Structure of the Device
A resonant tunnel diode (RTD) is a device that uses quantum effects and negative differential resistance (NDR). RTDs consist of a quantum well confined between barriers that are sufficiently thin that carriers, usually electrons, can tunnel into and out of the well. This action of exiting and coming back in is what makes it resonate and thus the name resonant tunneling diode.
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.
The RTD is made up of three layers, in this case GaAs, which is lightly doped, causes a reduction in capacitance, which leads to a decreased switching time. This occurs in the middle of GaAlAs barrier. The epitaxial growth, the growth of the crystalline substance that mimic the orientation of the crystalline substrate, causes improved purity levels in the crystalline substrate. The super lattice formed in the substrate is an artificial superconductor, having properties that are in high demand, composed of thin layers of varied semiconductors. The tunneling phenomena is demonstrated by a capable voltage affecting the holes present and is observable by the energy band gap of the device, which is a component of its chemical makeup. Resonance increases the maxima of the current, at the given voltage well and thereby the Fermi energy level corresponds to the quasi-stationary states of the rectangular voltage of a know drop for electrons. Furthermore, the chemical components used in the device must have small electron masses. A way to describe resonance, energy level, is to notice its peaks in the tunneling current, in fact a 50% change than that of a single layer barrier, near the aforementioned. Tunneling happens with great speed and thereby must be measured using the RC time constant of the material.
In order for tunneling to take effect, two things must occur:
1-The state must be filled on one side of the barrier and empty states on the other side of the barrier at the equal energy level.
2-The girth of the potential energy barrier must be very thin. In quantum mechanics terms, the width must be less than 100 angstroms.
RTD oscillates faster than the tunneling diode since it has a lower device capacitance and IV characteristics can be conducted by engineering with an adequate band gap.
This structure can be grown to by molecular beam heteroepitaxy. GaAs and AlAs in particular are used to form this structure. AlAs/InGaAs or InAlAs/InGaAs can be used.
A resonant-tunneling diode requires a band-edge discontinuity at the conduction band or valence band to form a quantum well and, thus, necessitates heteroepitaxy. The most common combination used is GaAs-AlGaAs. The middle quantum-well thickness is typically around, and the barrier layers range from 1.5 to. Symmetry of the barrier layers is not required so their thickness can be different. A typical resonant tunneling diode structure with analytical band edge model as used in simulations is depicted in Figure 5.1.
Figure 5.1: Conduction band edge of the RTD for different voltages. A linear voltage drop is assumed over a distance of 40 nm.The well layer (GaAs) and the barrier layers (AlGaAs) are all undoped, and they are sandwiched between heavily doped, narrow energy-gap materials, which usually are the same as the well layer. Adjacent to the barrier layers are thin layers of undoped spacers to ensure that dopants do not diffuse to the barrier layers.
The region between the two barriers defines a virtual quantum well since the electrons can escape the well confinement by tunneling. The resonant tunneling diode (RTD) is thus an open quantum system in which the electronic states are scattering states with a continuous distribution in energy space, rather than bound states with a discrete energy spectrum. Under these circumstances quasi-bound states (resonant states) are formed in the quantum well which accommodate electrons for a time that is characteristic for the double-barrier structure. So-called resonant tunneling through the double-barrier structure occurs when the energy of the electrons flowing from the emitter coincides with the energy of the quasi-bound state, in the quantum well. The effect of the external bias is to sweep the alignment of the emitter and quasi-bound states.
For many applications, negative differential resistance (NDR) devices should have a large peak current and a small valley current, where the latter is the minimum current following the peak current as the magnitude of the voltage increases. Therefore, an important figure of merit for an NDR device such as the RTD is the peak to valley ratio (PVR). For good devices with thin AlAs barriers, PVRs close to 4:1 and peak current densities in excess of may be obtained at (see [FG01], page 96), although not in the same structures, since there is usually a trade-off between these two parameters in terms of device design.
Because tunneling is inherently a very fast phenomenon that is not transit-time limited, the resonant-tunneling diode is considered among the fastest devices ever made. On the other hand, using resonant-tunneling diodes, it is more difficult to supply high current and the output power of an oscillator is limited.
Integration of RTDs with MOSFETs provides high speed operation due to the inherently fast tunneling process, and negative differential resistance regime (NDR) that provides at least two stable operating points (i.e., multiple valued logic) when combined with MOSFETs [RO001].
The resonant tunnel devices for logic applications include resonant tunnel transistors (RTT) and hybrid devices incorporating resonant tunneling diodes and one or more FETs (RTD-FET). RTD designs can offer a reduction in circuit component count by up to 40% when compared with the equivalent CMOS logic family.
The major problem is the extreme sensitivity of device characteristics to the thickness of the tunneling well as the tunneling current depends exponentially on the thickness of the tunnel barrier. The challenge to the process engineer is to match device properties across the wafer.
Overall, the resonant tunneling devices may be useful niche applications requiring high speed and low dynamic range, provided the manufacturing issues associated with uniformity of the tunneling barrier can be resolved. Thus, RTDs are on the verge of commercialization. Potential practical applications are high-speed microwave systems and novel digital logic circuits.
A quantum well, in the general use of this term, is a potential structure, which spatially confines the electron. According to quantum mechanics, an electron subjected to potential confinement has its energy quantized and a discrete energy spectrum would be expected for the electron system. However, the electron remains free to move in the perpendicular direction. This results in the creation of a two-dimensional electron gas of quasi-bound states.
Resonant tunneling refers to tunneling in which the electron transmission coefficient through a structure is sharply peaked about certain energies. The emergence of these peaks can be qualitatively explained by introducing infinite walls as boundary conditions. It is usually possible to do this far from the quantum well itself. Then the calculation of the quantized energy levels in a quantum well of arbitrary shape is the solution of an Eigen value problem. For electrons with an energy corresponding approximately to the virtual resonant energy level of the quantum well, the transmission coefficient is close to unity. That is, an electron with this resonant energy can cross the double barrier without being reflected. This resonance phenomenon is similar to that taking place in the optical Fabry-Perot resonator or in a microwave capacitively coupled transmission-line resonator.
The outlined physics gives a simple theory of quantum transport through quantum wells from elementary quantum mechanics. However, for simulation, we model resonant tunneling using open systems theory.
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.
A Z shaped bi-stability from the RTD, gives to the creation of non-moving and moving vertical schemes of charge and tunneling current that has been built up. The pattern are switching waves and are composed of two regions, that compromise the built up change and tunneling current, both the high and low, an the boundary travels through the quantum well. The IC characteristics will not show a negative differential resistance. A high frequency will be generated that gives electric oscillations. Where frequency cuts out is the inverse of the RC factor of the RTD circuit.
The Hall Effect, take place in RTDs. It can be observed that there is a pronounced anisotropy of the electrical properties that happen in tilted magnetic fields at low temperature. It can be deciphered as having a strong inclination for striped phase in electrons.