Solid-State Electronics Chapter 29

Solid-State Electronics Chapter 29 In this chapter you will: Distinguish among electric conductors, semiconductors, and insulators. Examine how pure semiconductors are modified to produce desired electric properties. Compare diodes and transistors.

Solid-State Electronics Chapter 29: Solid-State Electronics Section 29.1: Conduction in Solids Section 29.2: Electronic Devices Chapter 29

Conduction in Solids Describe electron motion in conductors and semiconductors. Compare and contrast n-type and p-type semiconductors. In this section you will: Section 29.1

Conduction in Solids All electronic devices own their origins to the vacuum tubes of the early 1900s. In vacuum tubes, electron beams flow through space to amplify and control faint electric signals. Vacuum tubes are big, require lots of electric power, and generate considerable heat. They have heated filaments, which require the replacement of the tubes after one to five years. Conduction in Solids Section 29.1

Conduction in Solids In the late 1940s, solid-state devices were invented that could do the jobs of vacuum tubes. These devices are made of materials, such as silicon and germanium, known as semiconductors. Conduction in Solids Section 29.1 The devices amplify and control very weak electric signals through the movement of electrons within a tiny crystalline space.

Conduction in Solids Because very few electrons flow in them and they have no filaments, devices made from semiconductors operate with a low power input. They are very small, do not generate much heat, and are inexpensive to manufacture. The estimated useful life of these devices is 20 years or more. Conduction in Solids Section 29.1

An atom from which an electron has broken free is said to contain a hole. A hole is an empty energy level in the valence band. The atom now has a net positive charge. An electron from the conduction band can jump into this hole and become bound to an atom once again. When a hole and a free electron recombine, their opposite charges neutralize each other. Conduction in Solids Semiconductors Section 29.1

Conduction in Solids The electron, however, has left behind a hole at its previous location. Thus, as in a game of musical chairs, the negatively charged, free electrons move in one direction, and the positively charged holes move in the opposite direction. Pure semiconductors that conduct as a result of thermally freed electrons and holes are called intrinsic semiconductors. Semiconductors Section 29.1 Because so few electrons or holes are available to carry charge, conduction in intrinsic semiconductors is very low, making their resistances very high.

Conduction in Solids The conductivity of intrinsic semiconductors must be increased greatly to make practical devices. Dopants are electron donor or acceptor atoms that can be added in low concentrations to intrinsic semiconductors. Doped Semiconductors Section 29.1 Dopants increase conductivity by making extra electrons or holes available. The doped semiconductors are known as extrinsic semiconductors.

If an electron donor with five valence electrons, such as arsenic (As), is used as a dopant for silicon, the product is called an n- type semiconductor. The adjoining figure shows a location in the silicon crystal where a dopant atom has replaced one of the silicon atoms. Four of the five As valence atoms bind to neighboring silicon. Conduction in Solids n-type Semiconductors Section 29.1

The fifth electron is called the donor electron. The energy of this donor electron is so close to the conduction band that thermal energy can easily move the electron from the dopant atom into the conduction band, as shown in the figure. Conduction in n-type semiconductors is increased by the availability of these extra donor electrons to the conduction band. Conduction in Solids n-type Semiconductors Section 29.1

If an electron acceptor with three valence electrons, such as gallium (Ga), is used as a dopant for silicon, the product is called a p-type semiconductor. When a gallium atom replaces a silicon atom, one binding electron is missing, creating a hole in the silicon crystal, as shown in the adjoining figure. Electrons in the conduction band can easily drop into these holes, creating new holes. Conduction in Solids p-type Semiconductors Section 29.1

Conduction in Solids The electric conductivity of intrinsic and extrinsic semiconductors is sensitive to both temperature and light. An increase in temperature of a semiconductor allows more electrons to reach the conduction band, and conductivity increases and resistance decreases. One semiconductor device, the thermistor, is designed so that its resistance depends very strongly on temperature. The thermistor can be used as a sensitive thermometer and to compensate for temperature variations of other components in an electric circuit. Thermistors also can be used to detect radio waves, infrared radiation, and other forms of radiation. Thermistors Section 29.1

Conduction in Solids Other useful applications of semiconductors depend on their light sensitivity. When light falls on a semiconductor, the light can excite electrons from the valence band to the conduction band in the same way that other energy sources excite atoms. Thus, the resistance decreases as the light intensity increases. Extrinsic semiconductors can be tailored to respond to specific wavelengths of light. These include the infrared and visible regions of the spectrum. Light Meters Section 29.1

Electronic Devices Describe how diodes limit current to motion in only one direction. Explain how a transistor can amplify or increase voltage changes. In this section you will: Section 29.2

Electronic Devices Section 29.2 Click image to view movie. Diodes

Electronic Devices A Diode in a Simple Circuit A silicon diode, with I/V characteristics like those shown below, is connected to a power supply through a 470-Ω resistor. The power supply forward-biases the diode, and its voltage is adjusted until the diode current is 12 mA. What is the power supply voltage? Section 29.2

Step 1: Analyze and Sketch the Problem Electronic Devices Section 29.2 A Diode in a Simple Circuit

Draw a circuit diagram connecting a diode, a 470-Ω resistor, and a power supply. Indicate the direction of current. Electronic Devices Section 29.2 A Diode in a Simple Circuit

Identify the known and unknown variables. Section 29.2 Known: I = A V d = 0.70 V R = 470 Ω Unknown: V b = ? Electronic Devices A Diode in a Simple Circuit

Step 2: Solve for the Unknown Electronic Devices Section 29.2 A Diode in a Simple Circuit

The voltage drop across the resistor is known from V = IR, and the power supply voltage is the sum of the resistor and the diode voltage drops. Section 29.2 Electronic Devices A Diode in a Simple Circuit

Substitute I = A, R = 470 Ω, V d = 0.70 V Section 29.2 Electronic Devices A Diode in a Simple Circuit

Step 3: Evaluate the Answer Electronic Devices Section 29.2 A Diode in a Simple Circuit

Electronic Devices Are the units correct? The power supply’s potential difference is in volts. Is the magnitude realistic? It is in accord with the current and the resistance. Section 29.2 A Diode in a Simple Circuit

The steps covered were: Electronic Devices Step 1: Analyze and Sketch the Problem –Draw a circuit diagram connecting a diode, a 470-Ω resistor, and a power supply. Indicate the direction of current. Step 2: Solve for the Unknown Step 3: Evaluate the Answer Section 29.2 A Diode in a Simple Circuit

Electronic Devices Diodes made from combinations of gallium and aluminum with arsenic and phosphorus emit light when they are forward- biased. When electrons reach the holes in the junction, they recombine and release the excess energy at the wavelengths of light. These diodes are called light-emitting diodes, or LEDs. Some LEDs are configured to emit a narrow beam of coherent, monochromatic laser light. Such diode lasers are compact, powerful light sources. Light-emitting Diodes Section 29.2

They are used in CD players, laser pointers, and supermarket bar-code scanners, as shown in the adjoining figure. Diodes can detect light as well as emit it. Light falling on the junction of a reverse- biased pn-junction diode creates electrons and holes, resulting in a current that depends on the light intensity. Section 29.2 Electronic Devices Light-emitting Diodes

Electronic Devices Section 29.2 Click image to view movie. Transistors and Integrated Circuits

Electronic Devices The current gain from the base circuit to the collector circuit is a useful indicator of the performance of a transistor. Although the base current is quite small, it is dependent on the base-emitter voltage that is controlling the collector current. Current Gain Section 29.2

Electronic Devices For example, if V B in the figure below is removed, the collector current will drop to zero. If V B is increased, the base current, I B, increases. The collector current, I C, will also increase, but many times more (perhaps 100 times or so). The current gain from the base to the collector ranges from 50 to 300 for general-purpose transistors. Current Gain Section 29.2

Electronic Devices In a tape player, the small voltage variations from the voltage induced in a coil by magnetized regions on the tape are amplified to move the speaker coil. In computers, small currents in the base-emitter circuits can turn on or turn off large currents in the collector-emitter circuits. In addition, several transistors can be connected together to perform logic operations or to add numbers together. In these cases, they act as fast switches rather than as amplifiers. Current Gain Section 29.2

Electronic Devices An integrated circuit, called a microchip, consists of thousands of transistors, diodes, resistors, and conductors, each less than a micrometer across. Microchips Section 29.2 All these components can be made by doping silicon with donor or acceptor atoms. A microchip begins as an extremely pure single crystal of silicon, 10–30 cm in diameter and 1–2 m long. The silicon is sliced by a diamond-coated saw into wafers less than 1-mm thick. The circuit is then built layer by layer on the surface of this wafer.

Electronic Devices By a photographic process, most of the wafer’s surface is covered by a protective layer, with a pattern of selected areas left uncovered so that they can be doped appropriately. The wafer is then placed in a vacuum chamber. Vapors of a dopant such as arsenic enter the machine, doping the wafer in the unprotected regions. Microchips Section 29.2

Electronic Devices By controlling the amount of exposure, the engineer can control the conductivity of the exposed regions of the chip. This process creates resistors, as well as one of the two layers of a diode or one of the three layers of a transistor. The protective layer is removed, and another one with a different pattern of exposed areas is applied. Microchips Section 29.2

Electronic Devices Then the wafer is exposed to another dopant, often gallium, producing pn-junctions. If a third layer is added, npn- transistors can be formed. The wafer also may be exposed to oxygen to produce areas of silicon dioxide insulation. A layer exposed to aluminum vapors can produce a pattern of thin conducting pathways among the resistors, diodes, and transistors. Microchips Section 29.2

Thousands of identical circuits, usually called chips, are produced at one time on a single wafer. The chips are then tested, sliced apart, and mounted in a carrier; wires are attached to the contacts; and the final assembly is then sealed into a protective plastic body. Section 29.2 Electronic Devices Microchips

The tiny size of microchips, shown below, allows the placement of complicated circuits in a small space. Because electronic signals need only travel tiny distances, this miniaturization has increased the speed of computers. Chips now are used in appliances and automobiles as well as in computers. Section 29.2 Electronic Devices Microchips

Semiconductor electronics requires that physicists, chemists, and engineers work together. Physicists contribute their understanding of the motion of electrons and holes in semiconductors. Physicists and chemists together add precisely controlled amounts of dopants to extremely pure silicon. Engineers develop the means of mass-producing chips containing thousands of miniaturized diodes and transistors. Together, their efforts have brought our world into this electronic age. Section 29.2 Electronic Devices Microchips

Section Check Which of the following statements about the resistance of a pn- junction diode is true when it is forward-biased and reverse-biased? Question 1 Section 29.2 A. A diode acts as a high value resistor when reverse-biased and a low value resistor when forward-biased. B. A diode acts as a low value resistor when reverse-biased and a high value resistor when forward-biased. C. A diode’s resistance is not affected by forward or reverse biasing. D. Resistance of a diode is zero, when it is reverse-biased and has a high value when it is forward-biased.

Section Check Answer: A Answer 1 Section 29.2 Reason: When the diode is reverse-biased, almost no current passes through the diode; it acts like a very large resistor, almost an insulator. When the diode is forward-biased, it acts like a low value resistor and current passes through the diode.

Section Check Explain how a small change in the base voltage produces a large change in the collector voltage in an npn-transistor. Question 2 Section 29.2

Section Check In a npn-transistor, the two pn-junctions in the transistor can be thought of as initially forming two back-to-back diodes. The battery across collector and emitter, V C, keeps the collector more positive than the emitter. The base-collector diode is reverse-biased, with a wide depletion layer, so there is no current from the collector to the base. When the battery, V B, is connected, the base is more positive than the emitter. That makes the base-emitter diode forward-biased, allowing current I B from the base to the emitter. The very thin base region is part of both diodes in the transistor. The charges injected by I B reduce the reverse bias of the base-collector diode, permitting charge to flow from the collector to the emitter. A small change in I B thus produces a large change in I C. The collector current causes a voltage drop across resistor R C. Small changes in the voltage, V B, applied to the base produce large changes in the collector current and thus changes in the voltage drop across R C. As a result, the transistor amplifies small voltage changes into much larger changes. Answer 2 Section 29.2

Section Check Which of the following circuits illustrates the operation of a pnp- transistor? Question 3 Section 29.2 A. B.

Section Check Which of the following circuits illustrates the operation of a pnp- transistor? Question 3 Section 29.2 C. D. Continued

Section Check Answer: C Answer 3 Section 29.2 Reason: In a transistor, the base-collector diode is reverse-biased with a wide depletion layer, so there is no current from the collector to the base and the base-emitter diode is forward- biased allowing current to flow from the base to the emitter. The very thin base region is part of both the diodes in the transistor. The charges injected by I B reduce the reverse bias of the base-collector diode, permitting charge to flow from the collector to the emitter. A small change in I B thus produces a large change in I C. Only in option (C), the base-collector is reverse-biased and the base-emitter diode is forward-biased.

End of Chapter Solid-State Electronics Chapter 29