Lidar is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can be used to make digital 3-D representations of the target; the name lidar, now used as an acronym of light detection and ranging, was a portmanteau of light and radar. Lidar sometimes is called 3D laser scanning, a special combination of a 3D scanning and laser scanning, it has terrestrial and mobile applications. Lidar is used to make high-resolution maps, with applications in geodesy, archaeology, geology, seismology, atmospheric physics, laser guidance, airborne laser swath mapping, laser altimetry; the technology is used in control and navigation for some autonomous cars. Lidar originated in the early 1960s, shortly after the invention of the laser, combined laser-focused imaging with the ability to calculate distances by measuring the time for a signal to return using appropriate sensors and data acquisition electronics.
Its first applications came in meteorology, where the National Center for Atmospheric Research used it to measure clouds. The general public became aware of the accuracy and usefulness of lidar systems in 1971 during the Apollo 15 mission, when astronauts used a laser altimeter to map the surface of the moon. Although now most sources treat the word "lidar" as an acronym, the term originated as a combination of "light" and "radar"; the first published mention of lidar, in 1963, makes this clear: "Eventually the laser may provide an sensitive detector of particular wavelengths from distant objects. Meanwhile, it is being used to study the moon by'lidar'..." The Oxford English Dictionary supports this etymology. The interpretation of "lidar" as an acronym came beginning in 1970, based on the assumption that since the base term "radar" started as an acronym for "Radio Detection And Ranging", "LIDAR" must stand for "Light Detection And Ranging", or for "Laser Imaging, Detection And Ranging". Although the English language no longer treats "radar" as an acronym and printed texts universally present the word uncapitalized, the word "lidar" became capitalized as "LIDAR" or "LiDAR" in some publications beginning in the 1980s.
No consensus exists on capitalization, reflecting uncertainty about whether or not "lidar" is an acronym, if it is an acronym, whether it should appear in lower case, like "radar". Various publications refer to lidar as "LIDAR", "LiDAR", "LIDaR", or "Lidar"; the USGS uses both "LIDAR" and "lidar", sometimes in the same document. Lidar uses ultraviolet, near infrared light to image objects, it can target a wide range of materials, including non-metallic objects, rain, chemical compounds, aerosols and single molecules. A narrow laser beam can map physical features with high resolutions; the essential concept of lidar was originated by EH Synge in 1930, who envisaged the use of powerful searchlights to probe the atmosphere. Indeed, lidar has since been used extensively for atmospheric meteorology. Lidar instruments fitted to aircraft and satellites carry out surveying and mapping – a recent example being the U. S. Geological Survey Experimental Advanced Airborne Research Lidar. NASA has identified lidar as a key technology for enabling autonomous precision safe landing of future robotic and crewed lunar-landing vehicles.
Wavelengths vary to suit the target: from about 10 micrometers to the UV. Light is reflected via backscattering, as opposed to pure reflection one might find with a mirror. Different types of scattering are used for different lidar applications: most Rayleigh scattering, Mie scattering, Raman scattering, fluorescence. Suitable combinations of wavelengths can allow for remote mapping of atmospheric contents by identifying wavelength-dependent changes in the intensity of the returned signal; the two kinds of lidar detection schemes are "incoherent" or direct energy detection and coherent detection. Coherent systems use optical heterodyne detection; this is more sensitive than direct detection and allows them to operate at much lower power, but requires more complex transceivers. Both types employ pulse models: either high energy. Micropulse systems utilize intermittent bursts of energy, they developed as a result of ever-increasing computer power, combined with advances in laser technology. They use less energy in the laser on the order of one microjoule, are "eye-safe", meaning they can be used without safety precautions.
High-power systems are common in atmospheric research, where they are used for measuring atmospheric parameters: the height and densities of clouds, cloud particle properties, pressure, wind and trace gas concentration. Lidar systems consist of several major components. 600–1000 nm lasers are most common for non-scientific applications. The maximum power of the laser is limited, or an automatic shut off system which turns the laser off at specific altitudes is used in order to make it ey
Single-photon avalanche diode
A single-photon avalanche diode is a solid-state photodetector in which a photon-generated carrier can trigger a short-duration but large avalanche current. This avalanche is created through a mechanism called impact ionization, whereby carriers are accelerated to high kinetic energies through a large potential gradient. If the kinetic energy of a carrier is sufficient further carriers are liberated from the atomic lattice; the number of carriers thus increases exponentially in some cases, as few as a single carrier. This mechanism was observed and modeled by John Townsend for trace-gas vacuum tubes, becoming known as a Townsend discharge, being attributed to solid-state breakdown by K. McAfee; this device is able to detect low-intensity ionizing radiation, including: gamma, X-ray and alpha-particle radiation along with electromagnetic signals in the UV, Visible and IR. SPADs are able to distinguish the arrival times of events with a timing jitter of a few tens of picoseconds. SPADs, like avalanche photodiodes, exploit the incident radiation triggered avalanche current of a p–n junction when reverse biased.
The fundamental difference between SPADs and APDs is that SPADs are designed to operate with a reverse-bias voltage well above the breakdown voltage. This kind of operation is called Geiger-mode in the literature; this is in analogy with the Geiger counter. Since the 1970s, the applications of SPADs have increased significantly. Recent examples of their use include LIDAR, Time of Flight 3D Imaging, PET scanning, single-photon experimentation within physics, fluorescence lifetime microscopy and optical communications. Notable companies that have commercialized SPAD technology include: ST Microelectronics, Tower Jazz and Micro Photon Devices; the related technologies of solid-state silicon photomultipliers and multi-pixel photon counters have been commercialized and available through companies such as SensL and Hamamatsu. The history and development of SPADs and APDs shares a number of important points with the development of solid-state technologies such as diodes and early p–n junction transistors.
This history can be traced to the late 1890s and early 1900s, by Prof. Brendan O Callaghan, UCC, BEng, Donoughmore et al, however suitable references for the historical development of these devices, can be found for the years 1900 to 1969, along with a number of overview historical and technical reviews. John Townsend in 1901 and 1903 investigated the ionisation of trace gases within vacuum tubes, finding that as the electric potential increased gasious atoms and molecules could become ionised by the kinetic energy of free electrons accelerated though the electric field; the new liberated electrons were themselves accelerated by the field, producing new ionisations once their kinetic energy has reached sufficient levels. This theory was instrumental in the development of the thyratron and the Geiger-Mueller Tube; the Townsend Discharge was instrumental as a base theory for electron multiplication phenomena, within both Silicon and Germanium. However, the major advances in early discovery and utilisation of the avalanche gain mechanism were a product of the study of Zener breakdown, related breakdown mechanisms and structural defects in early silicon and germanium transistor and p–n junction devices.
These defects are critical in the history of APDs and SPADs. Investigation of the light detection properties of p–n junctions is crucial the early 1940s findings by Russel Ohl. Light detection in semiconductors and solids through the internal photoelectric effect is older with Foster Nix pointing to the work of Gudden and Pohl in the 1920s. In the 1950s and 1960s, significant effort was made to reduce the number of Microplasma breakdown and noise sources, with artificial microplasmas being fabricated for study, it became clear that the avalanche mechanism could be useful for signal amplification within the diode itself, as both light and alpha particles were used for the study of these devices and breakdown mechanisms. In the early 2000s, SPADs have been implemented within CMOS processes; this has radically increased their performance, has leveraged the analog and digital circuits that can be implemented alongside these devices. Notable circuits include photon counting using fast digital counters, photon timing using both time-to-digital converters and time-to-analog converters, passive quenching circuits using either NMOS or PMOS transistors in place of poly-silicon resistors, active quenching and reset circuits for high counting rates, many on-chip digital signal processing blocks.
Such devices, now reaching optical fill factors of >70%, with >1024 SPADs, with DCRs < 10Hz and jitter values in the 50ps region are now available with dead times of 1-2ns. Recent devices have leaveraged 3D-IC technologies such as through-silicon-vias to present a high-fill-factor SPAD optimised top CMOS layer with a dedicated signal processing and readout CMOS layer. Significant advancements in the noise terms for SPADs have been obtained by silicon process modelling tools such as TCAD, where guard rings, junction depths and device structures and shapes can be optimised prior to validation by experimental SPAD structure
Solid-state electronics means semiconductor electronics. The term is used for devices in which semiconductor electronics which have no moving parts replace devices with moving parts, such as the solid-state relay in which transistor switches are used in place of a moving-arm electromechanical relay, or the solid-state drive a type of semiconductor memory used in computers to replace hard disk drives, which store data on a rotating disk; the term "solid state" became popular in the beginning of the semiconductor era in the 1960s to distinguish this new technology based on the transistor, in which the electronic action of devices occurred in a solid state, from previous electronic equipment that used vacuum tubes, in which the electronic action occurred in a gaseous state. A semiconductor device works by controlling an electric current consisting of electrons or holes moving within a solid crystalline piece of semiconducting material such as silicon, while the thermionic vacuum tubes it replaced worked by controlling current conducted by a gas of particles, electrons or ions, moving in a vacuum within a sealed tube.
Although the first solid state electronic device was the cat's whisker detector, a crude semiconductor diode invented around 1904, solid state electronics started with the invention of the transistor in 1947. Before that, all electronic equipment used vacuum tubes, because vacuum tubes were the only electronic components that could amplify, an essential capability in all electronics; the replacement of bulky, energy-wasting vacuum tubes by transistors in the 1960s and 1970s created a revolution not just in technology but in people's habits, making possible the first portable consumer electronics such as the transistor radio, cassette tape player, walkie-talkie and quartz watch, as well as the first practical computers and mobile phones. Today all electronics are solid-state except in some applications such as radio transmitters, in which vacuum tubes are still used, some power industrial control circuits which use electromechanical devices such as relays. Additional examples of solid state electronic devices are the microprocessor chip, LED lamp, solar cell, charge coupled device image sensor used in cameras, semiconductor laser.
Condensed matter physics Laser diode Materials science Semiconductor device Solar cell Solid-state physics
Polycrystalline silicon called polysilicon or poly-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry. Polysilicon is produced from metallurgical grade silicon by a chemical purification process, called the Siemens process; this process involves distillation of volatile silicon compounds, their decomposition into silicon at high temperatures. An emerging, alternative process of refinement uses a fluidized bed reactor; the photovoltaic industry produces upgraded metallurgical-grade silicon, using metallurgical instead of chemical purification processes. When produced for the electronics industry, polysilicon contains impurity levels of less than one part per billion, while polycrystalline solar grade silicon is less pure. A few companies from China, Japan and the United States, such as GCL-Poly, Wacker Chemie, OCI, Hemlock Semiconductor, as well as the Norwegian headquartered REC, accounted for most of the worldwide production of about 230,000 tonnes in 2013.
The polysilicon feedstock – large rods broken into chunks of specific sizes and packaged in clean rooms before shipment – is directly cast into multicrystalline ingots or submitted to a recrystallization process to grow single crystal boules. The products are sliced into thin silicon wafers and used for the production of solar cells, integrated circuits and other semiconductor devices. Polysilicon consists of small crystals known as crystallites, giving the material its typical metal flake effect. While polysilicon and multisilicon are used as synonyms, multicrystalline refers to crystals larger than 1 mm. Multicrystalline solar cells are the most common type of solar cells in the fast-growing PV market and consume most of the worldwide produced polysilicon. About 5 tons of polysilicon is required to manufacture 1 megawatt of conventional solar modules. Polysilicon is distinct from monocrystalline silicon and amorphous silicon. In single crystal silicon known as monocrystalline silicon, the crystalline framework is homogenous, which can be recognized by an external colouring.
The entire sample is one single and unbroken crystal as its structure contains no grain boundaries. Large single crystals are rare in nature and can be difficult to produce in the laboratory. In contrast, in an amorphous structure the order in atomic positions is limited to short range. Polycrystalline and paracrystalline phases are composed of a number of smaller crystals or crystallites. Polycrystalline silicon is a material consisting of multiple small silicon crystals. Polycrystalline cells can be recognized by a visible grain, a "metal flake effect". Semiconductor grade polycrystalline silicon is converted to "single crystal" silicon – meaning that the randomly associated crystallites of silicon in "polycrystalline silicon" are converted to a large "single" crystal. Single crystal silicon is used to manufacture most Si-based microelectronic devices. Polycrystalline silicon can be as much as 99.9999% pure. Ultra-pure poly is used in the semiconductor industry, starting from poly rods that are two to three meters in length.
In microelectronic industry, poly is used both at the micro-scale level. Single crystals are grown using the Czochralski float-zone and Bridgman techniques. At the component level, polysilicon has long been used as the conducting gate material in MOSFET and CMOS processing technologies. For these technologies it is deposited using low-pressure chemical-vapour deposition reactors at high temperatures and is heavily doped n-type or p-type. More intrinsic and doped polysilicon is being used in large-area electronics as the active and/or doped layers in thin-film transistors. Although it can be deposited by LPCVD, plasma-enhanced chemical vapour deposition, or solid-phase crystallization of amorphous silicon in certain processing regimes, these processes still require high temperatures of at least 300 °C; these temperatures make deposition of polysilicon possible for glass substrates but not for plastic substrates. The deposition of polycrystalline silicon on plastic substrates is motivated by the desire to be able to manufacture digital displays on flexible screens.
Therefore, a new technique called laser crystallization has been devised to crystallize a precursor amorphous silicon material on a plastic substrate without melting or damaging the plastic. Short, high-intensity ultraviolet laser pulses are used to heat the deposited a-Si material to above the melting point of silicon, without melting the entire substrate; the molten silicon will crystallize as it cools. By controlling the temperature gradients, researchers have been able to grow large grains, of up to hundreds of micrometers in size in the extreme case, although grain sizes of 10 nanometers to 1 micrometer are common. In order to create devices on polysilicon over large-areas however, a crystal grain size smaller than the device feature size is needed for homogeneity of the devices. Another method to produce poly-Si at low temperatures is metal-induced crystallization where an amorphous-Si thin film can be crystallized at temperatures as low as 150 °C if annealed while in contact of another metal film such as aluminium, gold, or silver.
Polysilicon has many applications in VLSI manufacturing. One of its primary uses is as gate electrode material for MOS devices. A polysilicon gate's electrical conductivity may be increased by depositing a metal or a metal silicide (such as tungsten silici