Transcript RTD
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 Digital applications (HBT, HEMT, CMOS) • RTTs (Resonant Tunneling Transistors) • Conclusion 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 What is an RTD? • RTD: Two potential barrier sandwiching a well region. How does an RTD work? Peak current density: IP=ION Peak-to-valley current ratio (PVCR) = ION/IVALLEY Valley Current I Theory underestimates valley current because of: IP (i) scattering by phonons and impurities (ii) extra tunneling via impurity states in the barriers (iii) tunneling via X and L states IV (iv) disorder in alloy barriers V (v) interface steps and roughness III-V RTDs • GaAs family AlGaAs/GaAs/AlGaAs • InP family (IP=500 kA/cm2, PVCR=52) InGaAs/AlAs/InAs RTDs in other materials systems • IV Si0.7Ge0.3/Si/Si0.7Ge0.3 on a relaxed Si0.7Ge0.3 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 CaF2/CoSi2, PVCR=2 AlAs/ErAs/AlAs on GaAs substrate • Amorphous SiO2/Si/SiO2, Si3N4/Si/Si3N4 SiC/Si/SiC, PVCR=9.4 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” 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 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 Applications • Oscillator ------ NDR • Digital Logic ------ Bistability Applications — Oscillator LC Oscillator C L C L R C L R -R Rtot = Ideal Case w = 1/ LC Real Case One-port Oscillator w = 1/ LC 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! Inverter I VDD I VIN=LO VOUT=HI VIN VOUT VIN=HI VOUT=LO VOUT • 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 Memory cell VRTD Write Select Read Select RTD1 Write Data Storage Node RTD2 IRTD IRTD RTD2 RTD1 Read Data VLO Storage Node VHI 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 Multivalued Logic I R Voltage VOUT I RTD1 RTD2 VOUT • There is some difference between the two devices such that they reach the peak current at different applied biases. RTD/Transistor Monolithic IC • RTD-HEMT J. Hontschel, et al. RTD/Transistor Monolithic IC • RTD-HBT S. Thomas III, et al., J. Vac. Sci. Technol. B 18(5), Sep/Oct 2000 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 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 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 Collector Base Collector 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 Multivalued RTTs • Double-barrier structure in Emitter region Federico Capasso, et al., IEEE Trans. Electron Devices, Vol. 36, no. 10, Oct. 1989 Promising Future