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, f max ~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: I P =I ON Peak-to-valley current ratio ( PVCR) = I ON /I VALLEY

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 IPIP IVIV I V

7. III-V RTDs GaAs family  AlGaAs/GaAs/AlGaAs InP family ( I P =500 kA/cm 2, PVCR=52)  InGaAs/AlAs/InAs

8. RTDs in other materials systems IV  Si 0.7 Ge 0.3 /Si/Si 0.7 Ge 0.3 on a relaxed Si 0.7 Ge 0.3 bufffer 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  CaF 2 /CoSi 2, PVCR=2  AlAs/ErAs/AlAs on GaAs substrate Amorphous  SiO 2 /Si/SiO 2, Si 3 N 4 /Si/Si 3 N 4  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

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

13. Applications Oscillator ------ NDR Digital Logic ------ Bistability

14. Applications — Oscillator C L C LR C LR  = 1/ LC - R R tot = Ideal Case LC Oscillator Real Case One-port Oscillator  = 1/ LC

15. 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!

16. Inverter 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 V DD V IN V OUT I I V IN =HI V OUT =LO V IN =LO V OUT =HI

17. Memory cell 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 Write Data Read Data Write Select Read Select V RTD I RTD RTD1 RTD2 RTD1RTD2 V RTD I RTD Storage Node Storage Node V LO V HI

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

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

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

21. 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

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

23. Resonant Tunneling Transistors (RTTs) Emitter Base Collector 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

24. 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

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

26. Promising Future