Instead of having discrete energies as in the case of free atoms, the available energy states form bands. It is no understatement to say that the advent of quantum theory changed the world, for it made possible modern solidstate electronics. These semiconductors are classified as intrinsic and extrinsic based on the number of carriers. Crucial to the conduction process is whether or not there are electrons in the conduction band. We will discuss how doping semiconductors allows us to create a separation of charge, a voltage, within semiconductors and.
If the band gap is sufficiently small to allow electrons in the filled band below it to jump into the upper empty band by thermal excitation, the solid is known as a semiconductor. But when it overlaps on the valence band electrons can move freely into it. But first i would like to revise what basically semiconductors are. Conduction in semiconductors electrical resistivity and. It is the ability to control conductivity that make semiconductors useful as currentvoltage control elements. The lower bands are filled and higher bands are empty in a semiconductor. Electron and hole transport in semiconductors in this lecture you will learn.
Conductivity can be varied by several orders of magnitude. Semiconductors types, examples, properties, application. Semiconductors may cross this conduction band when they are excited. Semiconductors the electrons in a filled band cannot contribute to conduction, because with reasonable e fields they cannot be promoted to a higher kinetic energy. Conduction in semico drift of electrons and holes in an. The total current is the sum of the electron current i e due to thermally generated electrons and the hole current i h. While the subject of quantum theory can be arcane, certain aspects of it are nevertheless essential to understanding electrical conduction in semiconductors i cringe every time i read an introductory electronics textbook discuss. What carriers participate in conduction in semiconductors. Currentvoltage control is the key to switches digital logic including microprocessors etc, amplifiers, leds, lasers, photodetectors, etc. Fermidirac distribution the probability that a particular energy state. Semiconductor, any of a class of crystalline solids intermediate in electrical conductivity between a conductor and an insulator. Electrons at the bottom of the conduction band in a semiconductor obey the. Conduction band valence band valence electron leaping into conduction band.
Evaluate the conductivity by assuming that ve is the average velocity. In a metal conductor, it is the metal atoms that provide the electrons. Conduction in semiconductors electrical resistivity. Semiconductors diodes transistors phet interactive. Conduction in semiconductors engineering libretexts. While the subject of quantum theory can be arcane, certain aspects of it are nevertheless essential to understanding electrical conduction in semiconductors i cringe every time i read an introductory electronics textbook discuss electrons orbiting. Conduction occurs at higher temperature because the. Electrical conduction in semiconductors this worksheet and. Semiconductors semiconductors are those substances whose conductivity lies in between that of a conductor and insulator. The top or outermost band in the diagram is called the conduction band. In intrinsic semiconductors, current flows due to the motion of free electrons as well as holes.
Band theory of semiconductors engineering libretexts. Lecture 1 introduction to semiconductors and semiconductor. The role of the s quantum number will be considered subsequently. Semiconductors are semigood electrical conductors because although their valence band is completely filled, the energy gap between the valance band and the conduction band is not too large. The valence electrons which serve as charge carriers are located in the valence band, in the ground state the conduction band is. Metals, semiconductors, and insulators metals have free electrons and partially filled valence bands, therefore they are highly conductive a. Conduction in semiconductors free download as powerpoint presentation. In order to understand the operation of these devices, the basic mechanism of how currents. In semiconductor electronics, we are concerned mostly in the valence and conduction bands. Concentration of conduction electrons p in thermal equilibrium.
Sample learning goals recognize that the battery is the driving force in a circuit. Hence some electrons can bridge it to become charge carriers. A unique feature of the semiconductors is that they are bipolar in nature and in them, the current is transported by the electrons and holes. Just as there is no motion of liquid in a totally filled or. The semiconductors have the conductivity which is between the conductors and the insulators. A solid substance can conduct electric current only if it contains charged particles, electrons, which are free to move about and not attached to atoms. As the name suggests, semiconductor is a material which cannot conduct completely, instead its conduction ability lies between that of an insulator and a conductor. At higher temperatures, however, some electrons can be thermallypromoted into the conduction band. Description dope the semiconductor to create a diode. According to the band theory, semiconductors will actually act as insulators at absolute zero.
However at room temperature 300 o k, some of the covalent bonds in the crystal are broken due to available energy and this phenomenon makes the availability of free electrons in the crystal and hence conduction of semiconductor may be some extent possible at room temperature. Conductivity is the inverse of resistivity, 1 sm oct 12, 2016. If an electron has an energy level, which lies within this band, and is comparatively free to move around in the crystal, then it conducts electric current. Semiconductors are classified by the fully occupied valence band and unoccupied conduction band.
The difference between a semiconductors and an insulator is the magnitude of the energy gap. The pblock octet semiconductors are by far the most studied and important for technological applications, and are the ones that we will discuss in detail zincblende and wurtzitestructure semiconductors have 8 valence electrons per 2 atoms. Conduction in semico conduction in intrinsic semiconductors. Semiconductor, as the name suggests is a kind of material whose shows properties of both conductors and insulators.
Atomic orbitals although orbitals are defined mathematically over all space, one can visualize a. This filled band, however, overlaps with the next higher band, therefore they are conductive but with slightly higher resistivity than normal. The formation of a pn semiconductor junction is described and its conduction properties are discussed. Questions you should be able to answer by the end of todays lecture. Intrinsic semiconductor and extrinsic semiconductor. A degenerate semiconductor is a semiconductor with such a high level of doping that the material starts to act more like a metal than as a semiconductor at moderate doping levels the dopant atoms create individual doping levels that can often be considered as localized states that can donate electrons or holes by thermal promotion or an optical transition to the conduction or. Conductive behaviors in materials, defined by the parameter conductivity, are a primary factor in the development. In perfectly pure intrinsic semiconductors, the only way charge carriers can exist is for valence electrons to leap into the conduction band with the application of su. Above this temperature and yet still staying below the melting point of the solid, the metal would act as a semiconductor. Request pdf parallel conduction in semiconductors electronic transport characterization in semiconductor materials is becoming increasingly difficult as. Discuss insulators, conductors, and semiconductors and how they differ. Semiconductors are employed in the manufacture of various kinds of electronic devices, including diodes, transistors, and integrated circuits.
Some electrons elevated to conduction band by thermal energy. The schema consists of two energy bands valence and conduction band and the band gap. Extrinsic semiconductors are used extensively due to the ability to precisely tailor their conductivity by adding the impurities. Lecture 3 electron and hole transport in semiconductors. Semiconductors band gaps, colors, conductivity and. When a potential difference is placed across a substance, the resulting. Semiconductors ppt and pdf report free study mafia. In contrast to metals, whose electrical conductivity decreases with temperature the more intense lattice vibrations interfere with the transfer of momentum by the electron fluid, the. Semiconductors containing many mobile electrons and few holes are called ntype semiconductors because electrons carry negative n charge. Band theory of solids a useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Electrical conduction in solids explained in terms of these energy bands and forbidden gaps is called the band theory of solids. Conduction occurs at higher temperature because the electrons surrounding the semiconductor atoms can break away from their covalent bond and move freely about the lattice the conductive property of semiconductors forms the basis for understanding how we can use these materials in electrical devices.
Current flow in semiconductors university of north. Compare the ease of forming free conductionband electrons in a semiconductor material having lots of donor electrons, against that of an intrinsic pure. The electronic band structure is an energy schema to describe the conductivity of conductors, insulators, and semiconductors. Such devices have found wide application because of their compactness, reliability, power efficiency, and low cost. In an intrinsic semiconductor, n e n h n i where n e the free electron density in conduction band, n h the hole density in valence band, and n i the intrinsic carrier concentration. The energy required to break the covalent bond is about 0.
Electrical conduction in pure semiconductors occurs by means of electronhole pairs. Conduction in semico putting qn, we get j e this is simply ohms law, put in another form. This band theory explains why solids fall into three categories. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdws. Pdf conduction in semiconductors seema goel academia. In this section we will go over the conduction mechanisms for semiconductors, the way by which charged particles move through the material.
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