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