A 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 exhibits negative differential resistance regions. All types of tunneling diodes make use of 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. One area of active research is directed toward building oscillators and switching devices that can operate at terahertz frequencies.
Introduction
An 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 the heavily doped p–n junction in Esaki diodes, double barrier, triple barrier, quantum well, or quantum wire. The structure and fabrication process of Si/SiGe resonant interband tunneling diodes are suitable for integration with modern Si complementary metal–oxide–semiconductor (CMOS) and Si/SiGe heterojunction bipolar technology. One type of RTDs is 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. The forming of negative resistance will be examined in detail in operation section below. 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.
Operation The following process is also illustrated from rightside figure. Depending on the number of barriers and number of confined states inside the well, the process described below could be repeated.
Positive resistance region For low bias, as the bias increases, the 1st confined state between the potential barriers gets closer to the source Fermi level, so the current it carries increases.
Negative resistance region As the bias increases further, the 1st confined state becomes lower in energy and gradually goes into the energy range of bandgap, so the current it carries decreases. At this time, the 2nd confined state is still too high above in energy to conduct significant current.
2nd positive resistance region Similar to the first region, as the 2nd confined state becomes closer and closer to the source Fermi level, it carries more current, causing the total current to increase again.
Intraband 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). Considering 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 the 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.
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