Pdf theory of space charge limited currents xiaoguang zhang. Influence of diffusion on spacechargelimited current measurements in. However, the accurate fitting for the current voltage curves. Chen3 1school of electrical and electronic engineering, nanyang technological university. Well lookat the effect ofsurface roughness onemittance well derive the expressions for the space charge limited current andcome up with the ultimate. Electron field emission and the space charge limit. If space chargelimited current is modeled in frequency domain with single traps, then the dynamic picture of the traps can be obtained. Spacechargelimited current in a film electron devices. Conventional space charge and trap space chargelimited current model cannot differentiate between shallow and deep traps for both electron and hole.
But for assumption 1, space chargelimited current is further modified with new version as trapped space chargelimited current model and will be discussed in. Spacechargelimited current fluctuations in organic. Influence of diffusion on spacechargelimited current. Spacecharge is the charge density that exists in free space, such as inside a particle beam.
Pdf spacechargelimited current in a film researchgate. The current voltage plots display an exponential behavior with a fitting constant. Reference 12 gives a detailed discussion of childlaw emission, the bohm current density and the formation of a plasma meniscus. Well derive the cathode emission current and emittance for thermal, photo.
Space charge, electrical charge distributed through a threedimensional region. Modeling of spacecharge effects in 3d thermionic devices. Space charge is an inherent property of all vacuum tubes. Huge magnetoresistance in space charge regime attracts broad interest on nonequilibrium carrier transport under high electric field. The boundaries separating these three regions are charge neutral planes at x. The currentvoltage plots display an exponential behavior with a fitting constant. The following section discusses fowlernordheim tunneling, poolefrenkel emission, space charge effects as well as ballistic transport. Mechanisms of spacechargelimited current in solids. Space charge article about space charge by the free dictionary. Spacecharge effect definition of spacecharge effect by.
It is assumed that impenetrable barriers limit the current flow to a film of thickness d. This current value is known as the spacecharge limited current. For photoemission, well discuss the three types ofcathodes. Pdf spacechargelimited current is considered in a thinfilm nin diode. The negatively charged electrons moving to the right constitute a steady electric current density to the left, i. The practical case is taken of a wide bandgap, highresistivity material containing empty shallow trapping states but in which empty deep trapping states are eliminated by the mechanism of defect compensation described by l ongini and g reen 1956. Pdf criteria for spacechargelimited currents to be applied as a spectroscopical. To obtain the best experience, we recommend you use a more up to date browser. Space charge simion 2019 supplemental documentation.
Space charge limited current in schottky diode with single. Very short time accelerated ageing tests pdf, prague. Empirical formulas are given for future comparison with experimental results. Spacechargelimited current in germanium caltechthesis. The formation of space charge clouds is a well known problem that affects thermionic emitters in regimes of high current emission.
Influence of diffusion on space charge limited current measurements in organic semiconductors thomas kirchartz address. Influence of diffusion on space chargelimited current. Equation 2 is therefore replaced by the frenkel effect is likely to have observable consequences if the exponent pell2 is. Space charge limited current in a film electron devices, ieee transact ions on author. This has at times made life harder or easier for electrical engineers who used tubes in their designs. Spacecharge limited ion emission from a plasma surface this section brie. It is assumed that impenetrable barriers limit the current flow to a film of. Spacecharge limited current wikipedia audio article. In an electron tube, for example, a negative charge results because electrons that are emitted from the cathode do not travel instantaneously to the plate anode but require a finite time for the trip. An experimental verification of the theory of spacechargelimited hole current in germanium is presented.
Space charge limited current sclc as observed on diamond. Space charge limited current in schottky diode 407 where is charge carrier mobility. This work presents a twodimensional theory for calculating an upperbound for the space charge limited current of relativistic electron. In order to have monopolar scl current, the devices are prepared such that the injecting contact is ohmic, i. Current mechanisms through materials, which do not contain free carriers, can be distinctly different from those in doped semiconductors or metals. In geo, the ambient plasma thermal current is insufficient to discharge spacecraft surfaces rapidly. Because of the mathematical difficulties, this limit is typically estimated from a onedimensional theory. Space charge limited current fluctuations in organic semiconductors a. Because the electric field of injected electrons spreads out of the film, the level of injection is substantially higher than in the bulk case described by the classical mottgurney law. The presence of traps not only reduces the magnitude of spacechargelimited currents, but also is likely to distort the shape of the currentvoltage curve from an. Nonuniform space charge limited current injection into a nano. Space charge limited current is considered in a thinfilm nin diode.
Find out information about space charge limitation. Space charge limited current university of nebraska. In order for a space charge to arise, the concentrations of positive and negative charge carriers, such as ions and electrons in a plasma, must not be equal. Although some analytical models have been presented, suitable for 1d geometries, the modeling of the problem in complex 3d structures remains a challenge due to the mutual coupling between electron trajectories and field they produce.
The current is thus referred to as space charge limited current sclc and can be expressed as. Space charge limited current the theory of space charge limited current between plane parallel electrodes was first. It is in contrast to, for example, charge density on conductive electrode surfaces or the bound charges within insulative materials. When the above current is exceeded, a portion of the emitted current returns to the spacecraft, limiting its effectiveness. Spacechargelimited current and frenkel efect 153 the effective depth of the trap is considered to be reduced by this amount, and conse quently the proportion of carriers which are free is increased. Finite element method based simulation of unipolar space. You are using a browser version with limited support for css. The space charge profiles for 14 layer samples are similar and the twolayer sample is thus taken as an example, as shown in fig. Kotowski1,2 1physics department and national institute of matter physics infm, politecnico di torino, c. The current flowing through a vacuum between a cathode and an anode cannot exceed a certain maximum value, as a result of modification of the electric field. Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space either a volume or an area rather than distinct pointlike charges.
Pn junctions consist of a spacecharge region sandwiched by two quasineutral bulk regions. Space charge effect definition is the limitation of flow of plate current in an electron tube produced by repulsion exerted on electrons leaving the filament by the other electrons in the region between filament and plate. Figure s2 in supporting information file 1, the reversebias currentvoltage. The space charge limit is applied to the observed operation of rf photocathode guns. Space charge limited currents sclc were observed on diamond surfaces that were damaged by megaelectron volt mev ion implantations. It is observed that iv curve enters into space charge region at lower voltage in heavily doped samples, however, space charge limited current is absent in lightly doped samples. A current voltage characteristic is given which, though following the mottgurney law at low fields, predicts much greater currents than given by this law at high fields. In the geo space environment, the major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. For example, space charge significantly limited the practical application of triode amplifiers which led to further innovations such as the vacuum tube tetrode.
The childs law behavior of a space charge limited current that applies in a vacuum. Z i n i, where n i, is the concentration, z i, the charge of type i carriers, and e the charge of an electron. In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have been diffused away, or have been forced away by an electric field. The connection between the space charge limit and the lowest achievable emittance is derived and discussed. Space charge limited currentvoltage characteristics of. Space charge region an overview sciencedirect topics. Spacechargelimited current is considered in a thinfilm nin diode. Space charge is a concept in which excess electric charge is treated as a continuum of charge. This model typically applies when charge carriers have been emitted from some region of a solidthe cloud. This current value is known as the space charge limited current. It is easy to find the contact potential once we have the field in the space charge region the negative of the contact potential is the area under the electric field curve. The mechanisms of space charge limited scl current in solids are discussed. Department of physics and centre for plastic electronics, imperial college london, south. Space charge limited current how is space charge limited.
In this case, once all the traps are filled the characteristics approaches. Theory of space charge limited current in fractional dimen. Modeling ion extraction from a freeplasma surface with a. Spacechargelimited current in a film electrical and computer. E b space charge field moves with the beam center e. Accumulated space charge within a solid dielectric may modify the electric field, and this internal field, in turn, controls the space charge distribution. Geometry dependence of 2dimensional spacechargelimited. It is assumed that impenetrable barriers limit the current flow to a film. Space chargelimited current model for polymers intechopen. This work presents a twodimensional theory for calculating an upperbound for the spacecharge limited current of relativistic electron.
1369 212 1106 1064 1050 760 101 1410 720 1144 223 74 403 156 1018 917 433 1482 867 152 737 47 759 238 808 252 1369 269 61 655 122 596 915