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