Clemson University TigerPrints All Theses Theses 6-2008 THE ELECTRICAL CHARACTERIZATION OF TANTALUM CAPACITORS AS MIS DEVICES Brian Holman Clemson University, [email protected] Follow this and additional works at: https://tigerprints.clemson.edu/all_theses Part of the Electrical and Computer Engineering Commons Recommended Citation Holman, Brian, "THE ELECTRICAL CHARACTERIZATION OF TANTALUM CAPACITORS AS MIS DEVICES" (2008). All Theses. 393. https://tigerprints.clemson.edu/all_theses/393 This Thesis is brought to you for free and open access by the Theses at TigerPrints. It has been accepted for inclusion in All Theses by an authorized administrator of TigerPrints. For more information, please contact [email protected]. THE ELECTRICAL CHARACTERIZATION OF TANTALUM CAPACITORS AS MIS DEVICES A Thesis Presented to the Graduate School of Clemson University In Partial Fulfillment of the Requirements for the Degree Master of Science Electrical Engineering by Brian Holman August 2008 Accepted by: Dr. William R. Harrell, Committee Chair Dr. James E. Harriss Dr. Pingshan Wang 1 ABSTRACT Electrical characteristics of a new class of tantalum capacitor are presented. Specifically, this type of tantalum capacitor is manufactured by KEMET Electronics Corporation and utilizes Poly(3,4-ethylenedioxythiolphene) (PEDOT) as the cathode material. There are two capacitor varieties based on the polymerization method used for the PEDOT. One uses In-Situ polymerization, and the other uses Pre-Polymerization. Existing polymer Ta capacitors use In-Situ polymerization while Pre-Polymerization is a new technique of cathode application. We investigated both types of devices to determine what, if any, performance benefits were gained by using Pre-Poly. In a basic form Ta capacitors consist of a Ta anode, Ta2O5 dielectric, and PEDOT cathode polymerized to be a semiconductor. Based on the simplified representation of the capacitor materials these devices were investigated as MIS structures and C-V, I-t, and I-V measurements were made. C-V measurements were used to observe characteristics of MIS operation in the devices. Measurements were made from room temperature down to 100K in attempts to suppress the leakage current in these devices. I- t and I-V measurements were used to identify dominating leakage current mechanisms. The Poole-Frenkel Effect, the Schottky Effect, and Space-Charge-Limited Current were observed in In-Situ polymerized devices, while the Poole-Frenkel Effect, the Schottky Effect, and Fowler-Nordheim Tunneling were observed in Pre-Poly devices. Overall, both devices showed voltage dependent capacitance. The Pre-Poly devices generally had lower levels of leakage current. However, due to differing properties of the polymer in each case In-Situ devices exhibited less capacitance loss at low temperatures. ii ACKNOWLEDGMENTS I would like to thank my advisor, Dr. Harrell, for his help and guidance in completing my research and thesis. I would like to thank Dr. Yuri Freeman and KEMET Electronics Corporation for the opportunity to conduct this research and for financially funding the project. I would like to thank Dr. Harriss and Dr. Wang for their help in reviewing my work. iii TABLE OF CONTENTS Page TITLE PAGE....................................................................................................................i ABSTRACT.....................................................................................................................ii ACKNOWLEDGMENTS ..............................................................................................iii LIST OF TABLES..........................................................................................................vi LIST OF FIGURES .......................................................................................................vii CHAPTER I. INTRODUCTION .........................................................................................1 1.1 Tantalum Capacitors ..........................................................................1 1.2 Summary of Chapters ........................................................................1 II. REVIEW OF CAPACITOR THEORY.........................................................4 2.1 Parallel Plate Capacitors ....................................................................4 2.2 Electrolytic Capacitors.......................................................................5 2.3 Conclusion .........................................................................................7 III. TANTALUM CAPACITORS .......................................................................8 3.1 Evolution of Tantalum Capacitors.....................................................8 3.2 Construction.....................................................................................12 3.3 Reliability Issues..............................................................................16 3.4 Electrical Performance.....................................................................20 3.5 Conclusion .......................................................................................22 IV. MODERN TANTALUM CAPACITORS...................................................24 4.1 Modern ICP Cathode .......................................................................24 4.2 Future Trends...................................................................................28 4.3 MIS Theory......................................................................................30 4.3.1 Flatband................................................................................36 4.3.2 Accumulation.......................................................................37 iv Table of Contents (Continued) Page 4.3.3 Depletion..............................................................................38 4.3.4 Inversion ..............................................................................42 4.3.5 Capacitance-Voltage Curves................................................43 4.4 Tantalum MIS Devices ....................................................................46 4.5 Conclusion .......................................................................................48 V. CONDUCTION MECHANISMS ...............................................................49 5.1 The Poole-Frenkel Effect.................................................................49 5.2 Space-Charge-Limited Current........................................................53 5.3 Fowler-Nordheim Tunneling ...........................................................55 5.4 The Schottky Effect .........................................................................57 5.5 Other Possible Mechanisms.............................................................59 VI. TANTALUM CAPACITOR CHARACTERIZATION..............................61 6.1 Introduction......................................................................................61 6.2 Equipment and Procedures ..............................................................63 6.2.1 Equipment............................................................................63 6.2.2 Procedures............................................................................64 6.3 Types of Devices..............................................................................66 6.4 C-V Characteristics..........................................................................68 6.4.1 Low Temperature Measurements ........................................71 6.4.2 Temperature Dependence ....................................................75 6.4.3 Room Temperature Measurements......................................84 6.5 I-V Characteristics ...........................................................................87 6.5.1 Current-Time Measurements ...............................................88 6.5.2 Temperature Dependence ....................................................93 6.5.3 Leakage Mechanisms...........................................................97 6.6 Conclusion .....................................................................................115 VII. SUMMARY AND CONCLUSIONS ........................................................118 APPENDIX..................................................................................................................122 REFERENCES ............................................................................................................126 v LIST OF TABLES Table Page 5.1 SCLC current density equations for varying trap distributions............................................................................................55 6.1 Initial nominal capacitance versus 100K zero bias capacitance.............................................................................................76 vi LIST OF FIGURES Figure Page 2.1 Electrolytic capacitor – cathode plate extension through electrolyte ...................................................................................6 3.1 Tantalum pellet after pressing and sintering................................................13 3.2 Anode and cathode plate structures in tantalum pellet ................................15 3.3 Completed chip-type solid tantalum capacitor ............................................16 3.4 MnO2 as contributing force to cracks ..........................................................17 3.5 MnO2 self-healing properties.......................................................................18 3.6 Theoretical ignition failure sequence...........................................................19