Geometric diodes, also known as morphological diodes, use the shape of their structure and ballistic / quasi-ballistic electron transport to create diode behavior. Geometric diodes differ from all other forms of diodes because they do not rely on a depletion region or a potential barrier to create their diode behavior. Instead of a potential barrier, an asymmetry in the geometry of the material (that is on the order of the mean free path of the charge carrier) creates an asymmetry in forward vs reverse bias current (aka a diode).
Geometric diodes are formed from one continuous material (adding a caveat for 2D-electron gasses which are layered systems) that has an asymmetry in the structure on the order of the size of the charge carrier's mean free path (MFP). Typical room temperature MFPs range from single digit nanometers for metals [1] up to tens or hundreds of nms for semiconductors, [2] and even >1 micrometer in select systems. [3] [4] This means that to create a geometric diode, one must either use a high MFP material, or have a fabrication process that has nanometer precision in order to create the relevant geometries.
Geometric diodes are majority carrier devices that do not need a potential barrier. The diode behavior comes from an asymmetry in the shape of the structure (as shown in the figure). Quite simply geometric diodes can be thought of as funnels or lobster traps for charges; In one direction it is relatively easy for charges to flow, and in the reverse direction it is more difficult.
Additionally, it is ideal to have specular reflection of the charge carriers at the surface of the structure; however, this is not as critical as being small enough to be in a ballistic regime.
Because all other diodes create asymmetry in current flow through some form of a potential barrier, they necessarily have some degree of a turn-on voltage. Geometric diodes could theoretically achieve zero-bias turn-on voltage due to their lack of potential barrier. With zero-bias turn-on voltage, there is no DC bias that must be supplied to the device; therefor, geometric diodes could greatly reduce the power needed to operate a device. This could also be beneficial in that the diodes would be more sensitive to small signals. This is of course theoretical, and truly zero-bias diodes may be limited from being experimentally realized.
A second major advantage also stems from their lack of potential barrier and minority carriers. A potential barrier is a large source of capacitance in a diode. Capacitance serves to decrease a diodes frequency response by increasing its RC time. Geometric diodes lack of potential barrier means they can have ultra-low capacitance down to the attofarads. [5] A geometric diode's frequency response is limited not by RC time or minority carrier mobility, but by the flight time of the charge carriers through the structural asymmetry. [6] Therefore, geometric diodes can achieve frequency response into the THz. [5]
The ability for a geometric diode's electronic properties to be tuned by the geometry of the structure, the surface coating on the structure, and the properties of the material used offer a level customization that is unrealized in any other diode system.
Principles learned from geometric diodes and ballistic systems will be used in understanding technology as devices become increasingly small and exist at or below charge carrier MFPs.
The same benefits from the lack of potential barrier also come with their share of downsides. The main one being that the reverse bias current from a geometric diode can be quite high (anywhere from three to less than one orders of magnitude less than the forward bias current). Depending on the application, a high reverse bias can be tolerated though.
Typically geometric diodes are on the nano-scale, so that necessarily means that they have high resistances. However, depending on the fabrication process this can be mitigated by stringing many diodes in parallel.
Perhaps the largest hurdle for geometric diodes to overcome is the reliability of their fabrication and ability to scale it up. Geometric diodes are typically made using nanofabrication methods that do not scale up well, but with the increasing resolution of photolithography this may not be a problem for long.
Geometric diodes are linked to the phenomena of electron ratchets, and their histories are intermingled. [7] [8]
Early work on geometric diodes used 2D electron gasses (2DEG) at cryogenic temperatures because these material systems have a very long charge carrier MFP. [9] [10] One of the most studied structures is a four-terminal geometry that either had a single antidot at the center, or an array of antidots that forces charges down instead of up when current is supplied from either the left or right. [11] This system was initially demonstrated at cryogenic temperatures, [9] [12] [13] but then was able to operate at room-temperature [14] and rectify signals of 50 GHz. [14]
The four-terminal geometries have also been created in graphene and function at room-temperature. [15] [16] Additionally, a different, two-terminal geometry resembling the simple geometric diode schematic was demonstrated in 2013. [5] Optimum design for the ballistic diode based on graphene field-effect transistors in 2021 by Van Huy Nguyen. [17] This work showed rectification speeds at THz speeds.
Geometric diodes formed from etched Silicon nanowires were shown to operate at room-temperature in April 2020. [6] This work highlights the tunability of geometric diodes by thoroughly studying the effects of geometry on the diode's electronic properties. The work also demonstrated rectification up to an instrument-limited 40 GHz.
A diode is a two-terminal electronic component that conducts current primarily in one direction. It has low resistance in one direction and high resistance in the other.
The Schottky diode, also known as Schottky barrier diode or hot-carrier diode, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless and metal rectifiers used in early power applications can be considered primitive Schottky diodes.
An avalanche photodiode (APD) is a highly sensitive type of photodiode, which in general are semiconductor diodes that exploit the photoelectric effect to convert light into electricity. APDs use materials and a structure optimised for operating with high reverse bias, approaching the reverse breakdown voltage, such that charge carriers generated by the photoelectric effect are multiplied by an avalanche breakdown; thus they can be used to detect relatively small amounts of light.
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz), although the upper boundary is somewhat arbitrary and is considered by some sources as 30 THz. One terahertz is 1012 Hz or 1,000 GHz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm = 100 μm. Because terahertz radiation begins at a wavelength of around 1 millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy. This band of electromagnetic radiation lies within the transition region between microwave and far infrared, and can be regarded as either.
A rectenna is a special type of receiving antenna that is used for converting electromagnetic energy into direct current (DC) electricity. They are used in wireless power transmission systems that transmit power by radio waves. A simple rectenna element consists of a dipole antenna with a diode connected across the dipole elements. The diode rectifies the AC induced in the antenna by the microwaves, to produce DC power, which powers a load connected across the diode. Schottky diodes are usually used because they have the lowest voltage drop and highest speed and therefore have the lowest power losses due to conduction and switching. Large rectennas consist of arrays of many power receiving elements such as dipole antennas.
A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB. The value of ΦB depends on the combination of metal and semiconductor.
A p–n junction is a combination of two types of semiconductor materials, p-type and n-type, in a single crystal. The "n" (negative) side contains freely-moving electrons, while the "p" (positive) side contains freely-moving electron holes. Connecting the two materials causes creation of a depletion region near the boundary, as the free electrons fill the available holes, which in turn allows electric current to pass through the junction only in one direction.
A quantum well is a potential well with only discrete energy values.
Ballistic deflection transistors (BDTs) are electronic devices, developed since 2006, for high-speed integrated circuits, which is a set of circuits bounded on semiconductor material. They use electromagnetic forces instead of a logic gate, a device used to perform solely on specified inputs, to switch the forces of electrons. The unique design of this transistor includes individual electrons bouncing from wedge-shaped obstacles called deflectors. Initially accelerated by electric field, electrons are then guided on their respective paths by electromagnetic deflection. Electrons are therefore able to travel without being scattered by atoms or defects, thus resulting in improved speed and reduced power consumption.
In mesoscopic physics, a Coulomb blockade (CB), named after Charles-Augustin de Coulomb's electrical force, is the decrease in electrical conductance at small bias voltages of a small electronic device comprising at least one low-capacitance tunnel junction. Because of the CB, the conductance of a device may not be constant at low bias voltages, but disappear for biases under a certain threshold, i.e. no current flows.
In mesoscopic physics, ballistic conduction is the unimpeded flow of charge carriers, or energy-carrying particles, over relatively long distances in a material. In general, the resistivity of a material exists because an electron, while moving inside a medium, is scattered by impurities, defects, thermal fluctuations of ions in a crystalline solid, or, generally, by any freely-moving atom/molecule composing a gas or liquid. Without scattering, electrons simply obey Newton's second law of motion at non-relativistic speeds.
Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a MOS transistor, the switching characteristics of the transistor can be permanently changed. Hot-carrier injection is one of the mechanisms that adversely affects the reliability of semiconductors of solid-state devices.
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 often exhibits negative differential resistance regions.
In solid-state physics, a metal–semiconductor (M–S) junction is a type of electrical junction in which a metal comes in close contact with a semiconductor material. It is the oldest practical semiconductor device. M–S junctions can either be rectifying or non-rectifying. The rectifying metal–semiconductor junction forms a Schottky barrier, making a device known as a Schottky diode, while the non-rectifying junction is called an ohmic contact.
An optical rectenna is a rectenna that works with visible or infrared light. A rectenna is a circuit containing an antenna and a diode, which turns electromagnetic waves into direct current electricity. While rectennas have long been used for radio waves or microwaves, an optical rectenna would operate the same way but with infrared or visible light, turning it into electricity.
A carbon nanotube field-effect transistor (CNTFET) is a field-effect transistor that utilizes a single carbon nanotube (CNT) or an array of carbon nanotubes as the channel material, instead of bulk silicon, as in the traditional MOSFET structure. There have been major developments since CNTFETs were first demonstrated in 1998.
Single-walled carbon nanotubes in the fields of quantum mechanics and nanoelectronics, have the ability to conduct electricity. This conduction can be ballistic, diffusive, or based on scattering. When ballistic in nature conductance can be treated as if the electrons experience no scattering.
Piezo-phototronic effect is a three-way coupling effect of piezoelectric, semiconductor and photonic properties in non-central symmetric semiconductor materials, using the piezoelectric potential (piezopotential) that is generated by applying a strain to a semiconductor with piezoelectricity to control the carrier generation, transport, separation and/or recombination at metal–semiconductor junction or p–n junction for improving the performance of optoelectronic devices, such as photodetector, solar cell and light-emitting diode. Prof. Zhong Lin Wang at Georgia Institute of Technology proposed the fundamental principle of this effect in 2010.
Metal–insulator–metal (MIM) diode is a type of nonlinear device very similar to a semiconductor diode and capable of very fast operation. Depending on the geometry and the material used for fabrication, the operation mechanisms are governed either by quantum tunnelling or thermal activation.
A graphene lens is an optical refraction device. Graphene's unique 2-D honeycomb contributes to its unique optical properties.