Читать книгу Semiconductor Basics - George Domingo - Страница 14

To explain how semiconductors work, we start with the Bohr atom. Most readers are familiar with Bohr's planetary model of the atom. How Bohr came up with this model is a very interesting scientific historical path involving many famous scientists of the nineteenth and early twentieth centuries. Science goes one step at a time.

William Wollaston (1766–1826), shown on the left in Figure 1.2, was an English chemist who discovered a couple of atomic elements, including palladium and rhodium. Very early in the 1800s, he built the first spectrometer. Wollaston focused the light from the sun through a prism and, to his surprise, found black lines partitioning the spectra (Figure 1.3). What the heck was going on?

Figure 1.2 William Wollaston (left) looked at the sun's light through a prism and was the first to observe the missing lines.

Source: https://library.si.edu/image‐gallery/73731. Joseph von Fraunhofer (right) studied the missing lines with his spectrometer and named them A–K. where Hz, Hertz is the unit for frequency. https://www.kruess.com/en/campus/spectroscopy/history‐of‐spectroscopy/

Figure 1.3 The sun’s spectrum through a prism shows dark lines: wavelengths of light that seem to have disappeared.

Source: https://www.kruess.com/en/campus/spectroscopy/history‐of‐spectroscopy/.

Suppose you are roasting a chicken and carefully watching the dial of a digital thermometer inserted in the chicken's breast as the temperature increases from room temperature, 24 °C (degrees Celsius), to 80 °C, the recommended internal temperature of a well‐cooked chicken breast. As the temperature increases, suddenly the thermometer jumps from 39 °C to 41 °C, then from 56 °C to 58 °C, and finally from 66 °C to 68 °C. You wonder what is wrong with the thermometer: why don't the temperatures 40, 57, and 67 °C show up on the dial? They don't seem to exist. You buy a new thermometer, just to be sure, and find that exactly the same temperature values are missing. A third thermometer gives the same results. You place the same thermometers in soup, and the thermometers are well behaved, showing in succession the values 39, 40, and 41 °C. So, the thermometers work. The missing temperatures are no coincidence. There is something in that chicken that makes the temperature jump from one value to another without passing through the one in the middle.

Well, that was probably Wollaston's initial reaction. What separated the colors? Was the instrument lens dirty? He even considered the possibility that there were natural boundaries between certain colors. But why didn't these black boundaries appear when he pointed the spectrometer at a white light?

German physicist Joseph von Fraunhofer (1787–1826), on the right in Figure 1.2, studied these dark lines of the sun's spectrum in much more detail and actually named the missing lines with the letters A–K (not too imaginative; ancient astronomers would have found much more attention‐grabbing names).