Quantum Physics

28.14 Semiconductor Devices 929I (mA)504030IAp∆V+ –Forward bias(a)nI 0 20 mA2010–1.0 –0.5Reverse bias(b)0.5 1.0Forward biasFigure 28.28 (a) Schematic of a p–n junction under forward bias. (b) The characteristic curve for areal p–n junction.∆V (V)where q is the electron charge, k B is Boltzmann’s constant, and T is the temperaturein kelvins. Figure 28.28 shows an I–V plot characteristic of a real p–n junction,along with a schematic of such a device under forward bias.The most common use of the semiconductor diode is as a rectifier, a device thatchanges 120-V AC voltage supplied by the power company to, say the 12-V DC voltageneeded by your music keyboard. We can understand how a diode rectifies acurrent by considering Figure 28.29a, which shows a diode connected in serieswith a resistor and an AC source. Because appreciable current can pass throughthe diode in just one direction, the alternating current in the resistor is reduced tothe form shown in Figure 28.29b. The diode is said to act as a half-wave rectifier,because there is current in the circuit during only half of each cycle.Figure 28.30a shows a circuit that lowers the AC voltage to 12 V with a stepdowntransformer and then rectifies both halves of the 12-V AC. Such a rectifier iscalled a full-wave rectifier and when combined with a step-down transformer is themost common DC power supply around the home today. A capacitor added in parallelwith the load will yield an even steadier DC voltage.I(a)(b)Figure 28.29 (a) A diode in serieswith a resistor allows current to passin only one direction. (b) Thecurrent versus time for the circuitin (a).RtThe Junction TransistorThe invention of the transistor by John Bardeen (1908–1991), Walter Brattain(1902–1987), and William Shockley (1910–1989) in 1948 totally revolutionizedthe world of electronics. For this work, these three men shared a Nobel prize in1956. By 1960, the transistor had replaced the vacuum tube in many electronic applications.The advent of the transistor created a multitrillion-dollar industry thatproduced such popular devices as pocket radios, handheld calculators, computers,television receivers, and electronic games. In this section we explain how a transistoracts as an amplifier to boost the tiny voltages and currents generated in a microphoneto the ear-splitting levels required to drive a speaker.One simple form of the transistor, called the junction transistor, consists of asemiconducting material in which a very narrow n region is sandwiched betweentwo p regions. This configuration is called a pnp transistor. Another configurationis the npn transistor, which consists of a p region sandwiched between two n regions.Because the operation of the two transistors is essentially the same, we describeonly the pnp transistor. The structure of the pnp transistor, together with itscircuit symbol, is shown in Figure 28.31 (page 930). The outer regions are calledthe emitter and collector, and the narrow central region is called the base.The configuration contains two junctions: the emitter–base interface and thecollector–base interface.IABCD 1Transformer D 2(a)(b)Figure 28.30 (a) A full-wave rectifiercircuit. (b) The current versustime in the resistor R.ItR