resonant tunneling diodes (rtds) ni, man ee 666 advanced electronic devices april 26, 2005
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Resonant Tunneling Diodes(RTDs)
Ni, ManEE 666
Advanced Electronic DevicesApril 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
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
IP
IV
I
V
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
Electroninjection
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
Applications
• Oscillator ------ NDR• Digital Logic ------ Bistability
Applications — Oscillator
C L C L R C L R
= 1/ LC
- R
Rtot =
Ideal Case
LC Oscillator
Real Case One-port Oscillator
= 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
• 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
VDD
VINVOUT
I I
VOUTVIN=HIVOUT=LO
VIN=LOVOUT=HI
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
WriteData
ReadData
WriteSelect
ReadSelect
VRTD
IRTD
RTD1
RTD2
RTD1RTD2
VRTD
IRTD
StorageNode
Storage NodeVLO VHI
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
VOUT
VOUT
I
I
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)
EmitterBase Base
Collector 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
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