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1. 4 Conclusion When conventional models are not applicable for new material devices it is useful to develop a user defined model using the C Interpreter It provides users with a flexible method to comprehend the device behavior and to optimize its operation and structure The Simulation Standard Page 8 April May June 2008 Hints Tips and Solutions Stephen Wilson Applications and Support Engineer Q How do I create Circular and Cylindrical meshes in ATLAS A It has been possible for some time to create meshes with circular and cylindrical symmetry using the DevEdit device building tool This capability has been extended re cently to the ATLAS command language thus providing the ATLAS user with an alternative and more convenient way to construct devices with circular symmetry Possible applications are nanowires and mesa type structures For nanowires quantum transport models are available To specify a circular mesh with ATLAS2D the user includes the parameter CIRCULAR on the MESH state ment The properties of the MESH must then be given by using a number of R MESH and A MESH statement The radial mesh spacing is controlled by the R MESH statements and the angular mesh given by the A MESH statements To conform to the new MESH shape the parameters R MIN R MAX A MIN and A MAX have been added to the REGION ELECTRODE and DOPING statements build V3 36 0 R ectl in dir region rur 1 mater 31 3cu m z2. file which includes the function name An example C Interpreter source file is listed below Hinclude lt math h gt Hinclude lt stdio h gt double temp double curr double double my lo rc double v double ktq double time double didv double cap charge double double double double double double eps e0 epl ele theta gamma Dtime va L W D L 152 W 148 Dtime 100e 3 theta 51 0 sec April May June 2008 gamma 51 2e 3 sec epl 3 1 vc 1 887 D 10 02 e0 8 854e 12 LE v gt vc eps epl theta gamma exp Dtime sqrt v ve 1 0 else if v lt vc eps epl clc e0 eps L W 1e 6 D E curr v 10e9 didv 1 10e9 Cap elo charge cap v printi cle Fe F in cle printf charge eln charge return 0 In the calculation above a user defined two terminal current is defined by the following formula I F1 V t F2 V t dF dV The 1st term is the DC current and the 2nd term is the capacitive current MixedMode performs capacitance and total charge cal culation based on the user defined C Interpreter func tion A typical voltage driven response of unit pixel is shown Figure 2 97us SVG _ o_o _ _ _ _ _ _ _ 16 7ms 1 60 sec Figure 2 A typical TFT AMLCD unit pixel voltage driven response April May June 2008 Before transient simulation in MixedMode the DC char acterist
3. max 9 65 z min 5 3 z nex f5 material oxide min 1 55 r tmax 2 25 z mi7 0 Z max 9 1 mate ial oxide min 2 25 r max 2 3 z m3n 3 z n6x 1 material oxide regijo number 1 r reajor number 2 r regior number 3 r regior number 4 r reaior number 5 r regior namber 5 r electrode nare ge72 r electrode nare gea721 r max electrod2 nare ge72 r min 2 electrod2 hare 0u ce r min 0 electrod2 nare drain z mi9 9 1 Figure 6 Example ATLAS commands for the cylindrical wedge structure April May June 2008 ATLAS Example of cylindrical structure Materials a Conductor SiO 2 Silicon Figure 7 Example cylindrical structure by the LAT CHAR or RATIO LAT parameters An ex ample of a quite complicated 3d cylindrical geometry produced by the commands shown in Figure 6 is shown in Figure 7 The MAXANGLE parameter has been set to 90 degrees but the major angular spacing is 36 degrees ATLAS requires that the actual MAXANGLE used correspond to a whole number of major angular mesh spacings Thus the mesh spacing has been automatically corrected by ATLAS to 108 degrees This can be seen in a cutplane of the structure Tip Because the mesh is independent of position along the z axis the radial and angular boundaries of all the RE GION s and ELECTRODESs are included in the R MESH and A MESH statements irrespective of the z location of the REGION It is always recommended to have mesh points exactly co incident with region
4. 1rin 0 a nax 3E0 0 r na 0 8 region rur 2 rateral cxid2 ninel 3 met region rur 3 mater 3l e r r min 0 r mex 1 1 accroce nane gate a m 0 3 mex 22 5 min accroce nane gatel a nin 5 3 mex 6 5 mi accroce nane gate2 a rin 90 3 mex 112 actroce nane gate3 a min 13 amp 3 mex 157 aczroce nane gate4 a min 180 3 mex 202 aczroce nane gate5 a min 22 3 mex 247 actroce nane gate5 a min 2 7C 3 mex 292 aczroce nane gate a min 315 3 mex 337 accroce nane 9 cuad r rii r nax 1 2 Ma Mmm mon mmm VERSA IRA DS DI DI ESTEET mmmmrm mn ene e Tries Woes Aia Bahan en Pi fee ON TS A gt Gey 2 30 2C 30 30 96 9C 2C A A A E cs ds o o o o o do EE PAN method carriers 0 cave outf ect sctr colve ir t colve prev Figure 1 Example ATLAS commands for the electrical ca pacitance tomography cell April May June 2008 These permit the use of polar coordinates in delineating the edges of REGIONS and ELECTRODES The mini mum radius is specified by R MIN and the maximum by R MAX both in units of microns A MIN and A MAX specify the minimum and maximum angular ranges re spectively in units of degrees between 0 and 360 To give a first example for illustrative purposes only the set of ATLAS commands shown in Figure 1 results in the structure as shown in Figure 2 For best results the radial limits of the REGIONS and ELECTRODES align with R MESH locations and the angular limits align with the major spokes
5. 7ype concentration 1 Ce1 char 0 0005 r mn 332 r max 0 0035 3 1in 30 J a max 150 0 doping 93455131 n ype concentration 1 Ce1 char 0 0005 r tmin 3 332 r max 0 0035 a mn 0 3 max 30 0 doping 93455137 n ype concentration 1 Ce1 char 0 0005 r mn 3 332 r maxz 0 0035 a mn 153 a3 max 180 0 model bqo 1 Farmi ni errr srh fldmob solve init Figure 3 Example ATLAS commands for a semi circular structure ATLAS2D Nanoscale semi circular device Materials Silicon SiO2 Conductar Electrodes 0 003 Figure 4 Half of a symmetrical circular structure ATLAS2D Doping Profile in Semi circular device Net Doping cm3 Materials Silicon sio2 Conductor 0 004 Figure 5 Doping density in semi circular structure showing p n junctions The Simulation Standard The R MIN R MAX A MIN and A MAX parameters have also been implemented for the DOPING statement They apply to the analytic doping profiles UNIFORM GAUSSIAN and ERFC For GAUSSIAN and ERFC dop ing profiles the principal direction is the radial direc tion and the lateral direction is the tangential direction Figure 3 shows the use of these parameters This gives a semi circular structure as shown in Figures 4 and 5 The doping density along with junction positions are also shown Of arguably more use is the ability to create cylindrical structures in ATLAS3D This is effected by using the CYLINDRICAL parame ter on the MESH stat
6. K Anode Voltage V Figure 4 Simulated l V hysteresis loop for one cycle Mobility cm2 Vs References 1 S Lai T Lowrey OUM A 180nm nonvolatile memory cell element technology for standalone and embedded applications IEDM Praise kins E Tech Dig 2001 pp 36 5 1 36 5 4 Figure 3 Top Triangular voltage sweep applied Middle A 2 O aah aaa O e temperature change at a point near the center of the device 30 i Technology ech Dig pp 30 7 1 Bottom Mobility change at that point 7 4 Figure 2 shows the mobility change in an applied voltage cycle The top left picture is the initial SET state of high mobility The top right picture shows the mobility and temperature contours at 50ns the mobility decrease de pends on the temperature distribution and corresponds to a phase change from crystal to amorphous or to the breaking of conduction path The bottom left picture at 150ns shows that the mobility increases again the increase corresponds to re crystallization or re forma tion of the conduction path The bottom right is the state at 200ns in which mobility is kept at the SET state once reached at 150ns The mobility and temperature near the device center at X 0 6um Y 0 5um traced for one cycle are shown in Figure 3 The I V hysteresis curve of the device is shown in Figure 4 A typical hysteresis curve can be obtained by the simple temperature dependent mobility functions defined in Figure 1
7. LL Opaque Schottky Contac Figure 2 Photogeneration Contours in Amorphous Silicon So lar Cell Device amorphous or polycrystalline devices including thin film transistors TFT and TFT3D can be used with Luminous and Luminous3D to simulate thin film solar cells made from amorphous silicon Spectral DC and transient re sponses can be extracted 3 Simulating of Solar Cell Characteristics Here we will discuss the various aspects of solar cell characteristics that can be simulated by Silvaco TCAD tools Typical characteristics include collection efficiency spectral response open circuit voltage VOC and short circuit current ISC Figure 1 shows the simulated spectral response of a solar cell using the Luminous module This figure is obtained by varying the incident wavelength of ATLAS Photogeneration rate from angled light beam Figure 3 Photogeneration Rate from Angled Light Beam The Simulation Standard Figure 4 Solar Cell with Texture Surface a light source to extract the solar cell s spectral response From this figure the green curve is the equivalent cur rent from the light source the red curve is the available photo current generated by the light within the solar cell device and the blue curve is the actual terminal current Collection effiecieny inlcuding the effects of reflection can be caluclated by the ratio of these quantities It is possible to study the details of photo generation of car riers i
8. Simulation Standard Connecting TCAD To Tapeout A Journal for Process and Device Engineers Simulating Solar Cell Devices Using Silvaco TCAD Tools 1 Introduction Silvaco TCAD offers complete and well integrated simulation software for all aspects of solar cell technol ogy TCAD modules required for Solar Cell simulation include S Pisces Blaze Luminous TFT Device3D Luminous3D and TFT3D 1 The TCAD Driven CAD approach provides the most accurate models to device engineers Silvaco is the one stop vendor for all compa nies interested in advanced Solar Cell technology simu lation solutions 2 TCAD Modules For Solar Cell Technology Simulation Brief descriptions of the TCAD modules that can be used for solar cell technology simulation are listed below For more details of these modules please visit the Silvaco TCAD products website 2 S Pisces is an advanced 2D device simulator for silicon based technologies that incorporates both drift diffusion and energy balance transport equations Large selections of physical models are available for solar cell simulation which includes surface bulk mobility recombination impact ionization and tunneling models Blaze simulates 2D solar cell devices fabricated using advanced materials It includes a library of binary ter nary and quaternary semiconductors Blaze has built in models for simulating state of the art multi junction solar cell devices Device3D is a 3D devic
9. as defined by the A MESH spacing In some cases it may be possible to study a reduced an gular range rather than a full circle For this reason the MAXANGLE parameter has been introduced onto the MESH statement to allow the user to specify a wedge shaped device structure MAXANGLE should not be greater than 180 degrees The algorithm used by MESH CIRCULAR creates a constrained Delaunay mesh If the radial mesh spacing decreases with increasing radial position obtuse elements can sometimes be created The number of obtuse elements can typically be reduced by the use of the MINOBTUSE parameter also on the MESH statement ATLAS2D Example structure for ECT Vacuum SiO Air Conductor Electrodes Figure 2 Circular device structure for Electrical capacitance tomography cell The Simulation Standard mesh circular minob use max angle 180 r n 1 0 53a2 0005 r n 1 0 J 2 5332 0 0005 r n 150 3935 saac 0 0002 r n 130 333 saac 0 000 r n 10 334 5332 0 0001 a nl10 533518 a n 1 36 533218 region namel mate ial s7 con a min 0 z me 260 0 r rax 0 004 region name mate ial 0ox de a min 18 a mex 1E 2 0 r rin 0 0035 r max 3 004 electrod2 1ama J ain rrn 0 0035 a mir C C a rrax 18 0 electrod2 1ama 04 ce rrn 0 0035 a mir 1 2 0 a trax 180 0 electrod2 1am2 3a2e a m n 36 0 a max 144 C r rrin 0 0039 r tax 334 doping uniform 39 7Ype conc 0e17 mir C CC1 x rrax 0 001 y Max 3 3315 Hoping gaissiai p
10. boundaries April May June 2008 ATLAS3D Cutplane Mesh in x y plane of cylindrical device Materials Silicon Si02 Conductor Init iin Se AA 1 2 16 2 Figure 8 Cutplane of Cylindrical structure showing details of mesh Call for Questions If you have hints tips solutions or questions to contribute please contact our Applications and Support Department Phone 408 567 1000 Fax 408 496 6080 e mail support silvaco com Hints Tips and Solutions Archive Check our our Web Page to see more details of this example plus an archive of previous Hints Tips and Solutions www silvaco com Page 11 The Simulation Standard SILVACO USA Headquarters Silvaco International 4701 Patrick Henry Drive Bldg 6 Santa Clara CA 95054 USA Phone 408 567 1000 Fax 408 496 6080 salesOsilvaco com www silvaco com Contacts Silvaco Japan jpsales silvaco com Silvaco Korea krsales silvaco com Silvaco Taiwan twsales silvaco com Silvaco Singapore sgsales silvaco com Silvaco China cnsales silvaco com Silvaco UK uksales silvaco com Silvaco France frsales silvaco com Silvaco Germany desales silvaco com
11. e Once the photogeneration rates are obtained by the Luminous module ATLAS will then be able to simu late the terminal currents to obtain the IV characteris tics Figure 5 shows the IV characteristics of an amor phous silicon solar cell under AMO illumination In this figure ISC is the short circuit current and VOC is the open circuit voltage The ISC is extracted from the curve when the voltage is zero On the other hand the VOC can be extracted from the IV curve when the cur rent is zero Also the maximum current Im and maxi mum voltage Vm can be obtained from the maximum power rectangle as indicated in the figure By changing the illumination power of the light beam we can obtain a series of IV characteristics as a function Solar Cell Simulation depend on the Light Power Current Voltage of a Si Solar Cell under AMO Figure 6 Amorphous Silicon Solar Cell Simulation with Differ ent Light Power April May June 2008 Figure 7 Potential Distribution in a Solar Cell of the illumination power can be obtained This is shown in Figure 6 From this figure it can be seen that the short circuit current increase linearly with the increase of light power where the open circuit voltage begins to saturate with the increase of light power Three dimensional simulation of solar cells can be per formed to investigate effects such as electrical losses in the cell structure due to variation in the front metal grid finge
12. e simulator for silicon and other material based technologies The DC AC and time do main characteristics of a wide variety of silicon II V II VI and IV IV devices be analyzed Luminous and Luminous3D are advanced 2D and 3D simulator specially designed to model light absorption and photogeneration in non planar Solar Cell devices Ex act solutions for general optical sources are obtained using Volume 18 Number 2 April May June 2008 ATLAS Spectral Response of a Solar Cell Available photo current A Source photo current A Cathode Current A 0 7 Figure 1 Spectral Response of a Solar Cell geometric ray tracing This feature enables Luminous and Luminous3D to account for arbitrary topologies internal and external reflections and refractions polarization de pendencies and dispersion Luminous and Luminous3D also allows optical transfer matrix method analysis for coherence effects in layered devices The beam propaga tion method may be used to simulate coherence effects and diffraction TFT and TFT3D are advanced 2D and 3D device technol ogy simulators equipped with the physical models and specialized numerical techniques required to simulate Continued on page 2 INSIDE TCAD TFT AMLCD Pixel Simulation Material Modeling of Resistive Switching for Non Volatile Memories by ATLAS C Interpreter Hints Tips and Solutions SILVACO ATLAS PHOTOGENERATION IN AMORPHOUS SILICON SOLAR CE
13. ement in ATLAS3D A distinction must be made between the structures produced by ATLAS2D and ATLAS3D when using the CYLINDRI CAL parameter In ATLAS2D the Device is a body of revolution around the x 0 axis and consequently has no angular dependence In ATLAS3D using the CYLINDRICAL parameter allows a device with full radial angular and axial variations to be modelled To achieve this both the REGION and ELECTRODE state ments accept the R MIN R MAX A MIN and A MAX parameters as well as the Z MIN and Z MAX param eters The DOPING statement also accepts these pa rameters for analytical doping profiles The principal direction of the doping is in the z direction with the parameters CHAR PEAK DOSE START and JUNC TION applying to the z direction The lateral fall off in the radial and angular directions can be controlled nesh three d cylindrical nex angle 30 9 minabtise 1 0 0 stac 0 5 1 0 5 spac 2e 3 spac 9 035 spac 9 035 spac 0 25 spac 0 25 spac 0 03 iy Vs hay pb Vinee Mat Tee 3333333 pac 36 9 spac 35 0 A ARA AO AER Bop tntninrarnhtntin qn un a a li CA IA te Rea oo man 000 ooooooooD gt o M ira al a AO tm ra m Mm Mm m m Mm m m m IA A A A A A A AA gt ae gt Ge gt er gt Ge ae gt ae gt Ge gt ae gt a mex 2 3 material 3i icon min C 615 max 0 65 Z max 1 7 neterial oxide min C 05 r tmrax 0 65 z mi9 7 Z max 5 3 mate ial oxide min C 0 r
14. ics of the a Si H TFT is simulated to reproduce the experimental transfer curve and output curve Interface traps are specified for the bulk and front and back channel using continuous DEFECT and INTDE FECT statements Interface fixed charge is also included In a TFT LCD pixel simulation the following a Si H TFT model and circuit behavior should be considered 1 the charging state which is driven by the on current of an a Si H TFT 2 the holding state which is affected by the off current of an a Si H TFT 3 the voltage drop characteristics of an a Si H TFT and LC capacitance The MixedMode circuit description input deck is listed below begin Voom 6 0 5 vg 10 0 pulse 0 20 O le 6 le 6 40us 180us vd 3 0 0 pulse 0 10 O le 6 le 6 160us 320us atft 2 source l gate 3 drain infile a Si TFT str width 41 re 2 4 1 28k co 4 5 317r tle 5 109 cst 2 6 1 06p colg 4 0 125 bLC 5 6 infile lc cap lib function my le rc numeric vchange 0 5 dtmin le 9 imaxtr 50 Options print Load 1intile tft de log outfile tft tran O lus 320us end The Simulation Standard In Figure 3 the AMLCD pixel dynamics are correctly reproduced accordingly the source voltage shape shows pixel charging holding and voltage drop 5 Conclusion ATLAS TFT MixedMode is a useful tool for TFT AMLCD unit pixel simulation and predicts transient pixel characteristics with trap density of a Si H TFT and liquid crystal modeling thro
15. istive switching especially for RRAM materials are still under investiga tion so developing better models which can account for experimental I V curves of these devices are useful for comprehending the operation and optimizing both the operation and structure of the device For that purpose the C Interpreter is very helpful It enables the user to create their own models in order to investigate material and device behavior For example assuming that the phase change or conduc tion path destruction re formation is dependent on the material s temperature and its resistivity change can be expressed as a mobility change a user definable tem perature dependent C Interpreter mobility model can be used The Giga module is used to account for self treating effects Figure 1 is an example in which the mobility is described as a function of temperature depending on the range and the direction of the temperature and mobility change 3 Simulation Results The device structure simulated is very simple as shown in Figure 2 A resistive switching material with the user defined mobility model is sandwiched between two electrodes A bipolar triangular voltage sweep of 200ns shown is applied as shown at the top of Figure 3 The Simulation Standard Triangular Voltage sweep I V hysteresis loop Data from pcm log Data from pcm log Anode Current A Anode Voltage V Temperature at X 0 5um Y 0 3um Data from pem lop Temperature
16. n the solar cell device during light illumination This is very useful for simulation of multi junction devices Figure 2 shows an elevated contour plot of photogenera tion rate in a simple thin film amorphous silicon solar cell Note that in this figure the device has an opaque metal contact in the center of the structure Once photogenera tion rates are obtained terminal currents can be evaluated to determine the quantum efficiency of the solar cell One useful feature of the Luminous module is ray trac ing This feature enables the analysis of more advanced solar cells designs Besides studying the photogeneration rates due to a normal incident light beam the photogen eration rates due to an angled light beam can also be studied This is shown in Figure 3 For large area solar cell devices the surface of the cell will take the shape of inverted cone pyramid etc depending on the type of optics Figure 4 shows the photogenera tion in a silicon solar cell when light impinges on the cell which has pyramids on the surface From this figure it can be seen that the light path inside the semiconduc tor is diverted from its original path due to the pyramid surface This causes the contour of the photogeneration rates to be a saw tooth shape as shown in the right hand side of Figure 4 April May June 2008 a Si Solar Cell Simulation Plot of Solar Cell Figure 5 Current Voltage Characteristics of Amorphous Silicon Solar Cell Devic
17. on a device numerical modeling approach is attractive and predictive Silvaco s ATLAS MixedMode module enables users to predict device performance and also the circuit level behavior of transient switching characteristics in AMLCD pixel simulation Figure 1 shows conventional equivalent cir cuit diagram of the unit pixel 2 Liquid Crystal Capacitance Model In order to simulate transient behavior of the unit pixel in MixedMode a time and voltage dependent liquid crystal capacitance model is to be used E y Ep 0 y xp il The total amount of LC capacitance C is calculated from above e and the geometry of the LC cell as follows Es E LW Co I here L and W are total area of the LC cell which is con nected to each TFT and D is the thickness of the LC cell cell gap The parameters used in the simulation are listed in Table 1 Liquid Crystal Parameters E 152um W 148um 10 02um 51 0 mm s 51 2 ms mm 100ms 1 887V 3 1 D Y D time Ve PL Table 1 LC parameters in MixedMode simulation The Simulation Standard Page 4 source Figure 1 This figure shows AMLCD unit pixel 3 MixedMode Circuit Description In order to simulate liquid crystal capacitance with MixedMode a user defined two terminal function with C Interpreter is necessary Bxxx n n infile filename function function_name Bxxx is a user defined name and infile filename is the source
18. r geometry In such cases it is necessary to use ATLAS3D together with the 3D modules for solar cell simulation Figure 7 shows the 3D structure of a large area solar cell device The potential distribution in the solar cell device after the light illumination is displayed in this figure 4 Conclusion In conclusion Silvaco TCAD tools provide a complete so lution for researchers interested in solar cell technology It enables researchers to study the electrical properties of solar cells under illumination in both Two and Three dimensional domains The simulated properties include IV characteristics spectral response quantum efficiency photogeneration rates potential distribution etc In ad dition the software is also capable of simulating amor phous silicon solar cell devices and large area solar cells with texture surfaces Silvaco is the one stop vendor for all companies interested in advanced solar cell technol ogy simulation solutions References 1 ATLAS User s Manual Silvaco Santa Clara California USA 2 Silvaco ATLAS website http www silvaco com products device __ simulation atlas html The Simulation Standard TCAD TFT AMLCD Pixel Simulation 1 Introduction The main drawbacks of circuit level simulation are the many assumptions made of the device model For ex ample the a Si H TFT model assumes that the channel is uniform and ignores interface trap effects For more accurate circuit level simulati
19. to change amorphous phase to crystal phase or to re form conduction path SEdit yoji UL40 TCAD PCRAM COMP pom ib BRA Ee if tenp gt Treset Mobcal Mobcry 0 8 temp Treset if Mobcal lt Mobamo Mobcal Mobamo if Mob i gt Mobcal amp amp Mobs i gt Mobcry Mob i Mobcal if Mobs i lt Mobcry Mobcal Mobamo 0 3 temp Tset if Mobcal gt Mobcry Mobcal Mobcry Mob i Mobcal else if temp gt Tset Mobcal Mobamo 0 12 temp Tset if Mobcal gt Mobcry Mobcal Mobcry if Tmp i lt temp amp amp Mob i lt Mobcal Mob i Mobcal Mobs i Mobcal else if temp gt 300 0 Mob i Mob i else Mobcal Mobcry Mob i Mobcal Mobs i Mobcal Tmp i temp printf sg g g i g n x y temp i Mob i mup Mob i return 0 0 ok e UNIX Simucac 2008 y Figure 1 A description of a user defined mobility model as a function of temperature using C INTERPRETER April May June 2008 Initial SET state Switch to RESET state Data from pem_ini sol Data from pen_cl L sol Microns Microns Switch to SEI state Stay in SET state Data fron pem_c3_L sol Data from pen_c4_1 sol Figure 2 Mobility change at initial 50ns 150ns and 200ns 2 A Simple Material Model for Resistive Switching Operation The detailed mechanisms of the res
20. ugh a user defined two terminal device TCAD approach to pixel design and combined device level capacitance characteristics is necessary for both circuit and device performance References 1 Dynamic Characterization of a Si TFT LCD pixels Hitoshi Akoi ULSI Research Laboratory HP Labs Hewlett Packard Com pany 3500 Deer Creek Rd Palo Alto CA 94304 2 ATLAS User s Manual Silvaco Santa Clara California USA The Simulation Standard Page 6 ATLAS Data from tft_tr log charging drain 5e 05 0 0001 0 00015 0 0002 0 00025 0 0003 0 00035 Figure 3 TFT AMLCD pixel voltage April May June 2008 Material Modeling of Resistive Switching for Non Volatile Memories Using ATLAS C Interpreter Giga 1 Introduction Recently a variety of materials having large non volatile resistance change have been studied as potential candi dates for next generation non volatile memory devices For example chalcogenides for PCM Phase Change Memory 1 and perovskite oxides or transition metal oxides for RRAM Resistive Random Access Memory 2 etc The basic operation of these devices is as follows there are two states RESET and SET The RESET state is a high resistance state obtained by applying a sufficiently high electrical pulse to change crystal phase to amorphous phase for PCM or to break the conduction path for RRAM The SET state is a low resistance state obtained by applying a lower and longer pulse
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