Geometric diodes, also known as morphological diodes, use the shape of their structure and

ballistic Ballistics may refer to: Science * Ballistics, the science that deals with the motion, behavior, and effects of projectiles ** Forensic ballistics, the science of analyzing firearm usage in crimes ** Internal ballistics, the study of the proce ...

/ 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 In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...

or a

potential barrier In quantum mechanics, the rectangular (or, at times, square) potential barrier is a standard one-dimensional problem that demonstrates the phenomena of wave-mechanical tunneling (also called "quantum tunneling") and wave-mechanical reflection. ...

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 In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as a ...

of the charge carrier) creates an asymmetry in forward vs reverse

bias current In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...

(aka a diode).

Creating a geometric 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

(MFP). Typical room temperature MFPs range from single digit nanometers for metals up to tens or hundreds of nms for semiconductors, and even >1 micrometer in select systems. 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 Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray of light emerges from the reflecting surface at the same angle to the surf ...

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.

Advantages and disadvantages of geometric diodes

Advantages

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. 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. Therefore, geometric diodes can achieve frequency response into the THz. 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.

Disadvantages

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 In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect ...

this may not be a problem for long.

Experimental examples

Geometric diodes are linked to the phenomena of electron ratchets, and their histories are intermingled.

2DEG

Early work on geometric diodes used 2D electron gasses (2DEG) at cryogenic temperatures because these material systems have a very long charge carrier MFP. 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. This system was initially demonstrated at cryogenic temperatures, but then was able to operate at room-temperature and rectify signals of 50 GHz.

Graphene

The four-terminal geometries have also been created in

graphene Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure.

and function at room-temperature. Additionally, a different, two-terminal geometry resembling the simple geometric diode schematic was demonstrated in 2013. Optimum design for the ballistic diode based on graphene field-effect transistors in 2021 b
Van Huy Nguyen
This work showed rectification speeds at THz frequencies.

Nanowires

Geometric diodes formed from etched

Silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

nanowires were shown to operate at room-temperature in April 2020. 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.

References

{{Reflist Diodes Nanoelectronics