Resonant tunnelling diode

A resonant tunnel diode (RTD) is a device which uses quantum effects to produce negative differential resistance (NDR). As an RTD is capable of generating a terahertz wave at room temperature, it can be used in ultra high-speed circuitry. Therefore the RTD is extensively studied.

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.

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.

Resonant Tunneling

thumb|300px|right|Adouble-barrier potential profile with a particle incident from leftwith energy less than the barrier height.

In quantum tunneling through a single barrier, the transmissioncoefficient, 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 thetransmission coefficient (as a function of the incoming particleenergy) using any of the standard methods.It turns out that, for certain energies, the transmission coefficient isequal to one, i.e. the double barrier is totally transparent for particletransmission.This phenomenon is called resonant tunneling ref|Bohm51.It is interesting that while the transmission coefficient of a potential barrier is always lower than one (and decreases with increasing barrierheight and width), two barriers in a row can be completely transparent forcertain energies of the incident particle.

Resonant tunneling also occurs in potential profiles with more than twobarriers ref|Tsu73.

The potential profiles required for resonant tunneling can be realizedin semiconductor system using heterojunctions which utilize semiconductorsof different types to crease potential barriers or wells in the conductionband or the valence band.It is worth noting that resonant tunnel diodes are "intraband" tunnel diodes,see also resonant interband tunnel diodes.

III-V Resonant Tunnel Diodes

Resonant tunnel diodes are typically realized in III-V compound materialsystems, where heterojunctions made up of various III-V compoundsemiconductors are used to create the double or multiple potentialbarriers in the conduction band or valence band.Reasonably high performance III-V resonant tunnel diodes have beenrealized. But such devices have not entered mainstream applications yet because the processing of III-V materials is incompatible with Si CMOStechnology 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 devicesref|Slight07 - see also [http://userweb.elec.gla.ac.uk/i/ironside/RTD/RTDOpto.html]

Si/SiGe Resonant Tunnel Diodes

Resonant tunnel diodes can also be realized using the Si/SiGe materialssystem.But the performance of Si/SiGe resonant tunnel diodes was limited due tothe limited conduction band andvalence band discontinuities between Si and SiGe alloys.Resonant tunneling of holes through Si/SiGe heterojunctions was attemptedfirst because of the typically relatively larger valence banddiscontinuityin Si/SiGe heterojunctions than the conduction band discontinuity.This has been observed, but negative differential resistancewas only observed at low temperatures but not at room temperatureref|Gennser90.Resonant tunneling of electrons through Si/SiGe heterojunctionswas obtained later, with a limited peak-to-valley current ratio (PVCR)of 1.2 at room temperature ref|Ismail91. Subsequent developments haverealized Si/SiGe RTDs (electron tunneling) with a PVCR of 2.9 with a PCD of 4.3kA/cm² ref|See01a and a PVCR of 2.43 with a PCD of 282kA/cm²at room temperature ref|See01b.

See also Si/SiGe resonant tunnel diode.

References

# David Bohm, Quantum Theory, Prentice-Hall, New York, 1951.
# R. Tsu and L. Esaki, Tunneling in a finite superlattice, Appl. Phys. Lett. 22, 562 (1973).
# U. Gennser, V.P. Kesan, S.S. Iyer, T.J. Bucelot, and E.S. Yang, J. Vac. Sci. Technol. B 8, 210 (1990).
# K. Ismail, B.S. Meyerson, and P.J. Wang, Electron resonant tunneling in Si/SiGe double barrier diodes, Appl. Phys. Lett. 59, 973 (1991).
# P. See, D.J. Paul, B. Hollander, S. Mantl, I. V. Zozoulenko, and K.-F. Berggren, High Performance Si/Si_1-xGe_x Resonant Tunneling Diodes, IEEE Electron Device Letters 22, 182 (2001).
# P. See and D.J. Paul, The scaled performance of Si/Si_1-xGe_x resonant tunneling diodes, IEEE Electron Device Letters 22, 582 (2001).
#Slight, T. J.; Ironside, C. N., “Investigation into the integration of a resonant tunnelling diode and an optical communications laser: model and experiment”,IEEE J. Quant. Elec. 43, 7, 580-587, 2007.http://dx.doi.org/10.1109/JQE.2007.898847

See also

*Resonant interband tunnel diode
*Si/SiGe resonant interband tunnel diode