Physics
Semiconductor Diode
A semiconductor diode is a two-terminal electronic component that allows current to flow in one direction while blocking it in the opposite direction. It is made of semiconductor materials such as silicon or germanium. When a forward voltage is applied, the diode conducts, allowing current to pass through. This property makes diodes useful for rectification and switching in electronic circuits.
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11 Key excerpts on "Semiconductor Diode"
eBook - PDFAnalog Circuit Techniques
With Digital Interfacing
- T. H. Wilmshurst(Author)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
5 Semiconductor Diode and power supplies Summary In Exercise 4.4 of the last chapter the Semiconductor Diode was found to be a device which passed current in one direction but not the other. Also • for significant current to flow in the forward direction a voltage of ≈ 600mV is required, • for applied voltages ≈ 600mV the transition from non-conduction to conduction is not quite abrupt, • as the frequency of the input signal is increased there is evidence of the diode capacitance. The first objective of this chapter is to give an under-standing of these properties. The other is to describe the way the diode is used as the rectifier in various kinds of DC power supply. The final section is mainly example calculations and detailed derivations, but with one or two experiments. 5.1 Semiconductors A semiconductor is a material with conducting properties intermediate between those of an insulator such as glass, and a conductor such as copper. The most commonly used semiconductor today is sili-con. Fig. 5.1 shows the atomic structure of a semicon-ductor. Here each atom has an outer shell with four elec-trons in it. These combine with the neighbouring electrons to constitute the complete 'covalent bonded' structure shown. The structure has zero overall charge, because the negative charge of the electrons is exactly balanced by the positive charge of the lattice of nuclei. Also the 'complete' nature of the structure means that none of the electrons is available for conduction. Thus so far the material is an insulator. Fig. 5.1 Covalent bonding of Silicon. Balls on a tray model The mathematical model describing this situation most completely is quantum theory. Another more homely model, that conforms reasonably well with the findings of the quantum theory, is the 'balls on a tray' model of Fig. 5.2. Fig. 5.2 Balls on a tray model of silicon semiconductor, locked in at absolute zero so no conduction.
- Christopher Lavers, Edmund G.R. Kraal(Authors)
- 2014(Publication Date)
- Thomas Reed(Publisher)
An amplifier using semiconductors rather than vacuum is in principle possible. William Shockley (1939) Semiconductors This chapter introduces the Semiconductor Diode , a device comparable in function to the thermionic diode in the electronic circuits as discussed in Chapter 10. Semiconductor materials have properties allowing their use for many device applications in response to temperature, pressure, stress, light, magnetic and electric fields. As such they are used frequently as sensors and transducers. The term ‘semiconductor’ includes all solids whose electrical conductivity lies between that of metals and insulators. Germanium and silicon, and gallium arsenide are perhaps the most important semiconductor materials of current working device choice, but graphene, a substance made from pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite but only one atom thick, may provide novel new commercial devices in the near future. However, before investigating the electrical properties of current materials such as germanium and silicon, it is worth recalling the history of semiconductor development in electrical work. The electrical properties of semiconductor materials will then be examined, without which diode and transistor operation will not be understood. SOLID-STATE ELECTRONICS: THE DIODE 11 ‘Solid-state’ devices, as their name might suggest, are not vacuum devices like the thermionic valve, and it is their ability to control conductivity without the need for bulky yet delicate glass bulbs that make solid-state devices so attractive for most practical purposes. The rectifying properties of the diode had been grounded in the fundamental work of the late nineteenth century onwards also providing the advent of radio through the thermionic valve and the rectifying crystal. In 1833, Michael Faraday discovered that silver sulphide had a negative temperature coeffi cient of resistance.
eBook - PDFThe Physical Basis of Electronics
An Introductory Course
- D. J. Harris, P. N. Robson, P. Hammond(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
4. The Semiconductor Junction Diode and Transistor IN ORDER to understand the operation of a transistor, it is first necessary to be familiar with the mechanism of charge transport across a junction formed between a piece of «-type semiconductor and a piece of /?-type semiconductor. Such a junction is called a p-n junction and exhibits recti-fying properties, i.e. it appears to have a low resistance to current flow in one direction and a high resistance in the other direction. A p-n junction is shown schematically in Fig. 4.1 (a). Electrical connections are made to both ends. A transistor is essentially a sandwich of two p-n junctions. A thin wafer of «-type material between sections of /?-type material constitutes a p-n-p Emitter ^ „ . γ Collector λ // y/ A . (α) Base (b) Emitter Col lector 7 Base (c) F I G . 4.1. (a) A p-n junction diode, (b) A p-n-p junction transistor, (c) A n-p-n junction transistor. 63 64 The Physical Basis of Electronics Direction of electron flow shown by arrows F I G . 4.2. Current carrier motion around a circuit containing a block of «-type semiconductor. transistor, whilst the converse holds for an n-p-n transistor. Electrical con-nections are made to all three sections as shown in Figs. 4.1 (b) and 4.1 (c). It is essential to realize that in all these three structures it is not sufficient to place the various doped pieces of semiconductor in contact with each other. The whole block constituting a p-n junction, for example, must be a single crystal of silicon or germanium, one-half of the crystal being doped with donor impurity atoms, the other half with acceptor atoms. If separate pieces of /?-and «-type material were just placed in contact, then the dis-continuity in the lattice structure at the junction would produce effects which in all probability would mask the desired ones. A transistor is one single crystal with three regions of different doping.
eBook - PDF- Marc Thompson(Author)
- 2006(Publication Date)
- Newnes(Publisher)
43 CHAPTER 3 Review of Diode Physics and the Ideal (and Later, Nonideal) Diode Current Flow in Insulators, Good Conductors and Semiconductors In nature, from the point of view of the ease of producing current flow in a material, there are three broad classes of materials: insulators, conductors and semiconductors. Semiconductors and metals can support significant current flow but the charge movement mechanisms are dif-ferent in the two types of materials. How “good” an electrical conductor is can be quantified by material property electrical resistivity and/or its inverse, electrical conductivity. Electrical resistivity 2 is a measure of how well a given material conducts current. If there are lots of free charged carriers available, a material is deemed a good conductor. In This Chapter f The basics of bipolar devices are covered, including basic semiconductor physics, 1 the concepts of electron and hole flow in semiconductors, the differences between drift and diffusion flow, generation and recombination, and the effects of semiconductor doping on carrier concentrations. We finish with a discussion of the ideal diode, and illustrate how a diode can conduct forward current, but can also block reverse voltage. Detailed mathematical derivations are avoided wherever possible. However, enough mathematical detail is given so that the reader can discern the important scaling laws and functional dependencies of the ideal diode. At the end of the chapter we’ll discuss some of the factors that result in nonideal behavior in diodes. We’ll conclude with a discussion of load lines, a useful method for solving for the operating point of circuits with nonlinear devices. The load-line technique will be useful in later chapters in analyzing transistors. 1 W e will not go into the quantum mechanics of semiconductors, which provides the rigorous analyses. The simpler models developed in this chapter hopefully will give insight into the basics of semiconductor operation.
eBook - PDF- Giovanni Saggio(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
FIGURE 8.22 Traffic sign: one way directional arrow. Principles of Analog Electronics 274 There are several types of diodes, differing in geometric scaling, doping level, and electrodes. But emphasizing different physical aspects, their v -i function can be changed accordingly. As a consequence, there are diodes with electrical behav-ior that differs from the standard design. We refer to avalanche diodes , Esaki or tunnel diodes , thermal diodes , photodiodes , point-contact diodes , PIN diodes , Schottky diodes , super barrier diodes , step-recovery diodes , varicaps or varac-tor diodes , Peltier diodes , gun diodes , IMPATT diodes , and so on. In the follow-ing, we will treat two of the most important ones: the Zener diode (Section 8.5.4) and the LED (Section 8.5.6). Observation The diode has numerous applications in electronics. We’ll detail some of the most important in the following sections. But now, let’s consider that the diode protects any electronic equipment (a radio, a recorder, etc.). In the event that the power source (battery or mains electricity) is connected the wrong way round, the diode prevents the current from flowing, so avoiding damaging or even destroying electronic devices. FIGURE 8.23 Blood circulation in the heart. The directions are controlled by the cardiac valves. Semiconductor Components 275 Another example is in our cars: the alternator charges the battery when the engine is running, but when the engine stops, a diode prevents the battery from discharging through the alternator (Figure 8.24). 8.5.1 The Bias The resistance of the depletion region can be modified by adding a fixed electric field to the built-in one. This operation is termed biasing , that is, the application of an external DC voltage, typically by means of a battery, across the junction. The added electric field can be in the same or opposite direction as the built-in electric field, determining an increase or a decrease of the resistance of the depletion region.
eBook - PDF- M. G. Scroggie, S. W. Amos(Authors)
- 2013(Publication Date)
- Newnes(Publisher)
CHAPTER 9 Diodes 9.1 Electronic Devices We now have all the essential basic principles of circuits, and it is time to strike out along a new line—electronics. Although all the electric currents we have been discussing in the last seven chapters consisted of electrons in motion there has been no real need to consider them as such. It would have been just about as easy to deal with the subject using one of the theories of the electric current in vogue before electrons were discovered. But now we are going to study electronic devices, which could not be satisfactorily explained without reference to electrons. It is these devices that are responsible for the rapid and multifarious developments included under the broad name of electronics. There are two main divisions of electronic devices: those in which the currents pass through vacuum or gas, and those in which they pass through semiconducting solids. The first includes those called valves in Britain and tubes in America, and the second includes transistors. Although some simple semiconductor devices were used, without knowing how they worked, for several decades before 1948, it was not until that year, when the transistor was invented as a result of research into the electronic nature of semiconductors, that they began to overtake thermionic valves (as they are named more fully), which are reckoned to have started in 1904. Electronic devices can be, and often are, classified according to the number of their electrodes. An electrode is not very easy to define precisely, but roughly it is any part of the device where a current leaves or enters or by means of which it is electrically controlled. Often each electrode has a terminal or wire by which connection is made to a circuit, but sometimes electrodes are connected together inside the device. 9.2 Diodes The least number of electrodes is two, and devices so constituted are called diodes.
- Dale R. Patrick, Stephen W. Fardo(Authors)
- 2020(Publication Date)
- River Publishers(Publisher)
This leakage current is primarily caused by thermally gen- erated electron-hole pairs in the depletion region. Current carriers of this type are swept across the junction by the reverse-biased electric field. An increase in junction tem- perature causes a corresponding increase electron-hole pair generation. This in turn causes an increase in leakage current. The same effect takes place when light energy is applied to a P-N junction. Light travels in small bundles of energy called photons. When photons are absorbed by the P-N junction, their energy is used to free new electron-hole pairs. Increasing the intensity of light causes a reverse-bi- ased photodiode to become more conductive. Characteristically, the reverse current of a photodi- ode increases with the intensity of light. A reverse cur- rent-light intensity graph and its test circuit are shown in Figure 3-34. The source voltage V S is adjusted to a typical 108 Electricity and Electronics Fundamentals operating value. In this case –20 V. Note particularly that an increase in illumination level causes a linear rise in re- verse current. As shown, the value of I R is limited to only a few microamperes. As a rule, its current must be ampli- fied before it can be made useful. Photodiodes are gener- ally used to drive transistor amplifiers. Phototransistors are now available that have a photodiode and a transistor combined in a single device. VARACTOR DIODES In an ordinary junction diode, the depletion region is an area which separates the P and N material on each side of the junction. This area develops when the junction Figure 3-34. Characteristic (a) reverse current light intensity graph and (b) its test circuit. is initially formed. It represents a unique part of the diode that is essentially void of current carriers. In this regard, the depletion region serves as a dielectric medium or in- sulator. When a diode has bias voltage applied, its deple- tion region will change in width.
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
C H A P T E R 1 Semiconductors and Junction Diodes I N T R O D U C T I O N The most significant development in electronics since the Second World War has been the introduction of the transistor. This minute semiconductor device can amplify, oscillate and be used for switching and other purposes. In all its applications it has an efficiency much greater than that of a thermionic valve. Moreover the transistor has a longer life than a valve, is non-microphonic and is much cheaper than a valve. It is not surprising that transistors have superseded valves in most categories of electronic equipment. They will probably supplant valves completely in all except high-power applications. Early germanium transistors had limited output power and operated satisfactorily only up to a few M H z : moreover they had appreciable leakage currents which caused difficulties at high ambient temperatures. Later developments particularly the intro-duction of the silicon epitaxial planar transistor eliminated these limitations. Modern transistors can give more than 25 W output; they operate satisfactorily up to frequencies above 1,000 MHz, and leakage currents are no longer troublesome. The characteristics are however dependent on temperature and precautions are necessary to ensure stability of operating conditions and gain. By its physical structure the bipolar transistor is a current-controlled device, that is to say its output current is linearly related to the input current. The input resistance is low (and dependent on signal amplitude) and the output resistance is high. The asso-ciated circuits must be designed to operate with such resistances 1 2 Principles of Transistor Circuits D E F I N I T I O N OF A S E M I C O N D U C T O R The heart of a transistor consists of semiconducting material, e.g. germanium or silicon and the behaviour of the transistor largely depends on the properties of this material.
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos, Mike James(Authors)
- 2013(Publication Date)
- Newnes(Publisher)
The frequency of the current pulses so generated depends on the transit time through the n-layer and hence on its thickness. If the diode is mounted in a suitably tuned cavity resonator, the current pulses cause oscillation by shock excitation and r.f. power up to 1W at frequencies between 10 and 30GHz is obtainable. Pin Diode As its name suggests, this is a junction diode with a region of intrinsic semiconductor between the n- and p-regions. When such a diode is reverse-biased the intrinsic layer is depleted of carriers and the diode behaves as a capacitor. When it is forward-biased carriers are injected into the intrinsic region to give a forward resistance which varies linearly between, say, 1 ohm and lOkilohms with the current through Semiconductors and Junction Diodes 21 the device. This property makes the diode useful as a modulator or switch in microwave systems and at frequencies between 1 MHz and 20 GHz. Light-emitting Diodes (LEDs) When a pn junction is forward-biased electrons are driven into the p-region and holes into the η-region as shown in Fig. 1.10. Some of these charge carriers combine in the junction area and in some of the combinations energy is given out in the form of light. By using an alloy of gallium, arsenic and phosphorus as the semiconducting material the emitted light can be made any colour between red and green but maximum electrical-optical efficiency is obtained when the light is red. Lamps and digital displays using this principle are known as light-emitting diodes. Such lamps typically consume 25 mA at 4 V and have a very long life. Blue-emitting LEDs use more exotic semiconductors such as silicon carbide, and are more expensive and less efficient than red and green emitters. They have forward voltages in the region of 5 V. Laser Diodes When high-energy photons pass through a material some are absorbed by atoms, which acquire a higher energy level as a result.
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
The usual way of achieving a structure of this type is by treating one end of a single crystal of n-type semiconductor Semiconductors and Junction Diodes 11 with a Group-Ill impurity so as to offset the n-type conductivity at this end and to produce p-type conductivity instead at this point. Alternatively, of course, one end of a p-type crystal could be treated with a Group-V impurity to give n-type conductivity at this end. The semiconducting device so obtained is termed a junction diode, and the non-linear conducting properties can be explained in the following way. Behaviour of a pn Junction Fig. 1.7 represents the pattern of charges in a crystal containing a pn junction. The ringed signs represent charges due to the impurity atoms and are fixed in position in the crystal lattice: the unringed signs represent the charges of the free electrons and holes (majority carriers) which are liberated by the impurities. The n-region also contains a few holes and the p-region also a few free-electrons : these are minority carriers which are liberated by thermal dissociation of the covalent bonds of the semiconducting element itself. Even when no external connections are made to the crystal, there is a tendency, due to diffusion, for the free electrons of the n-region to cross the junction into the p-region: similarly the holes in the p-region tend to diffuse into the n-region. However, the moment any of these majority carriers cross the junction, the electrical neutrality of the two regions is upset: the n-region loses electrons and gains holes, both causing it to become positively charged with respect to the p-region. Thus a potential difference is established across the junction and this discourages further majority carriers from crossing the junction: indeed only the few majority carriers with sufficient energy succeed in crossing.
eBook - PDFElectronics
A Course Book for Students
- G. H. Olsen(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
This is accompanied by a rise in current and a shift to the operating point A. The diode is then in the Off condition. A positive-going pulse will cause a rise in the voltage across the diode and the diode will revert to its original condition. The diode is then said to be 'on'. Solid-state switches of this type may be operated at the rate of 1000 million times per second. They are therefore ideal for incorporation into digital computers, and much research is being undertaken in this connection. So far, factors which need not concern us here have prevented large-scale construction of tunnel diode computers. THE JUNCTION TRANSISTOR The first transistors emerged from an American company (the Bell Telephone Laboratories) in 1948. The American physicists Bardeen and Brattain were investigating the behaviour of point-contact diodes by probing the area near to the contact point with a second metal wire. Their object was to try to improve their understanding of the fundamental physics involved. Almost by accident they discovered that when probing the base semiconductor material, the current through the probes (one emitting current into the base material and the other collecting it from the base) was not only much greater than the current through the base material from the emitter probe, but that the two currents were proportional over a useful range. Unlike a rectifier, therefore, this new arrangement could be made to increase, i.e. amplify, currents. Prior to 1948 the only electronic amplifying devices known were thermionic triodes and pentodes, along with their variants. It is difficult to find any other invention that surpasses in importance the transistor; this device has revolutionised the whole of technology during the past twenty-five years. It was found that the point-contact transistor was not easy to reproduce in large numbers by mass production techniques.
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