23.5

Semiconductor Devices

Semiconductor devices such as diodes and transistors are widely used in modern electronics, and Figure 23-21 illustrates one application. The drawing shows an audio system in which small ac voltages (originating in a compact disc player, an FM tuner, or a cassette deck) are amplified so they can drive the speaker(s). The electric circuits that accomplish the amplification do so with the aid of a dc voltage provided by the power supply. In portable units the power supply is simply a battery. In nonportable units, however, the power supply is a separate electric circuit containing diodes, along with other elements. As we will see, the diodes convert the 60-Hz ac voltage present at a wall outlet into the dc voltage needed by the amplifier, which, in turn, performs its job of amplification with the aid of transistors.

Figure 23-21 In a typical audio system, diodes are used in the power supply to create a dc voltage from the ac voltage present at the wall socket. This dc voltage is necessary so the transistors in the amplifier can perform their task of enlarging the small ac voltages originating in the compact disc player, etc.

| n-Type and p-Type Semiconductors

The materials used in diodes and transistors are semiconductors, such as silicon and germanium. However, they are not pure materials because small amounts of “impurity” atoms (about one part in a million) have been added to them to change their conductive properties. For instance, Figure 23-22a shows an array of atoms that symbolizes the crystal structure in pure silicon. Each silicon atom has four outer-shell* electrons, and each electron participates with electrons from neighboring atoms in forming the bonds that hold the crystal together. Since they participate in forming bonds, these electrons generally do not move throughout the crystal. Consequently, pure silicon and germanium are not good conductors of electricity. It is possible, however, to increase their conductivities by adding tiny amounts of impurity atoms, such as phosphorus or arsenic, whose atoms have five outer-shell electrons. For example, when a phosphorus atom replaces a silicon atom in the crystal, only four of the five outer-shell electrons of phosphorus fit into the crystal structure. The extra fifth electron does not fit in and is relatively free to diffuse throughout the crystal, as part b of the drawing suggests. A semiconductor containing small amounts of phosphorus can therefore be envisioned as containing immobile, positively charged phosphorus atoms and a pool of electrons that are free to wander throughout the material. These mobile electrons allow the semiconductor to conduct electricity.

Figure 23-22 A silicon crystal that is (a) undoped, or pure, (b) doped with phosphorus to produce an n-type material, and (c) doped with boron to produce a p-type material.

The process of adding impurity atoms is called doping. A semiconductor doped with an impurity that contributes mobile electrons is called an n-type semiconductor, since the mobile charge carriers have a negative charge. Note that an n-type semiconductor is overall electrically neutral, since it contains equal numbers of positive and negative charges.
It is also possible to dope a silicon crystal with an impurity whose atoms have only three outer-shell electrons (e.g., boron or gallium). Because of the missing fourth electron, there is a “hole” in the lattice structure at the boron atom, as Figure 23-22c illustrates. An electron from a neighboring silicon atom can move into this hole, in which event the region around the boron atom, having acquired the electron, becomes negatively charged. Of course, when a nearby electron does move, it leaves behind a hole. This hole is positively charged, since it results from the removal of an electron from the vicinity of a neutral silicon atom. The vast majority of atoms in the lattice are silicon, so the hole is almost always next to another silicon atom. Consequently, an electron from one of these adjacent atoms can move into the hole, with the result that the hole moves to yet another location. In this fashion, a positively charged hole can wander through the crystal. This type of semiconductor can, therefore, be viewed as containing immobile, negatively charged boron atoms and an equal number of positively charged, mobile holes. Because of the mobile holes, the semiconductor can conduct electricity. In this case the charge carriers are positive. A semiconductor doped with an impurity that introduces mobile positive holes is called a p-type semiconductor.

| The Semiconductor Diode

A p-n junction diode is a device that is formed from a p-type semiconductor and an n-type semiconductor. The p-n junction between the two materials is of fundamental importance to the operation of diodes and transistors. Figure 23-23 shows separate p-type and n-type semiconductors, each electrically neutral. Figure 23-24a shows them joined together to form a diode. Mobile electrons from the n-type semiconductor and mobile holes from the p-type semiconductor flow across the junction and combine. This process leaves the n-type material with a positive charge layer and the p-type material with a negative charge layer, as part b of the drawing indicates. The positive and negative charge layers on the two sides of the junction set up an electric field , much like that in a parallel plate capacitor. This electric field tends to prevent any further movement of charge across the junction, and all charge flow quickly stops.

Figure 23-23 A p-type semiconductor and an n-type semiconductor.

Figure 23-24 At the junction between n and p materials, (a) mobile electrons and holes combine and (b) create positive and negative charge layers. The electric field produced by the charge layers is .

The physics of a semiconductor diode.

Suppose now that a battery is connected across the p-n junction, as in Figure 23-25a, where the negative terminal of the battery is attached to the n-material, and the positive terminal is attached to the p-material. In this situation the junction is said to be in a condition of forward bias, and, as a result, there is a current in the circuit. The negative terminal of the battery repels the mobile electrons in the n-type material, and they move toward the junction. Likewise, the positive terminal repels the positive holes in the p-type material, and they also move toward the junction. At the junction the electrons fill the holes. In the meantime, the negative terminal provides a fresh supply of electrons to the n-material, and the positive terminal pulls off electrons from the p-material, forming new holes in the process. Consequently, a continual flow of charge, and hence a current, is maintained.

Figure 23-25 (a) There is an appreciable current through the diode when the diode is forward biased. (b) Under a reverse bias condition, there is almost no current through the diode.

In Figure 23-25b the battery polarity has been reversed, and the p-n junction is in a condition known as reverse bias. The battery forces electrons in the n-material and holes in the p-material away from the junction. As a result, the potential across the junction builds up until it opposes the battery potential, and very little current can be sustained through the diode. The diode, then, is a unidirectional device, for it allows current to pass only in one direction.
The graph in Figure 23-26 shows the dependence of the current on the magnitude and polarity of the voltage applied across a p-n junction diode. The exact values of the current depend on the nature of the semiconductor and the extent of the doping. Also shown in the drawing is the symbol used for a diode . The direction of the arrowhead in the symbol indicates the direction of the conventional current in the diode under a forward bias condition. In a forward bias condition, the side of the symbol that contains the arrowhead has a positive potential relative to the other side.

Figure 23-26 The current-versus-voltage characteristics of a typical p-n junction diode.

A special kind of diode is called an LED, which stands for light-emitting diode. You can see LEDs in the form of small bright red, green, and yellow lights that appear on many electronic devices, such as computers, TV sets, and stereo systems. These diodes, like others, carry current in only one direction. Imagine a forward-biased diode, like that shown in Figure 23-25a, in which a current exists. An LED emits light whenever electrons and holes combine, the light coming from the p-n junction. Commercial LEDs are often made from gallium, suitably doped with arsenic and phosphorus atoms.

The physics of light-emitting diodes (LEDs).

A fetal oxygen monitor uses LEDs to measure the level of oxygen in a fetus’s blood. A sensor is inserted into the mother’s uterus and positioned against the cheek of the fetus, as indicated in Figure 23-27. Two LEDs are located within the sensor, and each shines light of a different wavelength (or color) into the fetal tissue. The light is reflected by the oxygen-carrying red blood cells and is detected by an adjacent photodetector. Light from one of the LEDs is used to measure the level of oxyhemoglobin in the blood, and light from the other LED is used to measure the level of deoxyhemoglobin. From a comparison of these two levels, the oxygen saturation in the blood is determined.

Figure 23-27 A fetal oxygen monitor uses a sensor that contains LEDs to measure the level of oxygen in the fetus’s blood. (Reprinted by permission of Nellcor Puritan Bennett, Inc., Pleasanton, California)

The physics of a fetal oxygen monitor.

The physics of rectifier circuits. Because diodes are unidirectional devices, they are commonly used in rectifier circuits, which convert an ac voltage into a dc voltage. For instance, Figure 23-28 shows a circuit in which charges flow through the resistance R only while the ac generator biases the diode in the forward direction. Since current occurs only during one-half of every generator voltage cycle, the circuit is called a half-wave rectifier. A plot of the output voltage across the resistor reveals that only the positive halves of each cycle are present. If a capacitor is added in parallel with the resistor, as indicated in the drawing, the capacitor charges up and keeps the voltage from dropping to zero between each positive half-cycle. It is also possible to construct full-wave rectifier circuits, in which both halves of every cycle of the generator voltage drive current through the load resistor in the same direction (see Conceptual Question 11).

Figure 23-28 A half-wave rectifier circuit, together with a capacitor and a transformer (not shown), constitutes a dc power supply because the rectifier converts an ac voltage into a dc voltage.

When a circuit such as that in Figure 23-28 includes a capacitor and also a transformer to establish the desired voltage level, the circuit is called a power supply. In the audio system in Figure 23-21, the power supply receives the 60-Hz ac voltage from a wall socket and produces a dc output voltage that is used for the transistors within the amplifier. Power supplies using diodes are also found in virtually all electronic appliances, such as televisions and microwave ovens.

| Solar Cells

The physics of solar cells. Solar cells use p-n junctions to convert sunlight directly into electricity, as Figure 23-29 illustrates. The solar cell in this drawing consists of a p- type semiconductor surrounding an n-type semiconductor. As discussed earlier, charge layers form at the junction between the two types of semiconductors, leading to an electric field pointing from the n-type toward the p-type layer. The outer covering of p-type material is so thin that sunlight penetrates into the charge layers and ionizes some of the atoms there. In the process of ionization, the energy of the sunlight causes a negative electron to be ejected from the atom, leaving behind a positive hole. As the drawing indicates, the electric field in the charge layers causes the electron and the hole to move away from the junction. The electron moves into the n-type material, and the hole moves into the p-type material. As a result, the sunlight causes the solar cell to develop negative and positive terminals, much like the terminals of a battery. The current that a single solar cell can provide is small, so applications of solar cells often use many of them mounted to form large panels, as Figure 23-30 illustrates.

Figure 23-29 A solar cell formed from a p-n junction. When sunlight strikes it, the solar cell acts like a battery, with + and − terminals.

Figure 23-30 The Helios Prototype flying wing is propelled by solar energy. The solar cells are mounted on the top of the wing. (Gamma Press, Inc.)

| Transistors

The physics of transistors. A number of different kinds of transistors are in use today. One type is the bipolar junction transistor, which consists of two p-n junctions formed by three layers of doped semiconductors. As Figure 23-31 indicates, there are pnp and npn transistors. In either case, the middle region is made very thin compared to the outer regions.

Figure 23-31 There are two kinds of bipolar junction transistors, pnp and npn.

A transistor is useful because it can be used in circuits that amplify a smaller voltage into a larger one. A transistor plays the same kind of role in an amplifier circuit that a valve does when it controls the flow of water through a pipe. A small change in the valve setting produces a large change in the amount of water per second that flows through the pipe. In other words, a small change in the voltage input to a transistor produces a large change in the output from the transistor.
Figure 23-32 shows a pnp transistor connected to two batteries, labeled VE and VC. The voltages VE and VC are applied in such a way that the p-n junction on the left has a forward bias, while the p-n junction on the right has a reverse bias. Moreover, the voltage VC is usually much larger than VE for a reason to be discussed shortly. The drawing also shows the standard symbol and nomenclature for the three sections of the transistor— namely, the emitter, the base, and the collector. The arrowhead in the symbol points in the direction of the conventional current through the emitter.

Figure 23-32 A pnp transistor, along with its bias voltages VE and VC. On the symbol for the pnp transistor, the emitter is marked with an arrowhead that denotes the direction of conventional current through the emitter.

The positive terminal of VE pushes the mobile positive holes in the p-type material of the emitter toward the emitter/base junction. Since this junction has a forward bias, the holes enter the base region readily. Once in the base region, the holes come under the strong influence of VC and are attracted to its negative terminal. Since the base is so thin (about 10−6 m or so), approximately 98% of the holes are drawn through the base and into the collector. The remaining 2% of the holes combine with free electrons in the base region, thereby giving rise to a small base current IB. As the drawing shows, the moving holes in the emitter and collector constitute currents that are labeled IE and IC, respectively. From Kirchhoff’s junction rule it follows that .
Because the base current IB is small, the collector current is determined primarily by current from the emitter . This means that a change in IE will cause a change in IC of nearly the same amount. Furthermore, a substantial change in IE can be caused by only a small change in the forward-bias voltage VE. To see that this is the case, look back at Figure 23-26 and notice how steep the current-versus-voltage curve is for a p-n junction: small changes in the forward-bias voltage give rise to large changes in the current.
With the help of Figure 23-33 we can now appreciate what was meant by the earlier statement that a small change in the voltage input to a transistor leads to a large change in the output. This picture shows an ac generator connected in series with the battery VE and a resistance R connected in series with the collector. The generator voltage could originate from many sources, such as an electric guitar pickup or a compact disc player, while the resistance R could represent a loudspeaker. The generator introduces small voltage changes in the forward bias across the emitter/base junction and, thus, causes large corresponding changes in the current IC leaving the collector and passing through the resistance R. As a result, the output voltage across R is an enlarged or amplified version of the input voltage of the generator. The operation of an npn transistor is similar to that of a pnp transistor. The main difference is that the bias voltages and current directions are reversed.

Figure 23-33 The basic pnp transistor amplifier in this drawing amplifies a small generator voltage to produce an enlarged voltage across the resistance R.

It is important to realize that the increased power available at the output of a transistor amplifier does not come from the transistor itself. Rather, it comes from the power provided by the voltage source VC. The transistor, acting like an automatic valve, merely allows the small, weak signals from the input generator to control the power taken from the source VC and delivered to the resistance R.
Today it is possible to combine arrays of tens of thousands of transistors, diodes, resistors, and capacitors on a tiny chip of silicon that usually measures less than a centimeter on a side. These arrays are called integrated circuits (ICs) and can be designed to perform almost any desired electronic function. Integrated circuits, such as the type in Figure 23-34, have revolutionized the electronics industry and lie at the heart of computers, cellular phones, digital watches, and programmable appliances.

Figure 23-34 Integrated circuit (IC) chips are manufactured on wafers of semiconductor material. Shown here is one wafer containing many chips. Some of the so-called smart cards in which the chips are used are also shown. (Courtesy ORGA Card Systems, Inc.)

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.