Resonant Tunneling Diodes (RTDs)

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Resonant Tunneling Diodes(RTDs)

• Ni, Man
• EE 666
• Advanced Electronic Devices
• April 26, 2005

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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

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Why RTDs?

• Intrinsic bistability and high-speed switching
  capability (e.g., 1 ps switch, fmax1 THz)
• Low power consumption
• Small device footprint
• Increased functionality

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What is an RTD?

• RTD Two potential barrier sandwiching a well
  region.

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How does an RTD work?
Peak current density IPION
Peak-to-valley current ratio (PVCR) ION/IVALLEY
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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
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III-V RTDs

• GaAs family
• AlGaAs/GaAs/AlGaAs
• InP family (IP500 kA/cm2, PVCR52)
• InGaAs/AlAs/InAs

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RTDs in other materials systems

• IV
• Si0.7Ge0.3/Si/Si0.7Ge0.3 on a relaxed Si0.7Ge0.3
  bufffer layer
• PVCR1.2 due to the low conduction-band offsets
  (lt 0.5 eV)
• II-VI
• HgCdTe/HgTe
• PVCR1.4
• Mixed Crystalline
• MnTe/InSb/MnTe, PVCR1.7 at 77 K
• CaF2/CoSi2, PVCR2
• AlAs/ErAs/AlAs on GaAs substrate
• Amorphous
• SiO2/Si/SiO2, Si3N4/Si/Si3N4
• SiC/Si/SiC, PVCR9.4

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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
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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
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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
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Applications

• Oscillator ------ NDR
• Digital Logic ------ Bistability

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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
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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
• Its all about Load lines!

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Inverter
VDD
I
I
VINLO VOUTHI
VOUT
VIN
VOUT
VINHI VOUTLO

• 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

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Memory cell
VRTD
IRTD
Write Select
Read Select
RTD1
RTD2
RTD1
Read Data
Write Data
Storage Node
IRTD
RTD2
VRTD
VLO
VHI
Storage Node

• 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

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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.

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RTD/Transistor Monolithic IC

• RTD-HEMT

J. Hontschel, et al.
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RTD/Transistor Monolithic IC

• RTD-HBT

S. Thomas III, et al., J. Vac. Sci. Technol. B
18(5), Sep/Oct 2000
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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
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RTD-CMOS
A 1-bit RTD/CMOS comparator 6 devices

• A 1-bit conventional CMOS comparator 18 devices

J. I. Bergman, et al., EDL, 1999
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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
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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

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Multivalued RTTs

• Double-barrier structure in Emitter region

Federico Capasso, et al., IEEE Trans. Electron
Devices, Vol. 36, no. 10, Oct. 1989
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Promising Future