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 increased, the terahertz stops 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.

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Resonant Tunneling

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. 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 ^[1]. 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.

Resonant tunneling also occurs in potential profiles with more than two barriers ^[2].

The potential profiles required for resonant tunneling can be realized in semiconductor system using heterojunctions which utilize semiconductors of different types to crease potential barriers or wells in the conduction band 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 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 tunnel diodes 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 and the cost is high.

Si/SiGe Resonant Tunnel Diodes

Resonant tunnel diodes can also be realized using the Si/SiGe materials system. But the performance of Si/SiGe resonant tunnel 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. This has been observed, but negative differential resistance was only observed at low temperatures but not at room temperature ^[3]. 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 ^[4]. Subsequent developments have realized Si/SiGe RTDs (electron tunneling) with a PVCR of 2.9 with a PCD of 4.3 kA/cm^{2 [5]} and a PVCR of 2.43 with a PCD of 282 kA/cm² at room temperature ^[6].

See also Si/SiGe resonant tunnel diode.

References

1. ^  David Bohm, Quantum Theory, Prentice-Hall, New York, 1951.
2. ^  R. Tsu and L. Esaki, Tunneling in a finite superlattice, Appl. Phys. Lett. 22, 562 (1973).
3. ^  U. Gennser, V.P. Kesan, S.S. Iyer, T.J. Bucelot, and E.S. Yang, J. Vac. Sci. Technol. B 8, 210 (1990).
4. ^  K. Ismail, B.S. Meyerson, and P.J. Wang, Electron resonant tunneling in Si/SiGe double barrier diodes, Appl. Phys. Lett. 59, 973 (1991).
5. ^  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).
6. ^  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).

See also

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