Resonant Tunneling Device
Kalpesh Raval
Outline
•
•
•
•
•
•
•
•
•
Diode basics
History of Tunnel diode
RTD Characteristics & Operation
Tunneling Requirements
Various Heterostructures
Fabrication Technique
Challenges
Application & Integration
Conclusion
Diode Basics
•
A diode is a two- terminal device having two active electrodes
(anode/cathode) at each end between which the signal of interest may
flow.
•
Function - The most common function of a diode is to allow an electric
current to pass in one direction (called the forward biased condition) and
to block the current in the opposite direction (the reverse biased
condition). Thus, the diode can be thought of as an electronic version of
a check valve.
•
Forward Bias - Forward-bias occurs when the P-type semiconductor material
is connected to the positive terminal of a battery and the N-type
semiconductor material is connected to the negative terminal. This makes the
P-N junction conduct.
•
Reverse Bias - If a diode is reverse biased, the voltage at the cathode is
higher than that at the anode. Therefore, no current will flow until the diode
breaks down. Connecting the P-type region to the negative terminal of the
battery and the N-type region to the positive terminal, produces the reversebias effect.
I-V Characteristic of conventional diode
Why Resonant Tunneling Device?
•
Are candidates of new functional devices applicable to new
IC technology in so-called “Beyond CMOS” region.
•
is central to the development of new types of semiconductor
nanostructure.
•
Are promising because of:
- capability for high-speed operation
- large Negataive Differentiate Resistance (NDR)
characteristics at room temperature, and
- adaptive design of device characteristics.
Why RTD?
• Lower sub-threshold swing can allow for lower
operating voltages to be used
• Negative differential resistance (NDR) properties
can be exploited to create simpler designs for bistable circuits, differential comparators, oscillators,
etc.
• Leads to chips that consume less power (few mAmps
to a few tenths of a volt)
History of Tunnel diode
•
Invented in August 1957 by Leo Esaki while working at Tokyo Tsushin
Kogyo (now known as SONY). It was a 1st quantum electron device.
•
In 1973, he received the Nobel Prize in Physics for discovering the
electron tunneling effect used in these diodes.
•
A tunnel diode (Esaki diode) can operate at (.3 – 300 GHz) & terahertz
(x10^12 Hz ) at room temperature by using quantum mechanical effects.
•
RTDs are formed as a single quantum well structure that is surrounded
by thin layer barriers known as a double-barrier structure.
•
Initially manufactured by SONY in 1958 followed by General Electric and
other companies from 1960.
•
RTDs are being manufactured in low volume today.
Resonant Tunnel Diode (I-V) Characteristic
•
is a property of electrical circuit elements composed of certain materials in
which, over certain voltage ranges, current is a decreasing function of voltage.
This range of voltages is known as a negative resistance region.
•
Large NDR region allows new types of circuit to be designed based on different
principles than those of conventional circuits.
•
RTD has an intrinsic bistability in the negative differential resistance region
which stems from the heavily doped P-N junction.
Operation
• Heavily doped P-N junction results in extremely
narrow depletion zone (one-millionth of an inch).
• 1000 impurity atoms for ten-million semiconductor
atoms vs. 1 impurity atom used in a normal diode.
• Leads to overlap of conduction and valence bands.
Operation
Tunnel Diode
•
Forward Bias Operation
-Tunnel diodes have a heavily doped p-n junction approximately 10 nm (100
A°) wide which results in overlap between energy bands.
- As voltage begins to increase, electrons at first tunnel through the very
narrow p-n junction barrier causing current to increase.
- As voltage increases further these states become more misaligned and the
current drops this is called negative resistance, because current decreases
with increasing voltage.
- As voltage increases yet further, the diode begins to operate as a normal
diode, where electrons travel by conduction across the p-n junction, and no
longer by tunneling through the p-n junction barrier.
Resonant Tunneling
•
Tunneling is a quantum mechanical phenomenon with no analogy
in classical physics.
•
Occurs when an electron passes through a potential barrier
without having enough energy to do so.
•
Resonant tunneling can be described by the transmission and
reflection processes of coherent electron waves through the
double barrier structure.
•
Can be seen as leaking of the amplitude through the potential
step.
http://en.wikipedia.org/wiki/File:EffetTunnel.gif
Tunneling
•
Even though the transmission coefficient or the tunneling
probability of an incident particle through the barrier is always less
than one and decreases with increasing barrier height and width,
two barriers in a row can be completely transparent for certain
energies of the incident particle. This phenomenon is called
resonant tunneling.
Tunneling Requirements
•
•
•
•
•
Need good (high quality) heterostructures
Quantum well layers
Tunnel layers with high energy-barriers
Requires junction to be abrupt
Above, currently difficult on Si substrate
Heterostructures
•
Heterostructures are structures with two or more interfaces at the
boundaries between the regions of different materials.
•
Compound semiconductors from Group (III – V) are widely used to
fabricate heterostructures. A double-barrier structure can be grown by
molecular beam heteroepitaxy (MBE).
•
Common compounds used are GaAs (gallium arsenide) and AlAs
(aluminum arsenide)
•
Band offsets at the boundary of heterojunction can depend on the quality
of the interface, conditions of growth etc.
•
Types I, II and broken-gap lineup heterojunctions
•
Type I – The lowest conduction-band states occur in the same part of the
structure as the highest valence-band states. (GaAs/AlxGa1-xAs, is type I)
•
Type II – The lowest conduction-band minimum occurs on one side and
the highest valence-band maximum on the other with an energy separation
between the two is less than the lower of the two bulk bandgaps.
(AlAs/AlxGa1-xAs and some Si/SixGe1-x structures)
•
Broken-gap lineup – The bottom of the conduction band on one side drops
below the top of the valence band on the other. (InAs/GaSb gap of
150meV)
Molecular Beam Epitaxy Lab
The most important aspect of MBE is the slow deposition rate, typically
less than 1000 nm (1 micron) per hour, which allows the films to grow
epitaxially.
Conduction band profiles of a double barrier RTD
RTD Challenges
•
Scalability to below certain dimensions result in undesirable surface leakage
current.
•
Since the peak current through an RTD depends exponentially on the barrier
thickness, it is difficult to get device operation unless the gate also controls the
peak current.
•
Integration of a transistor with a pair of RTDs introduces delays to the fast bistable
switching times so, the operational speed of the integrated device can be slower
than the intrinsic speeds of the RTDs themselves.
•
The precise control of the layer thickness and properties of RTDs may require use
of commercial molecular beam epitaxy to achieve the required control.
•
A limited ION/IOFF ratio of 10 compare to the ratio of 105 required by CMOS digital
circuit designers.
•
Fabrication of silicon or silicon-germanium tunnel diodes with high peak-to-valley
current ratios.
•
Reasonably high performance III-V RTDs have been realized. But such devices
have not entered mainstream applications yet because the processing of III-V
materials is incompatible with Si CMOS technology.
•
Although, RTD devices have several issues to overcome as described earlier,
fabrication in a Si compatible material structure substantially reduces the integration
challenges.
•
Current research activity involving RTDs has been in the area of integration on the
silicon platform.
RTD on Si Substrates using Fluoride Alloy Heterostructures
•
RTDs composed of CaF2-barrier/CdF2-well/ CaF2-barrier
heterostructures are expected to be co-integrated with Si-Large Scale
Integration (LSI).
•
High speed LSI and low power consumption can be constructed in Siintegrated circuits.
Problems with compound fluoride structure
•
Thickness of the CaF2 barrier must be very thin to
obtain enough current density due to excessive barrier
height.
•
Since chemical reactivity between Si and CdF2 is very
high, the CdF2 well layer must be grown at very low
temperature.
•
This limits the overall growth temperature of the entire
heterostructure.
•
Results: poor yield of RTD and unstable electrical
properties.
Solution
•
Use CaxCd1-xF2 alloy for the overall structure and Cd-rich based alloy
separation layers to protect the RTD active region.
•
Cd-rich alloy can be grown with good crystallinity at higher temperature
than for pure CdF2 by adding a small amount of CaF2 to CdF2.
Solution
•
The alloy Ca0.5Cd0.5F2 showed the lower barrier height instead of CaF2 and
high PVCR.
•
Similarly, RTDs using the Cd-rich alloy for the well exhibited even larger
peak to valley current ratio at room temperature b/c of good crystallinity.
Result
Ca0.5Cd0.5F2
Ca0.1Cd0.9F2
Applications
•
OptoElectronic Integrated Circuits (OEICS) based on Group III-V RTDs
•
can be used as oscillators, high frequency amplifiers, frequency converters
and photodetectors.
•
MOBILE circuits used in ultra high-speed analog to digital converters (80
Gb/s). MOBILE type structures can be applied to multi-valued threshold
circuits and multi-threshold gates in general.
•
Resonant-tunneling transistors - a negative transconductance that is used in
logic XOR gate with only one transistor.
•
Recent works also explore spin-polarized resonant tunneling, which can be
useful for application in spintronic devices.
Integration of RTDs
•
A pair of RTDs can be integrated with CMOS gate to achieve bi-stable
operation. If connected in series with opposing polarities, results in MOBILE
configuration.
•
Trials of co-integration of the fluoride RTDs with Si-MOSFET have been
reported. However, stability of electrical properties of the RTDs is insufficient
(current drift) to realize circuit operations well at the moment. The origin is
presumably charging effects of carrier traps in heterostructures.
•
Further investigation to improve crystalline quality of the fluoride
heterostructures is necessary to achieve reliable circuit operation of this
device.
Devices
Conclusion
• Tunnel diodes are expected to add another
node in the road
• Three-terminal tunnel devices could add
several nodes at the end of CMOS-scaling
• Challenges are more practical than theoretical
• Uniformity of the tunneling barrier needs to
be improved
References
•
David Bohm, Quantum Theory, Prentice-Hall, New York, 1951.
•
Baym Gordon, Lectures on Quantum Mechanics, Addison-Wesley Company,
Canada, 1969.
•
H. Mizuta and T. Tanoue. “The Physics and applications of Resonant Tunnelling
Diodes.” Cambridge University Press, Cambridge 1995.
•
Koichi Maezawa. “Resonant Tunneling Diodes and Their Application to High-Speed
Circuits.” CSIC 2005 Digest, IEEE.
•
http://en.wikipedia.org/wiki/Resonant_tunnelling_diode
•
http://www.imec.be/esscirc/essderc-esscirc-2003/papers/all/182.pdf