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Homo-junction InGaAs Band-to-band Tunneling Diodes

Homo-junction InGaAs Band-to-band Tunneling Diodes. Cho, Woo-Suhl cho68@purdue.edu. Motivation. Moore’s law and MOSFET scaling. Downscaling of Transistors**. Moore’s law*. Transistor dimensions scale to improve performance, and reduce cost per transistor

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Homo-junction InGaAs Band-to-band Tunneling Diodes

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  1. Homo-junction InGaAs Band-to-band Tunneling Diodes Cho, Woo-Suhl cho68@purdue.edu

  2. Motivation Moore’s law and MOSFET scaling Downscaling of Transistors** Moore’s law* • Transistor dimensions scale to improve performance, and reduce cost per transistor • Increased packing density followed by Moore’s law  ** http://www.intel.com/technology/mooreslaw/ * http://en.wikipedia.org/wiki/Moore's_law/

  3. Motivation Dramatic Increase of Power Consumption • CMOS microprocessors have reached the maximum power dissipation level that BJT based chips had • New device concept or idea required * R. R. Schmidt, and B. D. Notohardjono, “High-End Server Low-Temperature Cooling”, IBM J. Res. & Dev., vol.46, No. 6, p. 739, 2002

  4. Motivation Power consumption in MOSFETs • Downscaling of MOSFETs • Leakage current usually fixed at IOFF=0.1μA/ μm • Increased transistor density per chip (>1 billion) • Increase of power consumption & heat generation * S. Borkar, “Getting Gigascale Chips: Challenges and Opportunities in Continuing Moore’s Law”, ACM Queue, vol. 1, No. 7, p. 26, 2003

  5. Motivation Limitations of MOSFET Scaling log(Id) ION ION IOFF IOFF Vg VT` VDD VDD • Limitations of scaling • Almost non-scalable supply voltage VDD • Physical limit of Sub-threshold Swing (SS) • Device with SS ≤ 60mV/dec is highly desired

  6. Motivation New Device Candidate: BTBT FETs BTBT FETs MOSFETs +Vg +Vg S Possible candidate to replace MOSFETs S EF EF D D • Minority carrier transport over the barrier • Diffusion of hot electrons • Depends on the thermal distribution of carriers • SS ≥ 60mV/dec limit • Majority carrier transport through the barrier • Band-to-band tunneling of cold electrons • Boltzmann tails are ignored • SS ≤ 60mV/dec possible

  7. Motivation Study of BTBT Diodes BTBT Diode BTBT FET No Gate Bias: OFF STATE Gate • Learn about the tunneling properties • Test the potential of a given material as a TFET • Test simulation model to design BTBT Gate oxide +Vg Source N+ source S D BTBT P+ I N+ P+ drain Drain Buried Oxide Substrate Positive gate bias: ON STATE • Horizontal structure • Difficult to get sharp interface • Need excellent channel control through gate contact • Low on current • Vertical structure • Sharp p-n interface can be more easily fabricated • Experimental data exist 2

  8. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  9. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  10. Approach Simulation Approach and Objective • Use full-band and atomistic quantum transport simulator based on the tight-binding model (OMEN) to model TDs • Ballistic transport using NEGF • Reproduce and understand experimental data • Homogeneous InGaAS tunneling diodes (TDs) fabricated and measured at Penn State, a partner in the MIND center 3

  11. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  12. Basic Physics Band-to-band Tunneling • High doping density • More degeneracy • High electric field • Small width barrier • Increase tunneling current EFP EFP EFN W EFN W P+ P+ N+ • Narrow band gap • Increase tunneling probability • Material property N+

  13. Basic Physics Use of InGaAs Indirect Eg Direct • Small band gap material: Si  Ge  III-V (InAs) • Indirect semiconductor Direct semiconductor • In0.53Ga0.47As: Lattice matched to InP

  14. Basic Physics I-V Characteristics of BTBT Diodes I Thermionic current Tunneling current P+ P+ P+ P+ P+ EFN IP Excess current (Gap state current) EC EFN EV EV EV EV EC NDR EFN EFN EC N+ EFP EFP EFP EFP EFP N+ IV N+ EFN N+ EC VP VV V Zener current N+ EC EV

  15. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  16. Penn State: InGaAs Diode Device Structure and Doping Profile Measured I-V Simulated device Fabricated device • I-V chracteristics of BTBT diodes • A InGaAs lattice matched to InP BTBT Diode • NA=1020/cm3, ND=5×1019/cm3 P+ 10nm NA=8×1019 3nm I N+ x 20nm ND=4×1019 In0.53Ga0.47As

  17. Junction Modeling Doping Profiles at the Junction Linear doping Abrupt doping 3nm 3nm D (N+) D (N+) S (P+) S (P+) x x ND=4×1019/cm3 10nm 10nm 20nm 20nm ND=4×1019/cm3 0 0 NA=8×1019/cm3 NA=8×1019/cm3

  18. Junction Modeling Effect of Junction Abruptness • Only Zener tunneling branch is shown • Step junction uses Rs closer to the estimated value (20Ω)

  19. Junction Modeling I-V Characteristics: Experiment vs Simulation I-V Characteristics: Experiment vs Simulation • Step junction is used • Zener current matched • Too low series resistance: RS=13.5Ω vs. Estimated value: RS=20Ω • Poor reproduction of forward-biased region • Low peak and valley currents • Thermionic current turns on at large bias • Investigate potential explanations for the observed disagreements 7

  20. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  21. Band Gap Narrowing Causes of BGN: High Doping Effects Impurity Bands E EC Donor Impurity Band ΔED EC EV ΔED ρDOS(E) • High doping level ≥ 1018/cm3 • D.O.S depends on the impurity concentration • Overlapping impurity states form an impurity band ~200meV BGN • Random distribution of impurities • Potential fluctuation of the band edges • Impurity states tails into the forbidden gap

  22. Band Gap Narrowing BGN Calculation Model • S. C. Jain, and D. J. Roulston, Solid-State Electronics, vol. 34, No. 5, p. 453, 1990 Jain-Roulston model* Before BGN After BGN ΔEC(min) ΔEC(maj) P+ S (P+) D (N+) Eg1 Eg S (P+) D (N+) Eg2 EF ΔEV(maj) ΔEV(min) N+ • Advantages • Compact model calculated based on many-body theory • Compute BGN as function of doping concentrations (N), and material parameters (A, B, C) • Compute band shifts in major and minor bands separately for all materials • No need for experimental fitting parameters

  23. Band Gap Narrowing BGN calculation for In0.53Ga0.47As n-In0.53Ga0.47As p-In0.53Ga0.47As ND=4e19/cm-3 NA=8e19/cm-3 ΔEg ΔEg ΔEV ΔEc ΔEV ΔEc • Not negligible shift in minor band • Less BGN than n-type material • Most shift occurs at conduction band * S. C. Jain, J. M. McGregor, and D. J. Roulston, and P.Balk, Solid-State Electronics, vol. 35, No. 5, p. 639, 1992. * James C. Li, Marko Sokolich, Tahir Hussain, and Peter M. Asbeck, Solid-State Electronics, vol. 50, p. 1440, 2006.

  24. Band Gap Narrowing Inclusion of BGN in Tight-Binding In0.53Ga0.47As before BGN In1-x1Gax1As-In1-x2Gax2As after BGN In1-x1Gax1As In1-x2Gax2As Eg1 0.6450eV S (P+) Eg2 0.5804eV D (N+) S (P+) S (P+) D (N+) D (N+) 0.75eV 23nm 10nm • Calculate new compositions of In and Ga from the reduced band gaps • Calculate tight-binding parameters from the empirical parameters of InAs and GaAs, and Bowing parameters • Shift band edges In0.64Ga0.36As In0.71Ga0.29As 11

  25. Penn State: InGaAs Diode The effect of BGN 1. 2. 2. 3. 1. • Closer to the experimental data: Effect of BGN • An increase of the series resistance • An increase of tunneling current including the peak current • An earlier turn-on of the thermionic current • Discrepancies: • Mismatch in NDR region, and low valley current • A shift of the thermionic current 1

  26. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  27. What can shift the thermionic current? • * Effect of doping variation • Influence of the donor concentration • Influence of the acceptor concentration

  28. Effect of Doping Variation (1) Variation of the donor concentration ND Experiment data ND=8e19/cm3 ND=4e19/cm3 ND=2e19/cm3 NA=8e19/cm3 P+ P+ P+ EF EF EF N+ N+ N+ • Higher tunneling current for higher ND • Increase in tunneling window ( ) • No shift of the thermionic current onset • No variation of potential barrier ( ) 8

  29. Effect of Doping Variation (2) Variation of the acceptor concentration NA Experiment data NA=4e19/cm3 NA=8e19/cm3 NA=1.2e20/cm3 ND=4e19/cm3 P+ P+ P+ EF EF EF N+ N+ N+ • Small increase in tunneling current for higher NA • Increase in tunneling window ( ) • Earlier turn-on of the thermionic current for lower NA • Lowered potential barrier ( ) • No strong influence 9 8

  30. What can increase the valley current? I Thermionic current Tunneling current IP Excess current (Gap state current) NDR IV VP VV V Zener • * Excess current • Existence of excess current via gap states • Influence of excess current

  31. Excess Current Source of Excess Current (Ix) E Conduction band EC A EC C qV Tail states V EFN Eg P+ B EV EV Valence band EFP N+ E ρDOS(E) x • Tunneling + Energy loss mechanism through gap states* • Gap States are mostly originated from the band edge tails • A: Tails of acceptor levels extending to the forbidden gap • B: Tails of donor levels extending to the forbidden gap * A. G. Chynoweth, W. L. Feldmann, and R. A. logan, Phys. Rev, vol. 121, p. 684, 1961

  32. Excess Current (1) Existence of Ix: Intrinsic I-V data σ • No series resistance is included • Purely thermionic current beyond the valley in the simulation data • Lower slope of the experiment data (σ≈⅓ of q/kT) at the valley confirms the existence of Ix • Assume that there is a dominant Ix around the valley

  33. Excess Current Excess Current Calculation * D. K. Roy, Solid-State Electron., vol. 14, p.520, 1971 • Exponential nature of the excess current* • Linear increase of the currents beyond the valley

  34. Excess Current (2) The Effect of Excess Current • Effects of excess current (BGN is included) • Increased current around and beyond the valley • Closer match to the experiment results

  35. Penn State: InGaAs Diode Effect of BGN V=-0.4V Efl Efr (Ix included) V=0.95V VV=0.64V VP=0.35V Efr Efr Efr Efl Efl Efl

  36. Temperature Dependence (3) The Effect of Temperature • Effects of temperature • 20meV more BGN occurs at room temperature • Increase of peak and NDR region currents

  37. Outline • Approach • Basic Physics of Tunneling Diodes • Band-to-band Tunneling • I-V Characteristic of BTBT Diodes • InGaAs Diodes • Junction Modeling and Effects of Junction Abruptness • Solution to Increase Tunneling Currents • Band Gap Narrowing Effect and Modeling • Solution to Shift the Onset of Thermionic Current • Effects of Doping Variation • Excess Current • Temperature Dependence • Summary and Future Work

  38. Summary • Investigate the performances of homogeneous InGaAs III-V band-to-band-tunneling (BTBT) diodes • Study the tunneling properties of a given material and its potential as a BTBT Field-Effect Transistors (TFETs) • Use full-band and atomistic quantum transport solver based on tight-binding to simulate BTBT diodes • Coherent tunneling (no e-ph) • Compare the simulation results to experimental data from Penn State RESULTS OBJECTIVE • BGN provides good agreement with experimental data for tunneling currents: Zener and peak currents • Excess current increase current around and beyond valley • Current in NDR region is not well captured • Solution: T-dependence, e-ph scattering APPROACH

  39. Conclusion & Future works • To investigate tunneling device, high doping effects such as BGN, and current via gap states should be considered • Electron-phonon scattering should be included to examine the effect on the increase of the current in the NDR region • Exploring some other scattering mechanisms that may explain the mismatches between the experiments and simulation results • Need the verification of the approach by analyzing another fabricated device • The approach can be applied to the analysis of other tunneling devices, such as the broken gap heterostructure diodes, and TFETs

  40. Acknowledgement Prof. Klimeck Prof. Lundstrom and Prof. Garcia Dr. Mathieu Luisier All NCN Students and Group Members Thank you!

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