Resonant tunnelling diode
Encyclopedia
Resonant-tunneling diode (RTD) is a diode
with a resonant-tunneling structure in which electrons can tunnel through some resonant states at certain energy levels. The current–voltage characteristic often exhibits negative differential resistance
regions.
All types of tunneling diodes make use of the quantum mechanical tunneling.
Characteristic to the current–voltage relationship of a tunneling diode is the presence of one or more negative differential resistance regions, which enables many unique applications. Tunneling diodes can be very compact and are also capable of ultra-high-speed operation because the quantum tunneling effect through the very thin layers is a very fast process.
tunneling diodes, which are essentially discrete diodes and therefore incompatible with modern Si integrated circuit technologies, the Si/SiGe resonant interband tunneling diodes are such that their structure and fabrication are suitable for integration with modern Si complementary metal-oxide-semiconductor (CMOS) and Si/SiGe heterojunction bipolar technology.
One type of RTDs are formed as a single quantum well
structure surrounded by very thin layer barriers. This structure is called a double barrier structure. Carriers such as electrons and holes can only have discrete energy values inside the quantum well. When a voltage is placed across an RTD, a terahertz wave is emitted which is why the energy value inside the quantum well is equal to that of the emitter side. As voltage is increased, the terahertz wave dies out because the energy value in the quantum well is outside the emitter side energy.Another feature seen in RTD structures is the negative resistance
on application of bias as can be seen in the image generated from Nanohub
.
This structure can be grown by molecular beam heteroepitaxy. GaAs
and AlAs
in particular are used to form this structure. AlAs/InGaAs or InAlAs/InGaAs can be used.
The operation of electronic circuits containing RTDs can be described by a Liénard system of equations, which are a generalization of the Van der Pol oscillator equation.
In quantum tunneling through a single barrier, the transmission coefficient, or the tunneling probability, is always less than one (for incoming particle energy less than the potential barrier height). Consider a potential profile which contains two barriers (which are located close to each other), one can calculate the transmission coefficient (as a function of the incoming particle energy) using any of the standard methods.
Tunneling through a double barrier was first solved in the Wentzel-Kramers-Brillouin (WKB) approximation by David Bohm in 1951, who pointed out that resonances in the transmission coefficient occur at certain incident electron energies. It turns out that, for certain energies, the transmission coefficient is equal to one, i.e., the double barrier is totally transparent for particle transmission. This phenomenon is called resonant tunneling. It is interesting that while the transmission coefficient of a potential barrier is always lower 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.
Later, in 1964, L. V. Iogansen discussed the possibility of resonant transmission of an electron through double barriers formed in semiconductor crystals. In the early 1970s, Tsu, Esaki, and Chang computed the two terminal current-voltage (I-V) characteristic of a finite superlattice, and predicted that resonances could be observed not only in the transmission coefficient but also in the I-V characteristic. Resonant tunneling also occurs in potential profiles with more than two barriers. Advances in the MBE technique led to observation of negative differential conductance (NDC) at terahertz frequencies, as reported by Sollner et al. in the early 1980s. This triggered a considerable research effort to study tunneling through multi-barrier structures.
The potential profiles required for resonant tunneling can be realized
in semiconductor system using heterojunctions which utilize semiconductors
of different types to create potential barriers or wells in the conduction
band or the valence band.
Most of semiconductor optoelectronics uses III-V semiconductors and so it is possible to combine III-V RTDs to make OptoElectronic Integrated Circuits (OEICS) that use the negative differential resistance of the RTD to provide electrical gain for optoelectronic devices.
/SiGe
materials system, Si/SiGe resonant interband tunneling diodes have also been developed which have the potential of being integrated into the mainstream Si integrated circuits technology.
(i) an intrinsic
tunneling barrier,
(ii) delta-doped injectors,
(iii) offset of the delta-doping planes from the heterojunction
interfaces,
(iv) low temperature molecular beam epitaxial
growth (LTMBE), and
(v) postgrowth rapid thermal annealing (RTA) for activation of dopants and reduction of density of point defects.
system, and PVCRs of up to 6.0 have been obtained. In terms of peak current density, peak current densities ranging from as low as 20 mA/cm2 and as high as 218 kA/cm2, spanning seven orders of magnitude, have been achieved. A resistive cut-off frequency of 20.2 GHz has been realized on photolithography defined SiGe RITD followed by wet etching for further reducing the diode size, which should be able to improve when even smaller RITDs are fabricated using techniques such as electron beam lithography.
s that is discussed in the next section, other applications of SiGe RITD have been demonstrated using breadboard circuits, including multi-state logic.
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
with a resonant-tunneling structure in which electrons can tunnel through some resonant states at certain energy levels. The current–voltage characteristic often exhibits negative differential resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
regions.
All types of tunneling diodes make use of the quantum mechanical tunneling.
Characteristic to the current–voltage relationship of a tunneling diode is the presence of one or more negative differential resistance regions, which enables many unique applications. Tunneling diodes can be very compact and are also capable of ultra-high-speed operation because the quantum tunneling effect through the very thin layers is a very fast process.
Introduction
RTD can be fabricated using many different types of materials (such as III-V, type IV, II-VI semiconductor) and different types of resonant tunneling structures (such as heavily doped pn junction in Esaki diodes, double barrier, triple barrier, quantum well, quantum wire or quantum dot). Compared to the EsakiLeo Esaki
Reona Esaki also known as Leo Esaki is a Japanese physicist who shared the Nobel Prize in Physics in 1973 with Ivar Giaever and Brian David Josephson for his discovery of the phenomenon of electron tunneling. He is known for his invention of the Esaki diode, which exploited that phenomenon...
tunneling diodes, which are essentially discrete diodes and therefore incompatible with modern Si integrated circuit technologies, the Si/SiGe resonant interband tunneling diodes are such that their structure and fabrication are suitable for integration with modern Si complementary metal-oxide-semiconductor (CMOS) and Si/SiGe heterojunction bipolar technology.
One type of RTDs are formed as a single quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
structure surrounded by very thin layer barriers. This structure is called a double barrier structure. Carriers such as electrons and holes can only have discrete energy values inside the quantum well. When a voltage is placed across an RTD, a terahertz wave is emitted which is why the energy value inside the quantum well is equal to that of the emitter side. As voltage is increased, the terahertz wave dies out because the energy value in the quantum well is outside the emitter side energy.Another feature seen in RTD structures is the negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
on application of bias as can be seen in the image generated from Nanohub
Nanohub
nanoHUB.org is science cyberinfrastructure comprising community-contributed resources and geared toward educational applications, professional networking, and interactive simulation tools for nanotechnology...
.
This structure can be grown by molecular beam heteroepitaxy. GaAs
Gaas
Gaas is a commune in the Landes department in Aquitaine in south-western France....
and AlAs
Alas
The term alas may refer to:* an interjection used to express regret, sorrow, or grief.* in geomorphology, a steep-sided depression formed by the melting of permafrost; it may contain a lake.* in zoology, a wing or winglike body part....
in particular are used to form this structure. AlAs/InGaAs or InAlAs/InGaAs can be used.
The operation of electronic circuits containing RTDs can be described by a Liénard system of equations, which are a generalization of the Van der Pol oscillator equation.
Intraband resonant tunneling
Tunneling through a double barrier was first solved in the Wentzel-Kramers-Brillouin (WKB) approximation by David Bohm in 1951, who pointed out that resonances in the transmission coefficient occur at certain incident electron energies. It turns out that, for certain energies, the transmission coefficient is equal to one, i.e., the double barrier is totally transparent for particle transmission. This phenomenon is called resonant tunneling. It is interesting that while the transmission coefficient of a potential barrier is always lower 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.
Later, in 1964, L. V. Iogansen discussed the possibility of resonant transmission of an electron through double barriers formed in semiconductor crystals. In the early 1970s, Tsu, Esaki, and Chang computed the two terminal current-voltage (I-V) characteristic of a finite superlattice, and predicted that resonances could be observed not only in the transmission coefficient but also in the I-V characteristic. Resonant tunneling also occurs in potential profiles with more than two barriers. Advances in the MBE technique led to observation of negative differential conductance (NDC) at terahertz frequencies, as reported by Sollner et al. in the early 1980s. This triggered a considerable research effort to study tunneling through multi-barrier structures.
The potential profiles required for resonant tunneling can be realized
in semiconductor system using heterojunctions which utilize semiconductors
of different types to create potential barriers or wells in the conduction
band or the valence band.
III-V resonant tunneling diodes
Resonant tunneling diodes are typically realized in III-V compound material systems, where heterojunctions made up of various III-V compound semiconductors are used to create the double or multiple potential barriers in the conduction band or valence band. Reasonably high performance III-V resonant tunneling diodes have been realized. Such devices have not entered mainstream applications yet because the processing of III-V materials is incompatible with Si CMOS technology and the cost is high.Most of semiconductor optoelectronics uses III-V semiconductors and so it is possible to combine III-V RTDs to make OptoElectronic Integrated Circuits (OEICS) that use the negative differential resistance of the RTD to provide electrical gain for optoelectronic devices.
Si/SiGe resonant tunneling diodes
Resonant tunneling diodes can also be realized using the Si/SiGe materials system. Both hole tunneling and electron tunneling have been observed. However, the performance of Si/SiGe resonant tunneling diodes was limited due to the limited conduction band and valence band discontinuities between Si and SiGe alloys. Resonant tunneling of holes through Si/SiGe heterojunctions was attempted first because of the typically relatively larger valence band discontinuity in Si/SiGe heterojunctions than the conduction band discontinuity for (compressively) strained Si1-xGex layers grown on Si substrates. Negative differential resistance was only observed at low temperatures but not at room temperature. Resonant tunneling of electrons through Si/SiGe heterojunctions was obtained later, with a limited peak-to-valley current ratio (PVCR) of 1.2 at room temperature. Subsequent developments have realized Si/SiGe RTDs (electron tunneling) with a PVCR of 2.9 with a PCD of 4.3 kA/cm2 and a PVCR of 2.43 with a PCD of 282 kA/cm2 at room temperature.Interband resonant tunneling diodes
Resonant interband tunneling diodes (RITDs) combine the structures and behaviors of both intraband resonant tunneling diodes (RTDs) and conventional interband tunneling diodes, in which electronic transitions occur between the energy levels in the quantum wells in the conduction band and that in the valence band. Like resonant tunneling diodes, resonant interband tunneling diodes can be realized in both the III-V and Si/SiGe materials systems.III-V RITDs
In the III-V materials system, InAlAs/InGaAs RITDs with peak-to-valley current ratios (PVCRs) higher than 70 and as high as 144 at room temperature and Sb-based RITDs with room temperature PVCR as high as 20 have been obtained. The main drawback of III-V RITDs is the use of III-V materials whose processing is incompatible with Si processing and is expensive.Si/SiGe RITDs
In SiSilicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
/SiGe
SiGe
SiGe , or silicon-germanium, is a general term for the alloy Si1−xGex which consists of any molar ratio of silicon and germanium. It is commonly used as a semiconductor material in integrated circuits for heterojunction bipolar transistors or as a strain-inducing layer for CMOS transistors...
materials system, Si/SiGe resonant interband tunneling diodes have also been developed which have the potential of being integrated into the mainstream Si integrated circuits technology.
Structure
The five key points to the design are:(i) an intrinsic
Intrinsic semiconductor
An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities...
tunneling barrier,
(ii) delta-doped injectors,
(iii) offset of the delta-doping planes from the heterojunction
Heterojunction
A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...
interfaces,
(iv) low temperature molecular beam epitaxial
Molecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
growth (LTMBE), and
(v) postgrowth rapid thermal annealing (RTA) for activation of dopants and reduction of density of point defects.
Performance
A minimum PVCR of about 3 is needed for typical circuit applications. Low current density Si/SiGe RITDs are suitable for low-power memory applications, and high current density tunndel diodes are needed for high-speed digital/mixed-signal applications. Si/SiGe RITDs have been engineered to have room temperature PVCRs up to 4.0. The same structure was duplicated by another research group using a different MBEMolecular beam epitaxy
Molecular beam epitaxy is one of several methods of depositing single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.-Method:...
system, and PVCRs of up to 6.0 have been obtained. In terms of peak current density, peak current densities ranging from as low as 20 mA/cm2 and as high as 218 kA/cm2, spanning seven orders of magnitude, have been achieved. A resistive cut-off frequency of 20.2 GHz has been realized on photolithography defined SiGe RITD followed by wet etching for further reducing the diode size, which should be able to improve when even smaller RITDs are fabricated using techniques such as electron beam lithography.
Applications
In addition to the realization of integration with Si CMOS and SiGe heterojunction bipolar transistorHeterojunction bipolar transistor
The heterojunction bipolar transistor is a type of bipolar junction transistor which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it that can handle signals of very high frequencies, up to several hundred...
s that is discussed in the next section, other applications of SiGe RITD have been demonstrated using breadboard circuits, including multi-state logic.
Integration with Si/SiGe CMOS and heterojunction bipolar transistors
Integration of Si/SiGe RITDs with Si CMOS has been demonstrated. Vertical integration of Si/SiGe RITD and SiGe heterojunction bipolar transistors was also demonstrated, realizing a 3-terminal negative differential resistance circuit element with adjustable peak-to-valley current ratio. These results indicate that Si/SiGe RITDs is a promising candidate of being integrated with the Si integrated circuit technology.See also
- Tunneling diode
- Schottky Barrier Quantum Well Resonant Tunneling Transistor (SBQWRTT)
External links
- For information on Optoelectronic applications of RTDs see http://userweb.elec.gla.ac.uk/i/ironside/RTD/RTDOpto.html.
- Resonant Tunneling Diode Simulation Tool on NanohubNanohubnanoHUB.org is science cyberinfrastructure comprising community-contributed resources and geared toward educational applications, professional networking, and interactive simulation tools for nanotechnology...
enables the simulation of resonant tunneling diodes under realistic bias conditions for realistically extended devices.