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Resonant Tunneling Diodes (RTDs)

Resonant Tunneling Diodes (RTDs). Ni, Man EE 666 Advanced Electronic Devices April 26, 2005. Outline. Introduction RTD basics RTDs in different material systems III-V IV, II-VI, etc. Molecular RTDs RITDs (Resonant Interband Tunneling Diodes) Applications High-frequency oscillator

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Resonant Tunneling Diodes (RTDs)

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  1. Resonant Tunneling Diodes(RTDs) Ni, Man EE 666 Advanced Electronic Devices April 26, 2005

  2. Outline • Introduction • RTD basics • RTDs in different material systems • III-V • IV, II-VI, etc. • Molecular RTDs • RITDs (Resonant Interband Tunneling Diodes) • Applications • High-frequency oscillator • Digital applications (HBT, HEMT, CMOS) • RTTs (Resonant Tunneling Transistors) • Conclusion

  3. Why RTDs? • Intrinsic bistability and high-speed switching capability (e.g., 1 ps switch, fmax~1 THz) • Low power consumption • Small device footprint • Increased functionality

  4. What is an RTD? • RTD: Two potential barrier sandwiching a well region.

  5. How does an RTD work? Peak current density: IP=ION Peak-to-valley current ratio (PVCR) = ION/IVALLEY

  6. Valley Current • Theory underestimates valley current because of: • (i) scattering by phonons and impurities • (ii) extra tunneling via impurity states in the barriers • (iii) tunneling via X and L states • (iv) disorder in alloy barriers • (v) interface steps and roughness I IP IV V

  7. III-V RTDs • GaAs family • AlGaAs/GaAs/AlGaAs • InP family (IP=500 kA/cm2, PVCR=52) • InGaAs/AlAs/InAs

  8. RTDs in other materials systems • IV • Si0.7Ge0.3/Si/Si0.7Ge0.3 on a relaxed Si0.7Ge0.3bufffer layer • PVCR=1.2 due to the low conduction-band offsets (< 0.5 eV) • II-VI • HgCdTe/HgTe • PVCR=1.4 • Mixed Crystalline • MnTe/InSb/MnTe, PVCR=1.7 at 77 K • CaF2/CoSi2, PVCR=2 • AlAs/ErAs/AlAs on GaAs substrate • Amorphous • SiO2/Si/SiO2, Si3N4/Si/Si3N4 • SiC/Si/SiC, PVCR=9.4

  9. Molecular RTDs • Small (~1.5 nm): ultra-dense IC • Natural nanometer-scale structure: identical in vast quantities James C. Ellenbogen, “A brief overview of nanoelectronic devices”

  10. Resonant Interband Tunneling Diodes (RITDs) • A hybrid of RTD and Esaki diode • Type II heterojunction RITD • p-n type I heterojunction double quantum well RITD • Type II heterojunction RITD Electron injection

  11. RITDs • p-n type I heterojunction double quantum well RITD PVCR = 144 H. H. Tsai, et al., IEEE EDL, Vol. 15, no. 9, Sep. 1994

  12. Applications • Oscillator ------ NDR • Digital Logic ------ Bistability

  13. Applications — Oscillator LC Oscillator L L R L R - R C C C Rtot = Ideal Case Real Case One-port Oscillator w = 1/ LC w = 1/ LC

  14. Applications — Digital Logic • Logic circuits ------ Bistability • Integration with transistors (HEMT, HBT, CMOS) is a requirement for a complete IC technology based on RTDs • Transitors: Input/output isolation, controllable gain • RTDs: increased functionality, enhanced circuit speed, reduced power consumption • It’s all about Load lines!

  15. Inverter VDD • Concept: A digital inverter cell with a low on-state current for low static power dissipation • Evaluation: The low on-state current also reduces the switching speed because the current stays low until the RTD again reaches resonance I I VIN=LO VOUT=HI VOUT VIN VOUT VIN=HI VOUT=LO

  16. Memory cell VRTD • Concept: A static memory cell with a low device count and low static power dissipation • Evaluation: Works and is fast, the difficulty is making RTDs reproducibly and integrating them with IC process IRTD Read Select RTD2 RTD1 Write Select RTD1 Read Data Write Data Storage Node IRTD RTD2 VLO VHI VRTD Storage Node

  17. Multivalued Logic I R Voltage VOUT RTD1 I RTD2 VOUT • There is some difference between the two devices such that they reach the peak current at different applied biases.

  18. RTD/Transistor Monolithic IC • RTD-HEMT J. Hontschel, et al.

  19. RTD/Transistor Monolithic IC • RTD-HBT S. Thomas III, et al., J. Vac. Sci. Technol. B 18(5), Sep/Oct 2000

  20. RTD-CMOS • Substantial improvement in speed, power dissipation, and circuit complexity over CMOS only circuits. • A hybrid integration process for RTD to be transferred and bonded to CMOS J. I. Bergman, et al., IEEE EDL, Vol. 20, no. 3, March 1999

  21. RTD-CMOS A 1-bit conventionalCMOS comparator: 18 devices A 1-bit RTD/CMOS comparator: 6 devices J. I. Bergman, et al., EDL, 1999

  22. Resonant Tunneling Transistors (RTTs) • Three-terminal (RTTs) vs two-terminal (RTDs) • Enhanced isolation between input and output • Higher circuit gain • Greater fan-out capacity • Greater Versatility in circuit functionality • Better suited for large circuits than RTD-only circuits Emitter Base Base Collector Collector

  23. Multivalued RTTs • Different quantum levels: different current peaks in I-V • Square well: not evenly spaced • Parabolic well: energy levels and the corresponding current peaks are all evenly spaced • Difficult to make the multiple peaks of comparable magnitude

  24. Multivalued RTTs • Double-barrier structure in Emitter region Federico Capasso, et al., IEEE Trans. Electron Devices, Vol. 36, no. 10, Oct. 1989

  25. Promising Future

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