Understanding Automotive Electronics 8th - Chapter 2

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This chapter is for the reader who has limited knowledge of electronics. It is intended to provide an overview of the subject so that discussions in later chapters about the operation and use of automotive electronics control systems will be easier to understand. The chapter discusses electronic dev
SEMICONDUCTOR DEVICES 25 silicon. When boron is used, the semiconductor material becomes a so-called p-type semiconductor When phosphorus is used, the semiconductor material becomes an n-type semiconductor In order to understand the operation of these transistors and diodes, it is helpful to understand the basic physical mechanism of electric conductivity in both n-type and p-type semiconductor materials The flow of an electric current through any material is due to the motion of electrons in the material in response to an applied electric field. This electric field results from the application of a voltage at the external terminals of the corresponding structure The variable called an electric field in this chapter is a component of the general theory that is known as " electromagnetic field theory. This theory forms the basis of modeling all electric phenomena. This electric field is represented by a vector that is known as electric field intensity and denoted as e in this book. Although the advanced details of electromagnetic field theory are beyond the scope of this book, somewhat simplified theoretical models are presented in later chapters(e. g., Chapter 5). For the purpose of explaining electric properties of semiconductor ma terials, we present the simplest model of electric field intensity in which the magnitude varies in pro portion to applied voltage and inversely with the distance between the electrodes to which the voltage is applied. The electrons that move in response to this electric field originate from the individual atoms that make up the material or a basic understanding of conductivity, it is helpful to refer to Fig. 2. 1 that depicts a relatively long, thin slab of semiconductor material across which a voltage is applied In this figure, the electric field intensity is a vector denoted as e that is x-directed. In this book vectors are indicated by a bar over the symbol for the vector as exemplified by the electric field inten sity e. a voltage v is applied to a pair of conducting(e. g, Cu)electrodes. For this relatively long, thin semiconductor material, the magnitude of the electric field intensity E is approximately constant over the semiconductor and is given approximately by E The vector e is given bv E=Ex where x=unit vector in the x direction E J Electrode Electrode FIG. 2. 1 Illustration of current conduction in semiconductor 26 CHAPTER 2 ELECTRONIC FUNDAMENTALS Also shown in Fig. 2. 1 is the current density vector which is also an x-directed vector The current density vector is proportional to the electric field intensity 丿=cE E where o is the conductivity of the material The magnitude of the current density is the current per unit cross-sectional area(which by the assumption of essentially constant E is constant) and is given by (2.2) Ac where Ac is the cross-Sectional area of the slab in the y, z plane. The reciprocal of conductivity is known as the resistivity p of the material (2.3) The explanation of electron flow in any material is based upon the " band theory of electrons This theory is a major component in modern atomic physics. According to this theory, the energy of the electrons associated with the atoms making up a material is constrained to certain ranges called bands Any given electron will have an energy within one of these bands, and no electron can have energy outside these bands. Within each band, the electrons can have only discrete energy levels, and only one electron can"occupy"a given energy level Consequently the number of electrons within each band for any atom is constrained to the number of"allowed""energy levels. An electron can only move in response to an applied electric field and contribute to current flow if there is an unoccupied energy level to which it can move as its energy changes due to the electric field intensity force acting All of the energy levels of the lower energy bands of an atom are filled such that there is no energy level to which an electron can move in response to an applied electric field. Thus, these lower band electrons cannot contribute to current flow in response to an applied voltage. The electrons in the out ermost band, known as the conduction band, are the least tightly bound and for a material such as si they are few in number relative to the number of energy levels in that band. These outer band electrons can move to an adjacent energy level and effectively move freely in response to an applied electric field. These electrons are called"free electrons. Doping Si with phosphorus impurity results in an excess of free electrons relative to pure Si. The doped material is said to be an"n-type? semiconductor and has a conductivity that is greater than the undoped Si The next lowest energy band from the outermost is called the "valence band" since it is associated with the chemical valence of the material (in this case Si. The energy levels of this band are nearly(but not completely) filled. However, doping a semiconductor with a p-type impurity(e. g, doping Si with boron) yields a relative excess of energy levels in this valence band. The resulting doped material is called a p-type semiconductor. Electrons in this band can move to the available energy levels created by doping in response to an electric field, thereby contributing to current flow. However, functionally, this p-type material beha aves as though it had excess of positively charged particles called" holes. The model for current flow in a semiconductor and the explanation of semiconductor devices use the fic itious holes and their response to an applied field as a basis for the contribution they make to current flow. The terminology used to describe these charge carriers is as follows: in n-type material electrons are called "majority carriers"and holes called"minority carriers"; the reverse is true in p-type material SEMICONDUCTOR DEVICES 27 Doping a semiconductor material changes the relative densities of holes and electrons However there is a basic relationship between these densities, which is preserved regardless of the doping con centrations. If one starts with an intrinsic semiconductor such as si that has an equal concentration of free"'electrons and holes (since each free electron leaves a hole" in the valence band for an intrinsic semiconductor), we denote this concentration n; =1. 5X 10 10 Doping Si with either a p-type or an n-type impurity changes the concentrations. Denoting electron density n, and hole density p, the following equation expresses the relationship between these concen trations under thermal equilibrium np (24) There is another basic aspect of semiconductor physics that plays a role in the electric characteristics of semiconductor electronic components. Whenever a voltage V is applied to a slab of semiconductor material, it creates an electric field that is represented by the electric field intensity vector E as de scribed above (in this text, the over bar for a variable is the notation indicating that the variable is a vector) In a semiconductor material, any electric field due to an external potential causes the electrons and holes to move with mean velocity vectors ve and Wh, respectively. These velocities are given by Wh=PhE where He is the electron drift mobility and uh is the hole drift mobility These mean velocities yield electron and hole current densities e and Jh, respectively: Jn=pgh where q is the charge on an electron(1.6X 10 coulomb). These relationships will appear in models for various components in this text Throughout this book, current flow is taken to be conventional current in which the direction of flow Is from positive to negative, whereas in reality, current consists of electron motion from negative to positive. This choice of current is merely convenient for notational purposes and has no effect on the validity of any circuit analysis or design DIODES The first electronic component to be considered is a device called a " diode a diode is a two-terminal electric device having one electrode that is called the anode (a p-type semiconductor)and another that is called the cathode(an n-type semiconductor ). A solid-state diode is formed by the junction between the anode and the cathode. In practice, a p-n junction is formed by diffusing p-type impurities on one of the intended junction and n-type impurities on the other side of a region of an intrinsic semi conductor (e.g, Si) The region in which the diode material changes from p-type to n-type material is called the p-n junction(or simply junction). The junction region is relatively short but plays a critical role in the diode operation When the junction is formed electrons in the vicinity of the junction migrate from the n-type to the p-type. Similarly, holes in the region migrate from p-type to n-type. This migration leaves behind 28 CHAPTER 2 ELECTRONIC FUNDAMENTALS a positively charged dopant ion on the n-side and a negatively charged dopant ion on the p-side over a region known as the depletion region that creates a charge distribution that in turn creates a potential difference between the two regions. In equilibrium conditions, this potential inhibits further current flow. This potential is known as the junction barrier potential since it acts more or less like a barrier to the current flow. a detailed model for the relationship between the charge distribution in the deple tion region and the potential (or equivalent voltage) is presented in the section of Chapter 5 on capacitor modeling. However, for the present discussion, it is only the existence of the barrier potential that is required for the operation of semiconductor device The current, which flows through the diode in response to an applied voltage, depends upon the polarity of the voltage and its magnitude. Fig. 2. 2 illustrates the schematic symbol for a p-n diode show ing the p-type(anode) and n-type(cathode) sides of the junction. If a voltage is applied with positive on the anode and negative on the cathode, it is said to be"forward biased. For the opposite polarity, the diode is said to be " reverse biased Forward bias reduces the junction barrier potential, thereby in creasing current flow. Reverse bias increases that potential, thereby inhibiting current flow The current through a forward-biased diode increases exponentially with applied voltage V, whereas the reverse-biased flow reaches a very low saturation current Is. A model for this current flow is Is(exp(v/nvr)-1) (2.5 where Is and n are parameters that are specific to a particular diode. The parameter Vr is called the thermal voltage and is given by where k is the boltzmann's constant, Tis the junction absolute temperature, and g is the electron charge At room temperature, Vr226 mv The parameter n is normally between 1 and 2, and Is is a few u amp Fig. 2.3 depicts this current flow vs diode junction applied voltage V. The reverse-bias current is too small to be shown Conventional current Anode P N Cathod FIG 2.2 Schematic symbol for p-n diode. SEMICONDUCTOR DEVICES 29 Ideal Actual 54 E42 18 6 0 0.2 0.6 1.4 1.8 Diode voltage (V) FIG. 2.3 Transfer characteristic. Diode transfer characteristics Although the model given above for diode voltage current characteristics is a very good represen- tation for a practical diode(provided that the reverse-bias voltage is below its breakdown voltage ), it is generally not necessary to represent the diode with this degree of accuracy for most circuit analysis or design purposes. Normally, it is sufficient for the voltage levels involved in automotive electronics to represent a p-n diode as a polarity-dependent switch as characteristic in associated figures. The switch can be modeled as being open for reverse bias and closed for forward bias. With this model, the diode current in the forward bias is limited by the external circuit components to which it is connected. The reverse-bias current is taken to be zero ZENER DIODE A special p-n junction diode having a unique reverse-bias characteristic called a zener diode has many applications in electronic circuits. The transfer characteristics for a zener diode are depicted in Fig. 2.4A The circuit symbol for a zener diode is depicted in Fig. 2. 4B in which the cathode has a unique shape The forward-bias characteristics are similar to any p-n junction diode. The reverse-bias current is extremely low for voltages-V2<VD<O. However, when the reverse voltage reaches -Vz, the current increases abruptly, while the voltage remains nearly constant at VD=-vz. The voltage V2 is called the zener voltage. The operation of any p-n junction diode at specific reverse-bias voltages(called avalanche voltages for an ordinary diode) abruptly increases. At this voltage level, the energy of charge carriers is suffi- cient to cause further ionization due to collisions that create more carriers and increase the reverse current by a large amount For an ordinary diode, the avalanche conduction causes sufficient heating to destroy the diode However, a zener diode is created with a special doping profile particularly in the vicinity of the elec trodes. The zener diode can sustain relatively large reverse bias currents while maintaining VD-V2 There are many applications in electronic circuits that require a fixed voltage level over a wide range of currents. These applications are discussed at various places in example circuits in later chapters 30 CHAPTER 2 ELECTRONIC FUNDAMENTALS (A) (B FIG. 2. 4 Zener diode transfer characteristics(a)and circuit symbol for zener diodes b) ELECTRO OPTICS Another important aspect of semiconductor materials and p-n diode junctions is the interaction between the material and optical power (light). Light is the propagation of energy in the form of quantum units called photons. Any given photon has a specific frequency of oscillation (). The speed of propagation of light depends upon the medium in which propagation occurs and the frequency of oscillation. In a vacuum, this speed (denoted as co) is essentially 3 x 10 m/s. In any other medium, c=Co/n where n=index of refraction for the material (n> 1). Light from an incandescent(high temperature)source contains a broad spectral distribution of frequencies that is characterized statistically by its power spec tral density. Since light propagates as an electromagnetic wave, the power spectral density can also be expressed as a function of wave length 1 where c=Co/n Thermally generated light has a spectrum that is spread over a relatively broad range of wave lengths On the other hand, light generated by a laser occupies a relatively narrow spectrum (ideally but not practically) at a single wavelength. Laser generated light is commonly used in vehicle clectronics Light that is incident on the surface of a semiconductor material is partially reflected and partially absorbed by the material. The relationship between incident, reflected, and absorbed light is a function of the type of material and the spectral distribution of the incident light. Any incident photon crossing ELECTRO OPTICS 31 the semiconductor material boundary can interact with various atoms that make up the material struc ture. For a photon to be absorbed by the semiconductor, the photon energy Ep must match the valence/ conduction band-gap energy such that the photon energy ionizes the atom, thereby creating a hole electron pair. This ionization process(called photoionization) causes the electric conductivity of the semiconductor material to increase in proportion to the absorbed light PHOTO CONDUCTOR A specially designed semiconductor structure that absorbs incident light as represented by the so-called light intensity (which is the optical power per unit of cross-sectional area) is termed a photoconductor. A simplified model for a photoconductor is based on the relatively simple configuration that is depicted in Fig. 2.5 The simplified configuration of the illustrative photoconductor consists of a slab of semiconductor material having a rectangular cross section of area Ac and of uniform thickness e with a pair of conducting electrodes e, and e2 attached at the ends as shown. The incident light is represented by optical intensity /. The electric conductivity o of the semiconductor is assumed to be uniform over the semiconductor and is given by D=KnI A voltage V is applied to the electrodes creating an electric field e within the material. In this simplified model, this electric field is assumed to be uniform over the material and given by where 2= unit vector in +z direction FIG. 2.5 Photoconductor configuration 32 CHAPTER 2 ELECTRONIC FUNDAMENTALS The current density vector is given by J=0(1)E /knl The total current flowing through the photoconductor i is given by J·ndls S=cross section of the material n= unit vector normal to s Since the vector E is assumed to be uniform over S, the current is given by A knva where A= cross-sectional area of photo conductor The conductance of the photo conductor G that is the reciprocal of its resistance is given by A G=kn-El That is, the conductance varies linearly(for the illustrative simplified example) with incident light in tensity. In effect, the photoconductor is a sensor for incident light intensity PHOTO DIODE In addition to photoconductive optical sensors, it is possible to fabricate a p-n junction optical sensor called a photodiode. As in the case of the photoconductor, the simplified physical configuration of a photodiode is somewhat similar to Fig. 2. 5 except that the region near eI is doped such that it is an n region and the region near e2 is doped to be p. In such a structure, the depletion region interacts with incident light in the same way as a photoconductor material such that photoionization creates hole electron pairs. The photodiode must be fabricated such that the depletion region is exposed to the in- cident light by having a transparent cover In light detecting applications, the photodiode is reverse biased such as is depicted for the circuit of Fig. 2.5 in which e1 is the cathode and e2 is the anode of the doped semiconductor slab In the absence of illumination of the p-n photodiode, the reverse-bias current is extremely small (ideally zero). Assuming that the incident light spectrum corresponds to the interband energy of the atoms in the depletion region and that a substantial portion of the incident light crosses the boundary of the semiconductor material

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