Physics
PN Junction
A PN junction is a boundary or interface between a region of semiconductor material doped with positive charge carriers (P-type) and a region doped with negative charge carriers (N-type). This junction forms a diode, allowing current to flow in one direction while blocking it in the opposite direction. It is a fundamental component in electronic devices such as diodes, transistors, and solar cells.
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11 Key excerpts on "PN Junction"
eBook - ePubElectronics
Basic, Analog, and Digital with PSpice
- Nassir H. Sabah(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
3 PN Junction and Semiconductor Diodes The purpose of this chapter is to explain the behavior of various types of semiconductor diodes, based on the physical principles enunciated in Chapter 2. The PN Junction is a basic constituent of a wide variety of semiconductor devices, not only diodes but also metal–oxide–semiconductor transistors and bipolar junction transistors, covered in Chapters 5 and 6, respectively. It is highly important, therefore, to acquire a good physical understanding of the behavior of the PN Junction. The PN Junction is first considered at equilibrium, which serves as a foundation for the discussion of the PN Junction under forward or reverse bias. The behavior of the PN Junction, and some associated phenomena, are explored in considerable detail, with a strong emphasis on fundamental physical principles. Mathematical derivations and topics that are more advanced are left to the Supplementary Examples and Topics on the website. The discussion then shifts to some special types of semiconductor diodes. The electrical properties of a PN Junction can be altered through irradiation by light, as in photodiodes and photocells. The reverse process of light emission by a PN Junction occurs in light-emitting diodes (LEDs). The current–voltage characteristic of the tunnel diode has an unusual negative resistance part that is useful for some applications. Metal–semiconductor contacts are not only unavoidable in semiconductor devices, but are also the main structure in some special-purpose diodes and transistors
eBook - PDF- Stefan Linder(Author)
- 2006(Publication Date)
- EPFL PRESS(Publisher)
CHAPTER 2 THE PN-JUNCTION The pn-junction is, without doubt, the most important building block in semicon- ductor technology. Most devices are produced by special arrangements of pn-junctions. Therefore, a thorough discussion of the pn-junction's physical prop- erties is a prerequisite to understand the operation of diodes, transistors, thyris- tors, etc. pn-junctions are formed by joining p- and n-doped regions. The point of con- tact itself is referred to as the metallurgical junction. Figure 2.1 illustrates the pro- cesses occurring in silicon at a pn-junction. Initially, we assume that the p- and n-doped regions are not in contact with one another (upper part of Figure 2.1). The impurity atoms (in this case boron in the p-region, and phosphorous in the n-region) are highlighted in the diagrams. Furthermore, we assume that the doping atoms are not ionized; this means that the boron atoms in the p-region have not incorporated an electron to complete their single-electron bond, and that the excess valence elec- trons of the phosphorous atoms in the n-region have not separated from their "host" atoms. When the p- and n-regions are joined and a small amount of thermal energy is added, two processes can be observed. The first is electron diffusion, and the second is the formation of an electric field. Initially, the thermal energy ionizes the weakly bound electrons of the phos- phorous atoms. The thermal energy also initiates random movement of the free electrons. When an electron arriving in the p-region enters the field of influence of a boron atom, it is captured and used by the atom to enhance its single-electron bond to a full covalent bond. Since all boron atoms capture electrons, a net negative charge is built up in the p-region. The density of charge corresponds to the concentration of boron atoms. In the n-region, a net positive charge arises due to the missing electrons from the ion- ized phosphorous atoms.
eBook - PDFBasic Electronics
Principles and Applications
- Chinmoy Saha, Arindam Halder, Debarati Ganguly(Authors)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
Chapter 3 P-N Junction and P-N Junction Diode 3.1 P-N JUNCTION All modern semiconductor devices and integrated circuits (IC), which are the backbone of modern day electronics, contain at least one junction between p -type and n -type semiconductor, commonly known as p-n junction. This p-n junction is the most fundamental semiconductor device as all other devices like various types of transistors (Bipolar Junction Transistor, Field Effect Transistor, Metal Oxide Semiconductor Field Effect Transistor, etc.) require p-n junctions of various configurations, which will be discussed in subsequent chapters. Various analog and digital operations and performance like rectification, amplification, switching, logic operations require this fundamental semiconductor device. In this chapter, we will discuss various issues related to the p-n junction followed by application of such junction diodes, called p-n junction diodes, for various operations. In chapter 2, we have been familiar with different types of semiconductors, classified in different aspects. We recall p-and n-type semiconductor, which are realized by doping an intrinsic semiconductor with different type of impurities to obtain large number of free carriers in the form of holes in p -type semiconductor and electrons in n -type semiconductor, respectively. However, these n -type and p -type semiconductors having electrons and holes in abundance are electrically neutral, as they are neutralized by the impurity ions which have created them. To be more specific, electrons are neutralized by associated donor ions, each of which has equal (opposite in sign) amount of charge as that of each electron, and holes are neutralized by acceptors ions, each of which has equal (opposite in sign) amount of charge as that of a hole, ensuring a charge neutrality.
eBook - ePubSemiconductor Basics
A Qualitative, Non-mathematical Explanation of How Semiconductors Work and How They are Used
- George Domingo(Author)
- 2020(Publication Date)
- Wiley(Publisher)
5 The pn‐JunctionOBJECTIVES OF THIS CHAPTER
After the digression in Chapter 4 , talking about one of the applications that can be understood with only the knowledge of semiconductor materials, its electrical properties and the concept of energy bands and energy gaps, we are now ready to see how by combining two different semiconductor types, one p and the other n, one with extra free holes and the other with extra free electrons at room temperature, we can create very useful devices.In this chapter I explain how a juxtaposition of one p‐ and one n‐type semiconductor creates a pn‐junction or a semiconductor diode, a device that lets the current go in just one direction. The pn‐junction is the fundamental concept that explains all the semiconductor devices we use today.5.1 The pn‐Junction
Consider two boxes, one full of sand and the other empty, the situation I show at the top of Figure 5.1 . What happens when I bring the two boxes together and remove any barrier between them? The sand from the left‐hand box spills over to the empty box, as I show in the lower part of the figure. The sand moves to the right because of a density gradient.Three points are very important:- First, the left‐hand box has lost sand and the right‐hand box has gained some.
- Second, there is a force that limits and prevents more sand from sliding toward the empty box. If instead of sand I had used water, both boxes would reach the same level. This does not happen with sand because of friction, that is, there is a force that prevents the sand moving further to the right.
eBook - PDFJunction Transistors
The Commonwealth and International Library: Applied Electricity and Electronics
- John. J. Sparkes, P. Hammond(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
CHAPTER 2 The PN Junction. Steady-state Properties IN THE first chapter we were concerned primarily with the properties of semiconductor material which was either n type or p type throughout. In this chapter we will consider what happens when the semiconductor type changes from p type to n type part way through the crystal, to form a PN Junction. Suppose we start with two separate regions in equilibrium, a/? region and an n region separated in space from one another, as in Fig. 2.1a. The/? region contains many holes and relatively few electrons and the net positive charge of mobile carriers is exactly equalled by the negative charge of the ionized acceptors fixed in the lattice. Thus it is electrically neutral, carries no field and, of course, no current flows. A similar description applies to the n region except that the dominant mobile carriers are electrons. Now we must imagine these two regions brought together in such a way that they form a single crystal. If they were simply pressed together there would be recombination at the interface and carriers would be unable to flow through from one region to the other without recombining, and many of the important properties of the PN Junction would not emerge. We shall consider later on what happens when significant recombination does occur in the transition region but for the moment we will assume that no recombination occurs anywhere in the junction. With these two regions abutting one another there will initially be large density gradients of both electrons and holes across 35 36 JUNCTION TRANSISTORS the interface. Holes will therefore diffuse from the p region into the n region and electrons will diffuse from the n region into the/? region as indicated in Fig. 2.1b. Both these processes p-type n-type + + + + + + + ++ + I +++++ Z Z Z (a) FIG.
eBook - PDF- Greg Parker(Author)
- 2004(Publication Date)
- CRC Press(Publisher)
CHAPTER SIX p-n junction I must start this chapter with a word of warning. The p-n junction is often described as the simplest semiconductor device with which to start learning electronics. I won't quibble over the fact that simpler device structures exist (photoconductors, for example) since , structurally speaking, the p-n junction is quite simple. However, be warned. Understanding how the p-n junction actually works physically is in fact quite difficult! For those of you who have done a little electronics before, and are now using this book for your further education studies, who think you know how a p-n junction works, I can assure you, you don't. This may actually be the toughest chapter in the book, tougher even than the quantum mechanics section . It is arguably the most important device chapter in the book since the humble p-n junction takes us on to an understanding of the LED, the photodiode, the LASER and, of course , the bipolar transistor. This is therefore a fundamentally important chapter for an understanding of electronic devices. 6.1 The I>-n junction in thermal equilibrium The p-n junction is the basic element of all bipolar devices, hence its importance. Its main electrical property is that it rectifies (allows current to flow easily in one direction only) . The p-n junction is often just called a diode, but we saw the confusion this might cause in the last chapter since diode just means a two-terminal device. However, during this chapter I shall frequently use the term 'diode ', meaning the p-n junction. There are many types of p-n junction diode used for many different applications, such as the following: (a) photodiode -light sensitive diode, (b) LED -light emitting diode, (c) varactor diode -variable capacitance diode, to name just a few. As we progress we shall see how all these devices work, but for the moment we'll stick with the basic p-n junction.
eBook - PDF- Behzad Razavi(Author)
- 2021(Publication Date)
- Wiley(Publisher)
• The diffusion current density is proportional to the gradient of the carrier concentra- tion and given by J tot = q(D n dn∕dx − D p dp∕dx). • A PN Junction is a piece of semiconductor that receives n-type doping in one section and p-type doping in an adjacent section. • The PN Junction can be considered in three modes: equilibrium, reverse bias, and for- ward bias. • Upon formation of the PN Junction, sharp gradients of carrier densities across the junction result in a high current of electrons and holes. As the carriers cross, they leave ionized atoms behind, and a “depletion region” is formed. The electric field cre- ated in the depletion region eventually stops the current flow. This condition is called equilibrium. • The electric field in the depletion results in a built-in potential across the region equal to (kT∕q) ln (N A N D )∕n 2 i , typically in the range of 700 to 800 mV. • Under reverse bias, the junction carries negligible current and operates as a capacitor. The capacitance itself is a function of the voltage applied across the device. • Under forward bias, the junction carries a current that is an exponential function of the applied voltage: I S [exp(V F ∕V T ) − 1]. • Since the exponential model often makes the analysis of circuits difficult, a constant-voltage model may be used in some cases to estimate the circuit’s response with less mathematical labor. • Under a high reverse bias voltage, PN Junctions break down, conducting a very high current. Depending on the structure and doping levels of the device, “Zener” or “avalanche” breakdown may occur. Problems 57 PROBLEMS Sec. 2.1 Semiconductor Materials and Their Properties 2.1. The intrinsic carrier concentration of ger- manium (GE) is expressed as n i = 1.66 × 10 15 T 3∕2 exp −Eg 2kT cm −3 , (2.127) where Eg = 0.66 eV. (a) Calculate n i at 300 and 600 K and compare the results with those obtained in Example 2-1 for Si.
eBook - PDF- Geoff Lewis(Author)
- 2013(Publication Date)
- Newnes(Publisher)
Operated in this way, the PN Junction thus has diode characteristics which, in many cases, can conveniently be considered simply as a piecewise linear device, with a relatively low î Barrier region Figure 32.1 PN Junction. forward resistance and very high or even infinite reverse resistance. However, the total voltage/current character-istic is more precisely described by the following equations, as logarithmic: I = I 0 exp —— or Κ = In — nkT q I 0 where / = forward current, I 0 = reverse saturation current q = electron charge (1.602 χ 10 19 coulombs) k = Boltzmann's constant (1.38 χ 10 23 J/K) V = voltage across junction Τ = absolute temperature η = constant typically close to unity. These equations hold true for any PN Junction and have been shown to be accurate to within 1 % over more than eight decades of current. 32.1.1 Bi-polar transistors Figure 32.2 shows the cross-section through one type of transistor. The surface of a piece of Ν type silicon which forms the substrate is coated with a thin insulating layer of silicon dioxide (S1O2). This operation is often performed by an evaporation process at a temperature of 300 to 400°C, and at low pressure in the presence of either silicon or silicon monoxide and oxygen. c Figure 32.2 Planar bi-polar transistor. 286 Semiconductor devices and technology A small area of the S1O2 is then etched away using a photographic and masking technique to leave a window in which the Ρ type base region will be formed. When this area is exposed to an atmosphere of a Ρ type gas such as boron, the exposed region will be converted into Ρ type semiconductor. The surface is again exposed to the S1O2 process so that the insulating layer is reformed. A smaller window is then etched into this layer and the area exposed to an Ν type impurity atmosphere such as phosphorus. This converts the exposed region back into Ν type semiconductor.
eBook - PDFElectrons in Solids
An Introductory Survey
- Richard Bube(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Rectifiers The application of p-n junctions to rectification of an alternating signal follows directly from the shape of the current-voltage curve, with well-defined forward and reverse characteristics. This application is shared with Schottky-barrier-type devices. 172 10. Junctions Amplifiers The junction transistor, which is typically a p-n-p structure as illustrated in Fig. 10.8, makes possible the amplification of electric power in a small solid-state device, previously possible only with vacuum tube circuits and related techniques. The transistor is a three-connection device, with one connection to each of the p-type regions (the emitter and the collector) and the third connection to the n-type base. An applied bias voltage between the emitter and the base biases this junction in the forward direction, and an applied bias voltage between the base and the collector biases that junction in the reverse direction. Under operating conditions, the n-type base is very narrow, only a fraction of a hole diffusion length ; the collector current is nearly equal to the emitter current and both currents are controlled by the emitter-base voltage while being essentially independent of the collector-base voltage. Since the same current flows in both the low-resistance forward-biased emitter-base junction as in the high-resistance reverse-biased col-lector-base junction, the transistor amplifies the input power into the output power. The narrow width of the n-type base is a critical design parameter since the base current, which represents a loss of current for the amplification process, results from recombination of injected holes in the base region before diffusion across the base-collector junction. Photodetectors If light falls on a reverse-biased p-n junction, photons with sufficient energy create electron-hole pairs.
eBook - ePub- Pradeep Fulay, Jung-Kun Lee(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
Forward bias: A voltage applied to a p-n junction in which the positive terminal of an external voltage supply is connected to the p-side of the p-n junction. This bias can overcome the built-in electric field and cause the p-n junction to conduct.Forward current (I F ):The current in a p-n junction under a forward bias.Generation current (I 0 ):The total current due to the drift of thermally generated carriers under the influence of an electric field in the depletion region. These go from the n-side to the p-side.Hole diffusion length (L p ):The average distance a hole can diffuse before recombining with an electron.Ideal diode equation: The equation that describes the I–V characteristics of an ideal p-n junction (Equation 5.63 ):I =I 0[ exp(− 1 ])q Vk BImpact ionization: A process in which a high-energy electron collides with other electrons, breaking bonds in a material and causing one of the valence electrons to move into the conduction band, which creates a hole. This process eventually leads to an avalanche breakdown in a p-n junction.Internal electric field: The electric field developed by positively charged donor ions, which are left behind as the electrons diffuse from the n-side to the p-side, and negatively charged acceptor ions, which are left behind as the holes diffuse from the p-side to the n-side.Knee voltage: The voltage on the I–V curve for a diode, at which the forward current begins to increase exponentially. This value is very close to the built-in potential (V 0 ) for the p-n junction.One-sided p-n junction: A p-n junction in which one side is very heavily doped. This side is indicated with a + superscript (e.g., a p+ -n junction is where the p-side is very heavily doped).Poisson’s equation: An equation relating the gradient of an electric field to the charge density and the dielectric permittivity of the material (ε), given by Equation 5.30 :Reverse bias:=d 2ϕ ( x )dx 2=ρ ( x )εd E ( x )d x
eBook - PDFSilicon, From Sand to Chips, Volume 1
Microelectronic Components
- Alain Vignes(Author)
- 2024(Publication Date)
- Wiley-ISTE(Publisher)
4 The PN Diode The discovery of the rectifier effect presented by a “PN Junction” by Russell Ohlin, 1940, was the founding act of William Shockley’s 1949 invention of the bipolar transistor, a component consisting of two PN diodes placed side by side. The PN diode is a current rectifier for low and high currents up to thousands of amperes. Due to its high switching speed, it is a component of logic circuits in computers and so-called electronic telephone exchanges. The PN diode is also one of the basic structures of microelectronics components: NPN bipolar transistors, memories and solar cells. The current in a diode, made up of a current of electrons and a current of holes, is controlled by the generation/recombination of electron–hole pairs, themselves controlled by metallic impurities and lattice defects of the base material. The development of purification and crystal growth processes for germanium and then silicon, thanks to the persistence of metallurgists, ensured the development of PN diodes and bipolar transistors. The purity (10N) and crystalline perfection achieved solved the problems of uniformity and reliability. The development of the zone fusion purification process ensured germanium’s dominance in the 1950s as the base material for NPN bipolar transistors. Germanium-based PN diodes are “leaky switches”; the higher the temperature, the more leaky they become. It was this property which, under pressure from the military, led to the development of processes for the purification and crystal growth of silicon. The invention of the floating-zone process made it possible to achieve the purification and crystal growth that would eventually, albeit slowly, establish silicon as the basic material for transistors.
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