Context: Did you ever make a secret code? One way to make a code is to represent each letter of the alphabet by a different number. Then you can send a coded message by writing words as strings of digits. This is similar to how information is encoded using an electric current. The voltage of the current is changed rapidly and repeatedly to encode a message, called an electronic signal. There are two different types of electronic signals: analog signals and digital signals. Both are illustrated in Figure 23.20. A digital signal consists of pulses of voltage, created by repeatedly switching the current off and on. This type of signal encodes information as a string of 0s (current off) and 1s (current on). This is called a binary ("two-digit") code. DVDs, for example, encode sounds and pictures as digital signals. An analog signal consists of continuously changing voltage in a circuit. For example, microphones encode sounds as analog signals. Electronic components are the parts used in electronic devices such as computers. The components transmit and change electric current. They are made of materials called semiconductors. A semiconductor is a solid crystalusually consisting mainly of siliconthat can conduct current better than an electric insulator but not as well as an electric conductor. Very small amounts of other elements, such as boron or phosphorus, are added to the silicon so it can conduct current. A semiconductor is illustrated in Figure 23.21. There are two different types of semiconductors: n-type and p-type. An n-type semiconductor consists of silicon and an element such as phosphorus that gives the silicon crystal extra electrons. An n-type semiconductor is like the negative terminal in a chemical cell. A p-type semiconductor consists of silicon and an element such as boron that gives the silicon positively charged holes where electrons are missing. A p-type semiconductor is like the positive terminal in a chemical cell. Electronic components contain many semiconductors. Types of components include diodes, transistors, and inte- grated circuits. Each type is described in Table 23.2. Electronic Component Diode Transistor Integrated Circuit (Microchip) Description A diode consists of a p-type and an n-type semicon- ductor placed side by side. When a diode is connected by leads to a source of voltage, electrons flow from the n-type to the p-type semiconductor. This is the only direction that electrons can flow in a diode. This makes a diode useful for changing alternating current to direct current. A transistor consists of three semiconductors, either p- n-p or n-p-n. Current cant flow through a transistor unless a small amount of current is applied to the center semiconductor (through the base). Then a much larger current can flow through the transistor from end to end (from collector to emitter). This means that a transmitter can be used as a switch, with pulses of a small current turning a larger current on and off. A transistor can also be used to increase the amount of current flowing through a circuit. You can learn more about transistors and how they work at this URL: http An integrated circuitalso called a microchipis a tiny, flat piece of silicon that consists of layers of elec- tronic components such as transistors. An integrated circuit as small as a fingernail can contain millions of electronic components. Current flows extremely rapidly in an integrated circuit because it doesnt have far to travel. You can learn how microprocessors are made at this URL: Many of the devices you commonly use are electronic. Electronic devices include computers, mobile phones, TV remotes, DVD and CD players, game systems, MP3 players, and digital cameras. All of these devices use electric current to encode, analyze, or transmit information. Consider the computer as an example of an electronic device. A computer contains microchips with millions of tiny electronic components. Information is encoded as 0s and 1s and transmitted as electrical pulses. One digit (either 0 or 1) is called a bit, which stands for "binary digit." Each group of eight digits is called a byte. A gigabyte is a billion bytes that 
 Question: Silicon can conduct current when it contains very small amounts of
Output: boron or phosphorus.