Section 7.6 Diodes, Photodiodes, and LEDs
A p-n junction like that shown in Figure 7.8 forms what is know in electronics as a diode — a device that enables current to pass only in one direction. Diodes are used for rectifiers which turn AC (alternating current) power from the outlets in your house into DC (direct current) power needed for most devices. Diodes are used to protect sensitive electronic devices from accidental reverse voltages. And diodes form some of the fundamental elements in digital computer circuitry. 1
To understand how a p-n junction works as a diode, consider Figs. Figure 7.8 and Figure 7.9. The key is the depletion zone (DZ) at the junction. Since there are no mobile charge carriers in the DZ, then that part of the device is a good electrical insulator. For a p-n junction to conduct, the DZ has to be eliminated.
If you want to run an electrical current through a p-n junction, you need to apply an electric field. If the field points from the p-type to the n-type semiconductor (as in Figure 7.9a), the holes in the p-type material are pulled in the direction of the electric field toward the junction, and the electrons in the n-type material are pulled in the direction opposite the electric field, also toward the junction. Consequently, the DZ shrinks. If the field is strong enough 2 the holes and electrons meet at the junction, and the DZ is completely gone; consequently, the entire device conducts and the current can pass. So, a strong enough electric field in this forward-biased direction will eliminate the DZ and produce an electrical current.
On the other hand, if an electric field is applied pointing from the n-type to the p-type semiconductor, the field pushes the holes and mobile electrons away from the junction, making the DZ even larger. With the DZ intact, the p-n junction won't conduct electricity in this reverse-biased direction. So, a p-n junction works as a diode — it allows current to flow in one direction but not the other. 3
But more can be done with a diode (p-n junction) than just passing current only in one direction. When a forward-biased diode is conducting a current, mobile electrons and holes are continually annihilating each other at the junction. Each time an electron-hole pair annihilates, a photon is emitted, as shown in Figure 7.10a. This is the principle behind what are called light-emitting diodes (LEDs), which can be found in almost any piece of modern electronics. The little red and blue indicator lights on your cell phone are LEDs. The “flash” that your cell phone uses to take pictures at night (which you probably use more as a flashlight when walking back to your dorm at night) is an LED. Most scoreboards at games are made with LEDs. And you can now buy LED light bulbs which are significantly more efficient and reliable than standard, incandescent light bulbs.
The process works in reverse. If a photon with suitable wavelength and energy hits a p-n junction (Figure 7.10b), it can excite an electron from the valence to the conduction band, producing an electron-hole pair in the DZ. Some residual electric fields in the DZ 4 sweep this electron and hole away from the DZ and produce a small electrical current that can be measured. As a result, a diode can be used as a light detector — it is referred to as a photodiode when used this way. This is the basic idea behind the detectors used in modern digital cameras, including the camera in your cell phone.
Finally, an introduction to semiconductor physics would not be complete with a discussion of the transistor, the development of which completely revolutionized modern technology. A basic transistor can be made out of a sandwich of a p-type semiconductor between two pieces of n-type semiconductors (Figure 7.11). For your homework problems, you will explain (by considering the depletion zones at each of the two p-n junctions) how a transistor works. The result is a device that will not pass an electrical current in either direction unless a very small electrical current is provided at one of the p-n junctions. That very small, second current effectively “turns on” or “turns off” the larger current flowing lengthwise through the device. As you will also see in your homework, the current can also be turned on by light shining on the p-n junction, as shown in Figure 7.11b for a device referred to as a phototransistor.
The use of a transistor as an electronic “on-off switch” has tremendous implications for modern technolgoy. Without electronic switches, most of modern electronic technology would be useless. Simple examples of the use of transistor switches abound. A transistor switch turns on the blue indicator light on your phone when you have a new email or the red indicator light if your phone is charging. A transistor switch turns on and off the display of your phone or the LED light that you use to walk home at night. In a computer, billions of transistor switches are rapidly turning on and off small currents that enable the computer to perform its tasks.
Transistors are also very important for power amplification. As an example, your cell phone detects electromagnetic waves transmitted by a nearby cell tower, but the detected signal is very weak. A transistor amplifier makes the signal stronger so that your cell phone can decode the signal and produce a time-varying voltage which is then amplified by another transistor circuit to produce a sound loud enough for your ear to hear.
The npn transistor was the first truly electronic switch that didn't depend on vacuum-tube technology. 5 There are many advantages to transistors over vacuum tubes, the most significant being that they can be miniaturized and printed into integrated circuits. Current computer CPUs have over a billion transistors. Think about that: that is a lot of transistors.