WHAT IS A TRANSISTOR, AND WHY SHOULD WE CARE?
Back to Tony Stark and his transistorized suit of armor. When Tony needed to increase the repelling power of his magnetic turbo-insulator, he used a top-hat transistor. How are transistors able to amplify weak signals, making radios portable and repulsor rays powerful?
While semiconductors are useful as photoconducting devices, if this were their only application, no one would think to call this era the Silicon Age. The thing about semiconductors that makes them very handy to have around the house is that you can change their ability to conduct electricity by a factor of more than a million by intentionally adding a very small amount of chemical impurities. Not only that, but, depending on the particular impurity, you can either add excess electrons to the semiconductor or remove electrons from the filled auditorium of our earlier metaphor, thereby creating additional holes that can also conduct electricity. When a material with excess electrons is placed next to a semiconductor with additional holes, you have a solar cell, and if you then add a third layer with excess electrons on top of that, you’ve made a transistor.
It’s been known for a long time that the addition of certain chemicals can change the optical and electronic properties of insulators. After all, that’s how stained glass is made. Ordinary window glass has an energy gap that is larger than the energy of visible light, which is why it is transparent. But add a small amount of manganese to the glass when it is molten, and after cooling, the glass appears violet when light passes through it. Manganese has a resonant absorption right in the middle of the glass’s energy gap, as if we had parked some extra chairs on the stairways that connect the filled orchestra and the empty balcony. Particular wavelengths of visible light that would ordinarily pass through the material unmolested will now induce a transition in the manganese atoms added to the glass. In this way certain wavelengths are removed from the white light transmitted through the glass, giving the window material a color or “stain.” Different chemical impurities, such as cobalt or selenium, will add different colorations (blue and red, respectively) to the normally transparent insulator.
The same principle works for semiconductors, only the chemical impurities that we choose to add can either make it very easy to promote electrons to the balcony or to take electrons out of the filled auditorium, leaving holes in their place. A semiconductor for which the chemical impurities donate electrons is termed “n-type,” since the electrons are negatively charged, while those for which the impurities accept electrons from the filled lower states are called “p-t ype,” referring to the positively charged holes created. What’s special about such semiconductors with added impurities is not that their conductivity can be changed dramatically (if we wanted a more conductive material, we would just use a metal) but rather what happens when we put an n-type semiconductor next to a p-type semiconductor. The extra electrons and holes near the interface between these two different materials quickly recombine, but the chemical impurities, which also have an electrical charge, remain behind. The positively charged impurities in the n-type region and the negatively charged impurities in the p-type region create an electric field, just as exists between positive and negative charges in space. This electric field points in one direction. If I try to pass a current through the interface between the n-type and p-type semiconductors, it will move very easily in the direction of the field, and it will be very tough going opposing the field. Such a simple device is called a “diode” in the dark, and a “solar cell” when you shine light on it. When the p-n junction absorbs light, the light induced electrons and holes create a current, even without being connected to a battery. The charges are pushed by the internal electric field just as surely as if the device were connected to an external voltage source. A solar cell, therefore, can generate an electrical current through the combination of the light induced extra electrons and holes with the internal electric field left behind by the charged impurities. This is one of the very few ways to generate electricity that does not involve moving a wire through a magnetic field, and thus no fossil fuels need be consumed for this device to work.
A transistor takes the directionality of the electrical current of a diode and makes the internal electric field changeable. By doing this, the transistor can be viewed as a special type of valve, where an input signal determines how far the valve is opened, which in turn leads to either a large or small current flowing through the device. Returning to the water-flow analogy for electrical current from Chapter 17, a fire hose is attached to the city water supply and, as the valve connecting the hose to the faucet is opened, water flows through the hose. If the valve is barely cracked open, the flow will be very weak, and as the valve is opened wider and wider, the quantity of water exiting the hose increases. Usually I have to manually turn the handle of the valve to effect a change. Now imagine a valve that is connected to a second, smaller garden hose that brings in a small stream of water. How much or how little the valve is opened will depend on how much water the second hose brings to the valve. If I considered the water flow in the garden hose as my “signal,” then the resulting water flow out of the main fire hose is an amplified version of this signal.
In this way, a small voltage can be magnified without changing any of the time-dependent information encoded within it. When Iron Man needs to increase the current to his magnetic turbo-insulator a thousandfold, or amplify the current going to the servo-motors that drive the punching force of his suit, he uses transistors to take small input currents and increase their amplitude. Despite what Tony Stark would tell you, transistors don’t actually provide power, but they do enable the amplification of a small signal, increasing it many times. To do so they need a large reservoir of electrical charge, such as an exter
nal battery, just as in the water analogy the “transistor valve” would not amplify the weak input from the garden hose unless the output from the fire hose was connected to the city water supply. Consequently, rather than providing power, transistors actually use power, but the rate at which they use power in order to amplify a weak signal is much less than the old amplification technique (vacuum tubes) they replaced. This is why Iron Man would be in desperate need of a recharge after a taxing battle. Tony would frequently gasp that his transistors needed to recharge, but I’m sure that he actually meant to refer to the battery supply to his transistors. Such a slip of the tongue is forgivable—I’m sure I’d misspeak after going several rounds with the Titanium Man.
Before transistors, the amplification of a weak input current was performed by heated wires and grids that guided the motion of electrons across space. A current was run through a filament wire until it glowed white hot, and electrons were ejected from the metal and accelerated by a positive voltage applied to a plate some distance away, pulling these free electrons toward it. Between the filament and the collector plate is a grid (that is, a screen) that can act like a valve. If the input signal is applied to this grid, it would modulate the collected current, opening and closing the valve as in the water analogy. In order to avoid collisions with air molecules that would scatter the electron beam away from the collector electrode, these wires and grids were enclosed in a glass cylinder from which nearly all the air had been removed. These so-called vacuum tubes were large, used a great deal of power to heat the wires and run the collector plate, took a while to warm up when initially started, and were very fragile. A vacuum tube-powered Iron Man would hardly be invincible, as the sound of breaking glass would accompany his first and only adventure. Semiconductor-based transistors are small, low-power devices that are instantly available to amplify current and are compact and rugged. Even so, it took years before the transistor, invented in 1947, replaced the vacuum tube in most electronic devices.
The Physics of Superheroes: Spectacular Second Edition Page 33
