Quantum Physics

930 Chapter 28 Atomic PhysicsEmitter Base Collector–++–∆V eb+ –p––––+ +n+ ++ ++ +––––pEmitterCollectorI bI bbep n pI eI ccR(a)(b)Base+ –∆V ec(c)Figure 28.31 (a) The pnp transistor consists of an n region (base) sandwiched between twop regions (emitter and collector). (b) Circuit symbol for the pnp transistor. (c) A bias voltage V ebapplied to the base as shown produces a small base current I b that is used to control the collectorcurrent I c in a pnp transistor.Suppose a voltage is applied to the transistor so that the emitter is at a higherelectric potential than the collector. (This is accomplished with the battery labeledV ec in Figure 28.31c.) If we think of the transistor as two diodes back to back, wesee that the emitter–base junction is forward biased and the base–collector junctionis reverse biased. The emitter is heavily doped relative to the base, and as a result,nearly all the current consists of holes moving across the emitter–base junction.Most of these holes do not recombine with electrons in the base because it isvery narrow. Instead they are accelerated across the reverse-biased base–collectorjunction, producing the emitter current I e in Figure 28.31c.Although only a small percentage of holes recombine in the base, those that dolimit the emitter current to a small value because positive charge carriers accumulatingin the base prevent holes from flowing in. In order not to limit the emittercurrent, some of the positive charge on the base must be drawn off; this is accomplishedby connecting the base to the battery labeled V eb in Figure 28.31c. Thosepositive charges that are not swept across the base–collector junction leave thebase through this added pathway. This base current I b is very small, but a smallchange in it can significantly change the collector current I c . If the transistor isproperly biased, the collector (output) current is directly proportional to the base(input) current and the transistor acts as a current amplifier. This condition maybe writtenI c I bwhere , the current gain factor, is typically in the range from 10 to 100. Thus, thetransistor may be used to amplify a small signal. The small voltage to be amplifiedis placed in series with the battery V eb . The input signal produces a small variationin the base current, resulting in a large change in the collector current and hencea large change in the voltage across the output resistor.The Integrated CircuitInvented independently by Jack Kilby (b. 1923) at Texas Instruments in late 1958and by Robert Noyce at Fairchild Camera and Instrument in early 1959, the integratedcircuit has been justly called “the most remarkable technology ever to hitmankind.” Kilby’s first device is shown in Figure 28.32a. Integrated circuits have indeedstarted a “second industrial revolution” and are found at the heart of computers,watches, cameras, automobiles, aircraft, robots, space vehicles, and all sortsof communication and switching networks.In simplest terms, an integrated circuit is a collection of interconnected transistors,diodes, resistors, and capacitors fabricated on a single piece of siliconknown as a chip. State-of-the-art chips easily contain several million components in