A reverse-biased p-n junction consists of a region, known as the depletion region, that is essentially devoid of free charge carriers and where a large built-in electric field opposes flow of electrons from the n-side to the p-side (and of holes from p to n). The temporal response of MSM photodetectors is generally different under back and top illuminations. The APD exhibited a 3-dB bandwidth of over 9 GHz for values of M as high as 35 while maintaining a 60% quantum efficiency. For lightwave systems operating in the wavelength range of 1.3-1.6 μm, Ge or InGaAs APDs must be used. By integrating this equation, the multiplication factor defined as M = ie(d)/ie(0) is given by. The table below compares the operating characteristics of Si, Ge, and InGaAs APDs. The performance of waveguide photodiodes can be improved further by adopting an electrode structure designed to support traveling electrical waves with matching impedance to avoid reflections. During the late 1990s, a planar structure was developed for improving the device reliability. Figure (a) below shows the basic design. Both W and vd can be optimized to minimize τtr. Several techniques have been developed to improve the efficiency of high-speed photodiodes. ~ 100 ps, although lower values are possible with a proper design. As a result, when the incident wavelength is close to a longitudinal mode, such a photodiode exhibits high sensitivity. The resulting flow of current is proportional to the incident optical power. Such large fields can be realized by applying a high voltage (~ 100 V) to the APD. For the case αh < αe, τe = cAkAτtr, where cA is a constant (cA ~ 1). Here, we proposed a hybrid BP/lead sulfide quantum dot photodetector with a cascade-type energy band structure, which can greatly improve the performance of this photodetector compared with a single-layer absorber. Although higher APD gain can be realized with a smaller gain region when αh and αe are comparable, the performance is better in practice for APDs in which either αe >> αh or αh >> αe, so that the avalanche process is dominated by only one type of charge carrier. In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In one approach, a Fabry-Perot (FP) cavity is formed around the p-i-n structure to enhance the quantum efficiency, resulting in a laser-like structure. where M0 = M(0) is the low-frequency gain and τe is the effective transit time that depends on the ionization coefficient ratio kA = αh/αe. A 2-D array of photodetectors may be used as an image sensor to form images from the pattern of light before it. Values ~ 1 x 104 cm-1 are obtained for electric fields in the range 2-4 x 105 V/cm. By using the condition ih(d) = 0 (only electrons cross the boundary to enter the n-region), the boundary condition then is ie(d) = I. The physical origin of the diffusive component is related to the absorption of incident light outside the depletion region. The physical phenomenon behind the internal current gain is known as the impact ionization. The development of InGaAs-based MSM photodetectors, suitable for lightwave systems operating in the range 1.3-1.6 μm, started in the late 1980s, with most progress made during the 1990s. Bandwidths of up to 70 GHz were realized as early as 1986 by using a thin absorption layer (< 1 μm) and by reducing the parasitic capacitance Cp with a small size, but only at the expense of a lower quantum efficiency and responsivity. Similar to the case of semiconductor lasers, the middle i-type layer is sandwiched between the p-type and n-type layers of a different semiconductor whose bandgap is chosen such that light is absorbed only in the middle i-layer. Adding a light source to the device effectively "primed" the detector so that in the presence of long wavelengths, it fired on wavelengths that otherwise lacked the energy to do so. Since the depletion width W can be tailored in p-i-n photodiodes, a natural question is how large W should be. The major problem with the InGaAs is its relatively low Schottky-barrier height (about 0.2 eV). They are used when the amount of optical power that can be spared for the receiver is limited. Such APDs are called SAGM APDs, where SAGM indicates, Most APDs use an absorbing layer thick enough (about 1 μm) that the quantum efficiency exceeds 50%. Types of Photodiode. It is even possible to grade the composition of InGaAsP over a region of 10-100 nm thickness. [16], In 2014 a technique for extending semiconductor-based photodetector's frequency range to longer, lower-energy wavelengths. All Orders Get 5% Cash Reward. If W is the width of the depletion region and vd is the drift velocity, the transit time is given by, Typically, W ~10 μm, vd ~ 105 m/s, and τtr ~ 100 ps. Such devices exhibit a low dark-current density, a responsivity of about 0.6 A/W at 1.3 μm, and a rise time of about 16 ps. Working of PIN Photodiode. A gain-bandwidth product of 140 GHz was realized in 2000 using a 0.1-μm-thick multiplication layer that required < 20 V across it. Semiconductor photodetectors, commonly referred to as photodiodes, are the predominant types of photodetectors used in optical communication systems because of their small size, fast detection speed, and high detection efficiency. Part one covers materials, detector types, and devices, and includes discussion of silicon photonics, detectors based on reduced dimensional charge systems, carbon nanotubes, graphene, nanowires, low-temperature grown gallium arsenide, plasmonic, Si photomultiplier tubes, and organic photodetectors, while part two focuses on important applications of photodetectors, including microwave photonics, … The resulting current flow constitutes the photodiode response to the incident optical power in accordance with the equation we derived earlier. A particularly useful design, shown below, is known as reach-through APD because the depletion layer reaches to the contact layer through the absorption and multiplication regions. Engineers from the UCLA have Used graphene to design a new type of photodetector that can work with more types of light than its current state-of-the-art counterparts. Construction of PIN Photodiode. For indirect-bandgap semiconductors such as Si and Ge, typically W must be in the range 20-50 μm to ensure a reasonable quantum efficiency. This problem can be solved by placing the two metal contacts on the same (top) side of an epitaxially grown absorbing layer using an interdigited electrode structure with a finger spacing of about 1 μm. A hybrid approach in which a Si multiplication layer is incorporated next to an InGaAs absorption layer may be useful provided the heterointerface problems can be overcome. The bandwidth of such photodiodes is then limited by a relatively long transit time (τtr > 200 ps). In one scheme, the absorption and multiplication regions alternate and consist of thin layers (~ 10 nm) of semiconductor materials with different bandgaps. If top illumination is desirable for processing or packaging reasons, the responsivity can be enhanced by using a semitransparent metal contacts. Since absorption takes place along the length of the optical waveguide (~ 10 μm), the quantum efficiency can be nearly 100% even for an ultrathin absorption layer. In a different kind of photodetector, known as a metal-semiconductor-metal (MSM) photodetector, a semiconductor absorbing layer is sandwiched between two metal electrodes. These diodes are particularly designed to work in reverse bias condition, it means that the P-side of the photodiode is associated with the negative terminal of the battery and n-side is connected to the positive terminal of the battery. The quantum efficiency η can be made almost 100% by using an InGaAs layer 4-5 μm thick. A PN junction photodiode is made of two layers namely p-type and n-type semiconductor whereas PIN photodiode is made of three layers namely p-type, n-type and intrinsic semiconductor. The problem can be solved by using another layer between the absorption and multiplication regions whose bandgap is intermediate to those of InP and InGaAs layers. Various kinds of photodetectors can be integrated into devices like power meters and optical power monitors. In the case of 1.55-μm APDs, alternate layers of InAlGaAs and InAlAs are used, the latter acting as a barrier layer. In another approach, the structure is separated from the host substrate and bonded to a silicon substrate with the interdigited contact on bottom. The APD gain is quite sensitive to the ratio of the impact-ionization coefficients. The planar structure of MSM photodetectors is also suitable for monolithic integration. (b) Photocurrent versus voltage curves under various irradiation densities. 1. It was measured by using a spectrum analyzer (circles) as well as taking the Fourier transform of the short-pulse response (solid curve). SAGCM APDs improved considerably during the 1990s. This tutorial focuses on reverse-biased p-n junctions that are commonly used for making optical receivers. This problem can be solved in heterostructure APDs by using an InP layer for the gain region because quite high electric fields (> 5 x 105 V/cm) can exist in InP without tunneling breakdown. As discussed before, a FP cavity has a set of longitudinal modes at which the internal optical field is resonantly enhanced through constructive interference. In another approach, an optical waveguide is used into which the incident light is edge coupled. Diffusion is an inherently slow process; carriers take a nanosecond or longer to diffuse over a distance of 1 μm. Since the middle layer consists of nearly intrinsic material, such a structure is referred to as the p-i-n photodiode. The depletion-layer width depends on the acceptor and donor concentrations and can be controlled through them. The bandwidth of a p-n photodiode is often limited by the transit time τtr. GaAs-based MSM photodetectors were developed throughout the 1980s and exhibit excellent operating characteristics. The diffusion contribution can be reduced by decreasing the widths of the p- and n-regions and increasing the depletion-region width so that most of the incident optical power is absorbed inside it. Nonetheless, considerable progress has been made through the so-called staircase APDs, in which the InGaAsP layer is compositionally graded to form a sawtooth kind of structure in the energy-band diagram that looks like a staircase under reverse bias. A photodiode is a type of photodetector that is used to convert light into current so that optical power can be measured. 4. By 2002, the use of a traveling-wave configuration resulted in a GaAs-based device operating near 1.3 μm with a bandwidth > 230 GHz. Figure (a) below shows a mesa-type SAM APD structure. The decrease in M(ω) can be written as. Such APDs are quite suitable for making a compact 10-Gb/s APD receiver. This problem can be solved through back illumination if the substrate is transparent to the incident light. Photodetectors may be classified by their mechanism for detection:[2][unreliable source?][3][4]. The performance of p-i-n photodiodes can be improved considerably by using a double-heterostructure design. A photodiode is a PN-junction diode that consumes light energy to produce electric current. Nov 14, 2020, Attenuation in Fibers Electrons generated in the p-region have to diffuse to the depletion-region boundary before they can drift to the n-side; similarly, holes generated in the n-region must diffuse to the depletion-region boundary. In 1998, a 1.55-μm MSM photodetector exhibited a bandwidth of 78 GHz. By 2000, such an InP/InGaAs photodetector exhibited a bandwidth of 310 GHz in the 1.55-μm spectral region. Pleasanton, CA 94566 As kA << 1 for Si, silicon APDs can be designed to provide high performance and are useful for lightwave systems operating near 0.8 μm at bit rates ~100 Mb/s. In some cases, it is possible to operate a photodetector without dark current; however, there are tradeoffs. A packaged device had a bandwidth of 4 GHz despite a large 150 μm diameter. The magnitude of dark current depends on factors such as temperature, type of the photosensitive material, bias voltage, active area, gain, and more 3. It also shows the advantage of using a semiconductor material for which kA << 1. Indeed, such an APD receiver was used for a 10-Gb/s lightwave system with excellent performance. for imaging applications. Such devices exhibit a low dark-current density, a responsivity of about 0.6 A/W at 1.3 μm, and a rise time of about 16 ps. In contrast with a semiconductor laser, the waveguide can be made wide to support multiple transverse modes in order to improve the coupling efficiency. Some common and popular types of photodetectors are photodiodes, photoresistors, phototransistors and photomultipliers. It can provide high gain (M ≈ 100) with low noise and a relatively large bandwidth. In a GaAs-based implementation of this idea, a bandwidth of 172 GHz with 45% quantum efficiency was realized in a traveling-wave photodetector designed with a 1-μm-wide waveguide. There are a number of performance metrics, also called figures of merit, by which photodetectors are characterized and compared[2][3]. The avalanche process is initiated by electrons that enter the gain region of thickness d at x = 0. Similar to the structures of … The RC time constant τRC can be written as. An external quantum efficiency of ~70% and a gain-bandwidth product of 270 GHz were realized in such a 1.55-μm APD using a 60-nm-thick absorbing layer with a 200-nm-thick multiplication layer. As a result, a large electric field exists in the i-layer. The P-type layer, intrinsic layer and N-type layer are sandwiched to form two junctions NI junction and PI junction. Waveguide photodiodes have been used for 40-Gb/s optical receivers and have the potential for operating at bit rates as high as 100 Gb/s. The reason behind this requirement is discussed in other tutorials where issues related to the receiver noise are considered. It should be mentioned that the avalanche process in APDs is intrinsically noisy and results in a gain factor that fluctuates around an average value. For a 52-nm-thick field-buffer layer, the gain-bandwidth product was limited to MΔf = 120 GHz but increased to 150 GHz when the thickness was reduced to 33.4 nm. The APD gain then becomes infinite for αed = 1, a condition known as the avalanche breakdown. All detectors require a certain minimum current to operate reliably. In a 1997 experiment, a gain-bandwidth product of more than 300 GHz was realized by using such a hybrid approach. 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In various layers its 3-dB bandwidth measured as a barrier layer the impact ionization diffusive component can distort the response. On top of bulk Si to form images from the top realize efficient photodiodes. Silicon substrate with the SAGM structure a great challenge due to the average APD gain then infinite.

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