DOCSLIB.ORG

Light-Emitting Diodes (LED)

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Light-Emitting Diodes (LED) Chapter 9 Light-emitting diodes (LED) Electro luminescence diodes and semiconductor are mainly manufactured as double heterostructure diodes lasers in optical telecommunication systems. In this structure the active region, where the recombination of charge carriers takes place, is surrounded by two layers with a larger band gap WG. The name double het- erostructure is due to the fact that both sides of the active region are surrounded with different semiconductor material (e.g. InGaAs-InP in Fig. 9.1). p - I n P n- oder p- E l e k t r o d e n I n G a A s P U 0 n - I n P (aktive Zone) Figure 9.1: Example of a double heterostructure diode Fig. 9.1 shows such a diode. Basically, it is a p-n diode, which is operated in forward direction. The charge carriers recombine under radiation in the active region. Fig. 9.2 shows the energy band diagram of a double heterostructure diode in forward direction during operation. A double heterostructure is characterized by a small active region with the band gap energy WG2 which is much smaller than the band gap energy of the sandwich layers WG1 and WG3. In a pure pn-junction the inversion condition WF n − WF p > WG can only be reached for very high currents. The trick of the double-heterostructure is that both, the electrons in the conduction band as well as the holes in the valence band are limited by energy barriers of the structure. As a result, simultaneously very large concentrations of holes and electrons are gathered in the very thin active region d. This provides a high spontaneous and stimulated emission. The thickness of the active layer in LEDs is typically around d ≤ 1 µm. In semiconductor lasers, this thickness can be reduced even to a few nanometers (i.e. a few atomic layers) (see also: quantum-well lasers in chapter HL-STRUK). Thus, even for low injected current densities, a high charge carrier concentration in the active region can be achieved. 1 Introduction to fiber optic communications ONT/ 2 ------------ ------ W G 1 W G 2 W G 3 W F p W F n + + + +++ + + + + + + n - I n P I n G a A s p - I n P d Figure 9.2: Energy-band diagram under forward bias 9.1 Electro luminescence diodes The recombination in electro luminescence diodes is mainly determined by the spontaneous emission. It is proportional to the product of electron density n in the conduction band and hole density p in the valence band. Rsp ∼ fL(1 − fV ) ∼ n · p (9.1) More precisely: Rsp = V · B · n · p (9.2) −10 cm3 where B is the recombination coefficient in the magnitude of 10 s . Assuming that n and p are much higher than in the thermodynamic equilibrium, the spontaneous emission rate per volume is of the form (without current injection): R dn dp sp = − = − = B · n · p (9.3) V dt dt The rate equations for the both carriers is given by dp dn R I = = − sp + (9.4) dt dt V e · V where I is the injection current. The dynamic modulation characteristics of an LED can be described by the spontaneous emission lifetime τsp. If the spontaneous emission is e.g. defined as Rsp=V = n/τsp , the charge carrier density n decay, after switching off the injection current I ( I = 0 ), is given by: ( ) · − t n = n0 exp (9.5) τsp The maximum modulation frequency can thus be estimated by: 1 fg ≈ (9.6) 2π · τsp Introduction to fiber optic communications ONT/ 3 1. Undoped active region For an undoped active region, the charge neutrality n = p applies and therefore: dn R ! n = − sp = −B · n2 = − (9.7) dt V τsp with 1 τ = (9.8) sp B · n Hence, the mean lifetime decreases with increasing carrier injection. For a fast modulation a high carrier injection (or high n) is required. · 18 −3 · −10 cm3 Example: Assuming n = 3 10 cm and B = 2 10 s the lifetime of the spontaneous emission is τsp = 1:7 ns. Such an LED is thus modulatable up to about 100 MHz. 2. Doped active region In a doped active region n and p are not necessarily much larger than in the thermodynamic equilib- rium. For this reason, Eq. (9.7) has to be modified. (a) p-doping with the acceptor concentration NA p ≈ n + NA (9.9) dn = −B · n · p = −B · n · (n + N ) (9.10) dt A 1 ) τsp = (9.11) B · (n + NA) (b) n-doping with the donor concentration ND n ≈ p + ND (9.12) dp = −B · p · (p + N ) (9.13) dt D 1 ) τsp = (9.14) B · (p + ND) For ND ≫ p and NA ≫ n , respectively, the mean lifetime of a doped active region becomes largely independent of the charge carrier concentration. With increasing doping, the LED can be modulated Introduction to fiber optic communications ONT/ 4 more quickly. This, however, increases the non-radiative recombination with the lifetime τns. At high non-radiative recombination, it is reasonable, to define an effective lifetime τe: 1 1 1 = + (9.15) τe τsp τns in which for a high doping τe becomes relatively short. Therefore a high modulation frequency f = 1 is feasible (at the cost of efficiency). g 2π·τe Figure 9.3: Surface-emitting diode (Burrus-type LED) To ensure a good heat dissipation, it is important that the active region is located close to the heat sink. The current flow has to be limited to a spot size of the diameter dL. In Fig. 9.3 for example the active region consists of GaAs where the heterostructures are made of GaAlAs on a substrate of GaAs. Since in this case, for example, the substrate is not transparent for the emission wavelength, a well is etched into the substrate in order to bring in the fiber (Burrus-type LED). The light emission of the LED, shown in Fig. 9.3, is perpendicular to the layer sequence. Such an LED is also called a surface-emitting LED. Example: For a typical dimension with the data dL = 30 µm Diameter of the light spot d = 0:5 µm Thickness of the active region I = 100 mA Injection current n and τe ( = τsp should be determined by assuming τns = 1 ). π · 2 · · −10 3 The active volume is V = 4 dL d = 3:5 10 cm . Due to Eq. (9.4) and Eq. (9.7) and assuming d τe = τsp in steady state ( dt = 0): τ · I n = e (9.16) e · V According to Eq. (9.8), for an undoped region, Eq. (9.16) can be written as I n2 = (9.17) e · V · B Introduction to fiber optic communications ONT/ 5 · −10 cm3 · −19 With B = 2 10 s , e = 1:6 10 As and the assumed data above the charge carrier density results 18 −3 in n = 3 · 10 cm . Therefore the effective mean lifetime is τe = 1:7 ns corresponding to the limiting frequency of approximately fg ≈ 100 MHz . 9.2 Optical power output The efficiency of an LED - defied as the ratio of emitted optical power to electrical power input - is usually relatively low (in the order of a few percent). Because of the high refractive index of the active region ( n ≈ 3; 5 ) the largest part of the spontaneous emission is totally reflected and is not emitted out of the semiconductor. The non-radiative recombination also reduces the efficiency. Since the quantity of the non-radiative recombination depends on the doping, the power output of the diode also depends on the doping, leading to a dependency to the cutoff frequency fg of the diode (Fig. 9.4). Instead of the output power P it is customary to declare the radiance SR. It Figure 9.4: Output Power of diodes with different cutoff frequencies fg (Light spot diameter dL = 50 µm , I = 300 mA) W is defined as the output power per dihedral and solid angle unit and is of the dimension cm2sr ( sr ^= unit of the solid angle, actually dimensionless). The LED as surface-emitter has a Lambertian radiation pattern (circular radiation pattern in Fig. 9.3): S(γ) = SR cos(γ) (9.18) The total emitted power is thus Z π P = · d2 S(γ) dΩ (9.19) 0 4 L where the integration in Eq. (9.19) is determined by integrating over a half space. Introduction to fiber optic communications ONT/ 6 Hence (see also Fig. 9.5) π Z2 π π2 P = · d2 · S 2π sin(γ) cos(γ) dγ = · d2 · S (9.20) 0 4 L R 4 L R 0 d g d 2 s i n d g W = ×p g×g 1 Figure 9.5: The solid angle element dΩ W Example: Assuming SR = 100 cm2sr and dL = 50 µm the total emitted output power is P0 = 6:2 mW. 9.3 Coupling of an LED to a step-index fiber Initially, it is assumed that the diameter of the LED light spot is greater than the diameter of the fiber ( dL > 2a ). The LED and the fiber should be arranged as shown in Fig. 9.3. The numerical aperture of the fiber should be sufficiently small, so that a constant power density S(γ) can be assumed for all acceptance angles. With π · a2 Surface of the fiber end · 2 π AN acceptance angle the power P1 coupled into the step-index fiber is given by: · 2 · 2 · 2 P1 = SR π a AN (9.21) W Example: SR = 100 cm2sr , a = 25 µm , AN = 0:2 ) P1 = 0:25 mW (9.22) The coupling efficiency is determined by the ratio of the coupled power P1 to the total emitted power P0.
Load more

Recommended publications
  • Non-Equilibrium Carrier Concentrations
    ECE 531 Semiconductor Devices Lecture Notes { 06 Dr. Andre Zeumault Outline 1 Overview 1 2 Revisiting Equilibrium Considerations1 2.1 What is Meant by Thermal Equilibrium?........................1 2.2 Non-Equilibrium Carrier Concentrations.........................3 3 Recombination and Generation3 3.1 Generation.........................................3 4 Continuity Equations5 4.1 Minority Carrier Diffusion Equations...........................5 5 Quasi-Fermi Levels8 1 Overview So far, we have only discussed charge carrier concentrations under equilibrium conditions. Since no current can flow in thermal equilibrium, this particular case isn't much interest to us in trying to create electronic devices. Therefore, in this lecture, we discuss the situations and governing equations in which a charge carrier responds under non-equilibrium conditions. Topics to be covered include: • Recombination and Generation: An increase (generation) or decrease (recombination) in charge carrier concentrations away from or towards equilibrium. (e.g. due to absorption of light) • Current Continuity: Charge conservation, applied in the context of drift/diffusion. • Minority Carrier Diffusion Equations: Minority carrier diffusion in the presence of a large majority carrier concentration. • Quasi-Fermi Levels: Treating carrier densities under non-equilibrium conditions. 2 Revisiting Equilibrium Considerations 2.1 What is Meant by Thermal Equilibrium? When we discussed equilibrium charge carrier concentrations, it was argued that the production of charge carriers was thermally activated. This was shown to be true regardless of whether or not the semiconductor was intrinsic or extrinsic. When a semiconductor is intrinsic, the thermal energy necessary for charge carrier production is the bandgap energy EG, which corresponds to the energy needed to break a Silicon-Silicon covalent bond.
    [Show full text]
  • Multidisciplinary Design Project Engineering Dictionary Version 0.0.2
    Multidisciplinary Design Project Engineering Dictionary Version 0.0.2 February 15, 2006 . DRAFT Cambridge-MIT Institute Multidisciplinary Design Project This Dictionary/Glossary of Engineering terms has been compiled to compliment the work developed as part of the Multi-disciplinary Design Project (MDP), which is a programme to develop teaching material and kits to aid the running of mechtronics projects in Universities and Schools. The project is being carried out with support from the Cambridge-MIT Institute undergraduate teaching programe. For more information about the project please visit the MDP website at http://www-mdp.eng.cam.ac.uk or contact Dr. Peter Long Prof. Alex Slocum Cambridge University Engineering Department Massachusetts Institute of Technology Trumpington Street, 77 Massachusetts Ave. Cambridge. Cambridge MA 02139-4307 CB2 1PZ. USA e-mail: [email protected] e-mail: [email protected] tel: +44 (0) 1223 332779 tel: +1 617 253 0012 For information about the CMI initiative please see Cambridge-MIT Institute website :- http://www.cambridge-mit.org CMI CMI, University of Cambridge Massachusetts Institute of Technology 10 Miller’s Yard, 77 Massachusetts Ave. Mill Lane, Cambridge MA 02139-4307 Cambridge. CB2 1RQ. USA tel: +44 (0) 1223 327207 tel. +1 617 253 7732 fax: +44 (0) 1223 765891 fax. +1 617 258 8539 . DRAFT 2 CMI-MDP Programme 1 Introduction This dictionary/glossary has not been developed as a definative work but as a useful reference book for engi- neering students to search when looking for the meaning of a word/phrase. It has been compiled from a number of existing glossaries together with a number of local additions.
    [Show full text]
  • Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes
    Application Note, Rev. 2.0, Oct. 2006 Application Note No. 034 Carrier Lifetime and Forward Resistance in RF PIN-Diodes RF & Protection Devices Edition 2006-10-20 Published by Infineon Technologies AG 81726 München, Germany © Infineon Technologies AG 2009. All Rights Reserved. LEGAL DISCLAIMER THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND (INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN THIS APPLICATION NOTE. Information For further information on technology, delivery terms and conditions and prices please contact your nearest Infineon Technologies Office (www.infineon.com). Warnings Due to technical requirements components may contain dangerous substances. For information on the types in question please contact your nearest Infineon Technologies Office. Infineon Technologies Components may only be used in life-support devices or systems with the express written approval of Infineon Technologies, if a failure of such components can reasonably be expected to cause the failure of that life-support device or system, or to affect the safety or effectiveness of that device or system. Life support devices or systems are intended to be implanted in the human body, or to support and/or maintain and sustain and/or protect human life.
    [Show full text]
  • Introduction to Semiconductor Devices Presented by K.Pandiaraj Ece Department Kalasalingam University Previous Class Topics
    ECE201 – ELECTRON DEVICES INTRODUCTION TO SEMICONDUCTOR DEVICES PRESENTED BY K.PANDIARAJ ECE DEPARTMENT KALASALINGAM UNIVERSITY PREVIOUS CLASS TOPICS • Atom • Electron • Proton • Neutron • Orbit levels of electrons in an atom • Valence electrons • Free electrons • Energy band diagram • Valance band, Conduction band, Forbidden band • Insulators • Conductors • Semiconductors INTRODUCTION TO SEMICONDUCTOR • The material which has electrical conductivity between that of a conductor and that of an insulator is called as semiconductor. • silicon, germanium and graphite are some examples of semiconductors. • semiconductors are the foundation of modern electronics, including transistors, light-emitting diodes, solar cells etc. INTRODUCTION TO SEMICONDUCTOR • It has a forbidden gap of about 1 electron volt (ev). • The semiconductor behaves as an insulator at low temperature. • However, at room temperature some of the electrons in valence band gains enough energy in the form of heat and moves in to conduction band. when the valence electrons moves in to conduction band they becomes free electrons. • The conduction band electrons are responsible for electrical conductivity. • When the temperature is goes on increasing, the number of valence band electrons moving in to conduction band is also increases. this shows that electrical conductivity of the semiconductor increases with increase in temperature. • In semiconductors, electric current is carried by two types of charge carriers they are electrons and holes. HOLE • The absence of electron in a particular place in an atom is called as hole. • Hole is a electric charge carrier which has positive charge. the electric charge of hole is equal to electric charge of electron but have opposite polarity. • When a small amount of external energy is applied, then the electrons in the valence band moves in to conduction band and leaves a vacancy in valence band.
    [Show full text]
  • Investigation of the Spatial Dependence of Carrier Dynamics in Semiconductor Optoelectronic Devices
    University of South Carolina Scholar Commons Theses and Dissertations 2018 Investigation Of The pS atial Dependence Of Carrier Dynamics In Semiconductor Optoelectronic Devices Bobby G. Barker Jr. University of South Carolina Follow this and additional works at: https://scholarcommons.sc.edu/etd Part of the Chemistry Commons Recommended Citation Barker, B. G.(2018). Investigation Of The Spatial Dependence Of Carrier Dynamics In Semiconductor Optoelectronic Devices. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4696 This Open Access Dissertation is brought to you by Scholar Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. INVESTIGATION OF THE SPATIAL DEPENDENCE OF CARRIER DYNAMICS IN SEMICONDUCTOR OPTOELECTRONIC DEVICES by Bobby G Barker, Jr. Bachelor of Science University of Kentucky, 2013 Submitted in Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy in Chemistry College of Arts and Sciences University of South Carolina 2018 Accepted by: Andrew B. Greytak, Major Professor Donna Chen, Committee Member Natalia Shustova, Committee Member MVS Chandrashekhar, Committee Member Cheryl L. Addy, Vice Provost and Dean of the Graduate School © Copyright by Bobby G. Barker, Jr., 2018 All Rights Reserved. ii DEDICATION This dissertation is dedicated to my parents, who are always there to support me throughout my Ph.D. and my entire life. Their positive reinforcement, care, and education from a young age have set me up for success and I could not have had any successes without them. iii ACKNOWLEDGEMENTS I would like to express my deepest appreciation to my advisor Dr.
    [Show full text]
  • Simulations of the Physics and Electronics in 2D Semiconductor Pixel Detectors
    Simulations of the physics and electronics in 2D semiconductor pixel detectors Ashley Joy A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of University College London. Physics and Astronomy University College London July 6, 2017 Abstract In this thesis, simulations have been developed of entire 2D Semiconductor pixel detectors, including the physics and electronic responses. The most important con- cept of this is the separation of simulation into three distinct physical regimes, which should each be simulated in a different manner. The first stage is a particle level simulation of X-Ray photons and their sec- ondary particles interacting with the detector, and recording where interactions de- positing energy in the sensitive layer occur. This uses Geant4, as the energy range in use lies just within Geant4’s minimum energy bound. The second stage is a charge cloud simulation of the interaction energy con- version to electron hole pairs, and then the movement of these electrons or holes in the sensitive layer towards the collection area or pixels. This uses an analytic solution to the problem of a charge cloud moving and interacting with itself within a semiconductor, this work is based upon the earlier HORUS simulation and more detailed lattice simulations by R. F. Fowler, et al. The third stage is an electronics level simulation of how the collected charge is manipulated by the detector into a digital read out signal. This is done with a collection of functions representing the response of electronic components to a signal, that can be used in turn to represent the electronic response to an electrical signal.
    [Show full text]
  • Electrical Engineering Dictionary
    ratio of the power per unit solid angle scat- tered in a specific direction of the power unit area in a plane wave incident on the scatterer R from a specified direction. RADHAZ radiation hazards to personnel as defined in ANSI/C95.1-1991 IEEE Stan- RS commonly used symbol for source dard Safety Levels with Respect to Human impedance. Exposure to Radio Frequency Electromag- netic Fields, 3 kHz to 300 GHz. RT commonly used symbol for transfor- mation ratio. radial basis function network a fully R-ALOHA See reservation ALOHA. connected feedforward network with a sin- gle hidden layer of neurons each of which RL Typical symbol for load resistance. computes a nonlinear decreasing function of the distance between its received input and Rabi frequency the characteristic cou- a “center point.” This function is generally pling strength between a near-resonant elec- bell-shaped and has a different center point tromagnetic field and two states of a quan- for each neuron. The center points and the tum mechanical system. For example, the widths of the bell shapes are learned from Rabi frequency of an electric dipole allowed training data. The input weights usually have transition is equal to µE/hbar, where µ is the fixed values and may be prescribed on the electric dipole moment and E is the maxi- basis of prior knowledge. The outputs have mum electric field amplitude. In a strongly linear characteristics, and their weights are driven 2-level system, the Rabi frequency is computed during training. equal to the rate at which population oscil- lates between the ground and excited states.
    [Show full text]
  • MOSFET Device Physics and Operation
    1 MOSFET Device Physics and Operation 1.1 INTRODUCTION A field effect transistor (FET) operates as a conducting semiconductor channel with two ohmic contacts – the source and the drain – where the number of charge carriers in the channel is controlled by a third contact – the gate. In the vertical direction, the gate- channel-substrate structure (gate junction) can be regarded as an orthogonal two-terminal device, which is either a MOS structure or a reverse-biased rectifying device that controls the mobile charge in the channel by capacitive coupling (field effect). Examples of FETs based on these principles are metal-oxide-semiconductor FET (MOSFET), junction FET (JFET), metal-semiconductor FET (MESFET), and heterostructure FET (HFETs). In all cases, the stationary gate-channel impedance is very large at normal operating conditions. The basic FET structure is shown schematically in Figure 1.1. The most important FET is the MOSFET. In a silicon MOSFET, the gate contact is separated from the channel by an insulating silicon dioxide (SiO2) layer. The charge carriers of the conducting channel constitute an inversion charge, that is, electrons in the case of a p-type substrate (n-channel device) or holes in the case of an n-type substrate (p-channel device), induced in the semiconductor at the silicon-insulator interface by the voltage applied to the gate electrode. The electrons enter and exit the channel at n+ source and drain contacts in the case of an n-channel MOSFET, and at p+ contacts in the case of a p-channel MOSFET. MOSFETs are used both as discrete devices and as active elements in digital and analog monolithic integrated circuits (ICs).
    [Show full text]
  • Solar Energy Glossary of Terms
    Glossary of Terms, SOLAR 1 Glossary Absorber: In a photovoltaic device, the material that readily absorbs photons to generate charge carriers (free electrons or holes). AC: See alternating current. Activated Shelf Life: The period of time, at a specified temperature, that a charged battery can be stored before its capacity falls to an unusable level. Activation Voltage(s): The voltage(s) at which a charge controller will take action to protect the batteries. Adjustable Set Point: A feature allowing a user to adjust the voltage levels at which a charge controller will become active. Alternating Current (AC): A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease. Acceptor: A dopant material, such as boron, which has fewer outer shell electrons than required in an otherwise balanced crystal structure, providing a hole that can accept a free electron. AIC: See amperage interrupt capability. Air Mass (sometimes called air mass ratio): Equal to the cosine of the zenith angle or that angle from directly overhead to a line intersecting the sun. The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness). Ambient Temperature: The temperature of the surrounding area. Amorphous Semiconductor: A non‐crystalline semiconductor material that has no longrange order.
    [Show full text]
  • IEEE Taxonomy
    2017 IEEE Taxonomy Version 1.0 Created by The Institute of Electrical and Electronics Engineers (IEEE) 2017 IEEE Taxonomy IEEE Taxonomy: A Subset Hierarchical Display of IEEE Thesaurus Terms The IEEE Taxonomy comprises the first three hierarchical 'levels' under each term-family (or branch) that is formed from the top-most terms of the IEEE Thesaurus. In this document these term-families are arranged alphabetically and denoted by boldface type. Each term family's hierarchy goes to no more than three sublevels, denoted by indents (in groups of four dots) preceding the next level terms. A term can appear in more than one hierarchical branch and can appear more than once in any particular hierarchy. The IEEE Taxonomy is defined in this way so that it is always a subset of the 2017 IEEE Thesaurus. Aerospace and electronic systems ............Missiles ....Aerospace control ............Nuclear weapons ........Air traffic control ............Projectiles ........Attitude control ....Radar ........Ground support ........Airborne radar ....Aerospace engineering ........Bistatic radar ........Aerospace biophysics ........Cognitive radar ........Aerospace electronics ........Doppler radar ........Aerospace safety ........Ground penetrating radar ............Air safety ........Laser radar ........Aerospace simulation ........Meteorological radar ........Aerospace testing ........Millimeter wave radar ........Satellites ........Multistatic radar ............Artificial satellites ............MIMO radar ............Earth Observing System ........Passive
    [Show full text]
  • HALL Semiconductor Resistance, Band Gap, and Hall Effect
    ADVANCED UNDERGRADUATE LABORATORY HALL Semiconductor Resistance, Band Gap, and Hall Effect Revisions: September 2016, January 2018: Young-June Kim November 2011, January 2016: David Bailey October 2010: Henry van Driel February 2006: Jason Harlow November 1996: David Bailey March 1990: John Pitre & Taek-Soon Yoon Copyright © 2011-2018 University of Toronto This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. (http://creativecommons.org/licenses/by-nc-sa/3.0/) Introduction Solid materials are usually classified as metals or insulators, depending on their electrical conducting properties. Although there are numerous physical properties that distinguish metals from insulators, the critical difference between a metal and an insulator is the existence of band gap in the latter. The allowed energies of electrons inside solid materials are quantum mechanically restricted to certain ranges known as energy bands. For insulators the lower energy bands are completely filled at absolute zero temperature and are known as valence bands; the first band that is normally empty is known as the conduction band. The difference in energy between the top of the valence band and the bottom of the conduction band is known as the energy gap or band gap. Materials in which an energy band is always partially filled are metals, with the partially filled bands allowing electrons to move freely. When the band gap of an insulator is relatively small (usually less than 2 eV), electrons can be thermally activated across the small band gap and participate in the conduction. This type of insulator that can be made to conduct electricity is technologically very important and is generally referred to as a semiconductor.
    [Show full text]
  • Minority and Majority Charge Carrier Mobility in Cu2znsnse4 Revealed by Terahertz Spectroscopy Received: 30 April 2018 Hannes Hempel1, Charles J
    www.nature.com/scientificreports OPEN Minority and Majority Charge Carrier Mobility in Cu2ZnSnSe4 revealed by Terahertz Spectroscopy Received: 30 April 2018 Hannes Hempel1, Charles J. Hages1, Rainer Eichberger2, Ingrid Repins3 & Thomas Unold 1 Accepted: 12 September 2018 The mobilities of electrons and holes determine the applicability of any semiconductor, but their Published: xx xx xxxx individual measurement remains a major challenge. Here, we show that time-resolved terahertz spectroscopy (TRTS) can distinguish the mobilities of minority and majority charge carriers independently of the doping-type and without electrical contacts. To this end, we combine the well-established determination of the sum of electron and hole mobilities from photo-induced THz absorption spectra with mobility-dependent ambipolar modeling of TRTS transients. The method is demonstrated on a polycrystalline Cu2ZnSnSe4 thin flm and reveals a minority (electron) mobility of 128 cm2/V-s and a majority (hole) carrier mobility of 7 cm2/V-s in the vertical transport direction relevant for light emitting, photovoltaic and solar water splitting devices. Additionally, the TRTS analysis yields an efective bulk carrier lifetime of 4.4 ns, a surface recombination velocity of 6 * 104 cm/s and a doping concentration of ca. 1016 cm−3, thus ofering the potential for contactless screen novel optoelectronic materials. TRTS is a widely used pump-probe technique to measure the dispersive AC-mobility µe+h(f) of photo-excited charge carriers at THz frequencies, as well as the decay of the photoconductivity from femtoseconds to nanosec- onds shown in Fig. 1 1–3. Te outstanding benefts of this technique include its non-destructive and contactless nature as well as high time resolutions of <100 fs.
    [Show full text]