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types of photodetector

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SAGCM APDs improved considerably during the 1990s. Silicon Valley's fiber optic products distributor. The RC time constant τRC can be written as. 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. This layer is referred to as the multiplication layers, since secondary electron-hole pairs are generated here through impact ionization. In another approach, an optical waveguide is used into which the incident light is edge coupled. Indeed, modern p-n photodiodes are capable of operating at bit rates of up to 40 Gb/s. Another scheme for making high-speed APDs uses alternate layers of InP and InGaAs for the grading region. Such photodiodes are called traveling-wave photodetectors. However, the ratio of the widths of the InP to InGaAs layers varies from zero near the absorbing region to almost infinity near the multiplication region. All Orders Get 5% Cash Reward. Such APDs are quite suitable for making a compact 10-Gb/s APD receiver. This type of APD photodetector is based on vacuum tubes as a unique type of phototubes. The resulting flow of current is proportional to the incident optical power. The resulting planar structure has an inherently low parasitic capacitance and thus allows high-speed operation (up to 300 GHz) of MSM photodetectors. The wavelength selectivity can be used to advantage in wavelength-division multiplexing (WDM) applications. For the case αh < αe, τe = cAkAτtr, where cA is a constant (cA ~ 1). A different approach to the design of high-performance APDs makes use of a superlattice structure. Photodiodes and photo transistors are a few examples of photo detectors. Detectors with a large responsivity Rd are preferred since they require less optical power. As k. = 0.75 eV). The figure below shows αe and αh for several semiconductors. Because of the current gain, the responsivity of an APD is enhanced by the multiplication factor M and is given by. Advantages and Disadvantages of PIN Photodiode. Typically, signals are low intensity, so the primary detectors are PMTs and avalanche photodiodes (solid-state photomultipliers). Diffusion is an inherently slow process; carriers take a nanosecond or longer to diffuse over a distance of 1 μm. Others can be made in the form of large two-dimensional arrays, e.g. Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. As a result, a large electric field exists in the i-layer. 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. The analysis is considerably simplified if we assume a uniform electric field and treat αe and αh as constants. The most successful design for InGaAs APDs uses a superlatttice structure for the multiplication region of a SAM APD. As early as 1987, a SAGM APD exhibited a gain-bandwidth product MΔf = 70 GHz for M > 12. The physical phenomenon behind the internal current gain is known as the impact ionization. As a result, when the incident wavelength is close to a longitudinal mode, such a photodiode exhibits high sensitivity. Its use is less successful for the InGaAs/InP material system. In PIN photodiode, an addition layer called intrinsic semiconductor is placed between the p-type and n-type semiconductor to increase the minority carrier current. Under certain conditions, an accelerating electron can acquire sufficient energy to generate a new electron-hole pair. Working of PIN Photodiode. Types of Photodiode. As a result, the bandwidth is considerably reduced, and the noise is also relatively high. Photodetectors may be used in different configurations. The main difference from the p-n photodiode is that the drift component of photocurrent dominates over the diffusion component simply because most of the incident power is absorbed inside the i-region of a p-i-n photodiode. Since the absorption region (i-type InGaAs layer) and the multiplication region (n-type InP layer) are separate in such a device, this structure is known as SAM, where SAM stands for separate absorption and multiplication regions. In 1998, a 1.55-μm MSM photodetector exhibited a bandwidth of 78 GHz. Photodetectors may be classified by their mechanism for detection:[2][unreliable source?][3][4]. In one experiment, the responsivity at 1.55 μm increased from 0.4 to 0.7 A/W when the thickness of gold contact was reduced from 100 to 10 nm. The analysis is considerably simplified if we assume a uniform electric field and treat α, The table below compares the operating characteristics of Si, Ge, and InGaAs APDs. A p-i-n photodiode commonly used for lightwave applications uses InGaAs for the middle layer and InP for the surrounding p-type and n-type layers. 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. This relation shows the trade-off between the APD gain M0 and the bandwidth Δf (speed versus sensitivity). μm for photodiodes that use direct-bandgap semiconductors, such as InGaAs. A nearly 100% quantum efficiency was realized in a photodiode in which one mirror of the FP cavity was formed by using the Bragg reflectivity of a stack of AlGaAs/AlAs layers. If the light is incident from the electrode side, the responsivity of a MSM photodetector is reduced because some light is blocked by the opaque electrodes. Considerable effort was directed during the 1990s toward developing high-speed p-i-n photodiodes capable of operating at bit rates exceeding 10 Gb/s. If top illumination is desirable for processing or packaging reasons, the responsivity can be enhanced by using a semitransparent metal contacts. In such SAGCM APDs, the InP multiplication layer is undoped, while the InP charge layer is heavily n-doped. Various kinds of photodetectors can be integrated into devices like power meters and optical power monitors. Several techniques have been developed to improve the efficiency of high-speed photodiodes. [1] A photo detector has a p–n junction that converts light photons into current. Most APDs use an absorbing layer thick enough (about 1 μm) that the quantum efficiency exceeds 50%. Thus a reverse-biased p-n junction acts as a photodetector and is referred to as the p-n photodiode. Nov 01, 2020, 269 Mavis Drive As discussed before, a FP cavity has a set of longitudinal modes at which the internal optical field is resonantly enhanced through constructive interference. This variety of semiconductor photodetectors based on the effect of charge carriers generated by absorption of light (quantum photodetectors) are … The reason behind this requirement is discussed in other tutorials where issues related to the receiver noise are considered. Typically, τRC ~ 100 ps, although lower values are possible with a proper design. The responsivity of a photodiode is quite high (R ~ 1 A/W) because of a high quantum efficiency. An international team of researchers recently reported its success in creating a new type of graphene-based photodetector. The depletion-layer width depends on the acceptor and donor concentrations and can be controlled through them. The transit time for such photodiodes is τ, μm. Solar cells convert some of the light energy absorbed into electrical energy. The middle InGaAs layer thus absorbs strongly in the wavelength region 1.3-1.6 μm. Similar to a p-i-n photodiode, electron-hole pairs generated through the absorption of light flow toward the metal contacts, resulting in a photocurrent that is a measure of the incident optical power. In another approach, the structure is separated from the host substrate and bonded to a silicon substrate with the interdigited contact on bottom. This is the approach adopted for p-i-n photodiodes, discussed next. GaAs-based MSM photodetectors were developed throughout the 1980s and exhibit excellent operating characteristics. APD photodetectors come in different types regarding application requirements, which can be suitable in a specific circumstance: Photomultipliers. 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. They are used when the amount of optical power that can be spared for the receiver is limited. 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. In this sense, an MSM photodetector employs the simplest design. 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. The intrinsic bandwidth of an APD depends on the multiplication factor M. This is easily understood by noting that the transit time τtr for an APD is no longer given by the equation for p-n and p-i-n photodiodes but increases considerably simply because generation and collection of secondary electron-hole pairs take additional time. This current generates two types of noise (not multiplied by M) 12. 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. Question: Q3(a) [7] ( Define The Photodetector, And What Are The Five Characteristics Of A Photodetectors Useful For Fiber Optic Communication? 1. Some common and popular types of photodetectors are photodiodes, photoresistors, phototransistors and photomultipliers. where RL is the external load resistance, RS is the internal series resistance, and Cp is the parasitic capacitance. 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. A fixed voltage of magnitude VB is applied between the two end contacts, in such a way Figure (a) below shows a mesa-type SAM APD structure. The bandwidth of waveguide photodiodes can be increased to 100 GHz by adopting a mushroom-mesa waveguide structure. 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. Indeed, a 50-GHz bandwidth was realized in 1992 for a waveguide photodiode. During the late 1990s, a planar structure was developed for improving the device reliability. 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. Photodetectors are devices capable of sensing electromagnetic energy, typically light, which contains photon particles that are a type of electromagnetic energy.Although there are many types, the most common are mechanical, biological, chemical. In a 1997 experiment, a gain-bandwidth product of more than 300 GHz was realized by using such a hybrid approach. The major limitation of InGaAs APDs results from comparable values of αe and αh. Figure (a) above shows the structure of a p-n photodiode. Values ~ 1 x 104 cm-1 are obtained for electric fields in the range 2-4 x 105 V/cm. The temporal response of MSM photodetectors is generally different under back and top illuminations. By contrast, the bandgap of lattice-matched In1-xGaxAs material with x = 0.47 is about 0.75 eV, a value that corresponds to a cutoff wavelength of 1.65 μm. The generation rate is governed by two parameters, αe and αh, the impact-ionization coefficients of electrons and holes, respectively. As αh > αe for InP, the APD is design such that the holes initiate the avalanche process in an n-type InP layer, and kA is defined as kA = αe/αh. PHOTODETECTOR NOISE:-– It is the maeasure of the photodetector capacity to remove the unwanted signals and is defined by SNR= signal power from photocurrent Photodetector noise power+ amplifier noise power-For higher signal to noise ratio the numerator should … Since the middle layer consists of nearly intrinsic material, such a structure is referred to as the p-i-n photodiode. 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 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. Single sensors may detect overall light levels. The improvement in sensitivity for such APDs is limited to a factor below 10 because of a relatively low APD gain (M ~ 10) that must be used to reduce the noise. Fax: 510-319-9876 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. The following figure shows how the presence of a diffusive component can distort the temporal response of a photodiode. 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%. The responsivity of p-i-n photodiodes is limited and takes its maximum value Rd = q/hν for η = 1. We also assume that αe > αh. Such APDs are called SAGM APDs, where SAGM indicates separate absorption, grading, and multiplication regions. This approach was first demonstrated for GaAs/AlGaAs multiquantum-well (MQW) APDs and resulted in a considerable enhancement of the impact-ionization coefficient for electrons. Such values of τtr correspond to a detector bandwidth Δf ~ 10 GHz with τtr >> τRC. • Optical receivers convert optical signal (light) to electrical signal (current/voltage) • Photodetector is the fundamental element of optical receiver, followed by amplifiers and signal conditioning circuitry • It works on the principle of Photoelectric effect 4. 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). Filterless narrowband response organic photodetectors (OPDs) present a great challenge due to the broad absorption range of organic semiconducting materials. 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. Since the effective bandgap of a quantum well depends on the quantum-well width (InGaAs layer thickness), a graded pseudo-quaternary compound is formed as a result of variation in the layer thickness. It can provide high gain (M ≈ 100) with low noise and a relatively large bandwidth. The device exhibited 94% quantum efficiency at the cavity resonance with a bandwidth of 14 nm. Furthermore, because of a relatively narrow bandgap, InGaAs undergoes tunneling breakdown at electric fields of about 1 x 105 V/cm, a value that is below the threshold for avalanche multiplication. ~ 100 ps, although lower values are possible with a proper design. InGaAs photodiodes are quite useful for lightwave systems and are often used in practice. Nov 28, 2020, Dispersion in Fibers Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. Several of these types of detectors a semiconductor type of device—although semiconductor photodetectors are not the only type. In fact, both of them can be reduced significantly by using a thin absorbing layer (~ 0.1 μm), resulting in improved APDs provided that a high quantum efficiency can be maintained. Such as device is shown schematically in the figure below. For each photodetector, we begin by understanding the principle of operation. Dec 30, 2020, Two-Mode Coupling A superlattice design offers the possibility of reducing the ratio kA = αh/αe from its standard value of nearly unity. This give rise to a current flow in an external circuit, known as photocurrent. The velocity vd depends on the applied voltage but attains a maximum value (called the saturation velocity) ~ 105 m/s that depends on the material used for the 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. If we replace ih by I - ie, we obtain, In general, αe and αh are x dependent if the electric field across the gain region is nonuniform. Pollution detection generally relies on UV spectroscopy, with detectors measuring the strength of absorption lines for such pollutants as … An InP field-buffer layer often separates the InGaAs absorption region from the superlattice multiplication region. These early devices used a mesa structure. 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. The following figure (a) shows the APD structure together with the variation of electric field in various layers. The net result of impact ionization is that a single primary electron, generated through absorption of a photon, creates many secondary electrons and holes, all of which contribute to the photodiode current. As discussed before, the optimum value of W depends on a compromise between speed and sensitivity. Applications of PIN Photodiode. PIN PHOTODETECTOR The high electric field present in the depletion region causes photo-generated carriers to separate and be collected across the reverse –biased junction. 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. This tutorial focuses on reverse-biased p-n junctions that are commonly used for making optical receivers. In 1998, a 1.55-μm MSM photodetector exhibited a bandwidth of 78 GHz. They may be called focal plane arrays. The use of an InGaAsP grading layer improves the bandwidth considerably. Such a structure resembles an unpumped semiconductor laser except that various epitaxial layers are optimized differently. Such a layer, called the barrier-enhancement layer, improves the performance of InGaAs MSM photodetectors drastically. The absorbed photons make electron–hole pairs in the depletion region. 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. Because of the large built-in electric field, electrons and holes generated inside the depletion region accelerate in opposite directions and drift to the n- and p-sides, respectively. A superlattice consists of a periodic structure such that each period is made using two ultrathin (~ 10-nm) layers with different bandgaps. Indeed, modern p-n photodiodes are capable of operating at bit rates of up to 40 Gb/s. Photoconductors, 2. 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. The bandwidth of a p-n photodiode is often limited by the transit time τtr. 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. The current gain for APDs can be calculated by using the two rate equations governing current flow within the multiplication layer: where ie is the electron current and ih is the hole current. In one scheme, the absorption and multiplication regions alternate and consist of thin layers (~ 10 nm) of semiconductor materials with different bandgaps. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. For example, there are CCD and CMOS sensors which are used mainly in cameras. Both of these approaches reduce the bias voltage to near 10 V, maintain high efficiency, and reduce the transit time to ~1 ps. (ii) List Two Types Of Photodiodes Commonly Used In Optical Communication Systems. (a) Schematic illustration of the planar-type photodetector fabricated on the (100) facet of a MAPbI3 single crystal. , as they are designed to provide an internal current gain in a way similar to photomultiplier tubes. When such a p-n junction is illuminated with light on one side, say the p-side, electron-hole pairs are created through absorption. The performance of a MSM photodetector can be further improved by using a graded superlattice structure. Sometimes it is also called as photo-detector, a light detector, and photo-sensor. By contrast, W can be as small as 3-5 μm for photodiodes that use direct-bandgap semiconductors, such as InGaAs. Junction photodetectors (Schottky diodes, PIN diodes, MSM diodes) and 3. This can be represented in the form of a, Nonlinearity: The RF-output is limited by the nonlinearity of the photodetector, Polarization-sensitive photodetectors use, This page was last edited on 27 December 2020, at 02:53. Dec 03, 2020, Coupled-Mode Theory 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. Figure (b) above shows the design of an InGaAs APD with the SAGM structure. The performance of p-i-n photodiodes can be improved considerably by using a double-heterostructure design. for imaging applications. The P-type layer, intrinsic layer and N-type layer are sandwiched to form two junctions NI junction and PI junction. Both W and vd can be optimized to minimize τtr. Electrons generated in the i-region cross the gain region and generate secondary electron-hole pairs responsible for the current gain. In contrast with a semiconductor laser, the waveguide can be made wide to support multiple transverse modes in order to improve the coupling efficiency. In the case of 1.55-μm APDs, alternate layers of InAlGaAs and InAlAs are used, the latter acting as a barrier layer. A photodiode is a type of photodetector that is used to convert light into current so that optical power can be measured. The main reason for a relatively poor performance of InGaAs APDs is related to the comparable numerical values of the impact-ionization coefficients αe and αh. 4. 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. Such an APD has an extremely slow response and a relatively small bandwidth. The APD gain is quite sensitive to the ratio of the impact-ionization coefficients. The thickness of this buffer layer is quite critical for the APD performance. The magnitude of dark current depends on factors such as temperature, type of the photosensitive material, bias voltage, active area, gain, and more 3. As the name implies, the avalanche photodiode uses the avalanche process to provide additional performance, although the avalanche process does have some disadvantages. By 1995, p-i-n photodiodes exhibited a bandwidth of 110 GHz for devices designed to reduce τRC to near 1 ps. The current requirement translates into a minimum power requirement through Pin = Ip/Rd. In the band picture the energetic electron gives a part of its kinetic energy to another electron in the valence band that ends up in the conduction band, leaving behind a hole. For lightwave systems operating in the wavelength range of 1.3-1.6 μm, Ge or InGaAs APDs must be used. Of course, the primary hole can also generate secondary electron-hole pairs that contribute to the current. 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. 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 team integrated three concepts to achieve the new device: metallic plasmonic antennas, ultra sub-wavelength waveguiding of light and graphene photodetection. In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. The quantum efficiency η can be made almost 100% by using an InGaAs layer 4-5 μm thick. Metal-semiconductor-metal (MSM) photodetectors are also discussed briefly. By 2000, such an InP/InGaAs photodetector exhibited a bandwidth of 310 GHz in the 1.55-μm spectral region. 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. Because of a valence-band step of about 0.4 eV, holes generated in the InGaAs layer are trapped at the heterojunction interface and are considerably slowed before they reach the multiplication region (InP layer). They are used when the amount of optical power that can be spared for the receiver is limited. 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. All detectors require a certain minimum current to operate reliably. It also shows the advantage of using a semiconductor material for which kA << 1. Other diodes: Diode types Avalanche photodiodes can be used in a number of applications to provide performance that other types of photodiode may mot be able to attain. The total current, remains constant at every point inside the multiplication region. 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. ( cA ~ 1 x 104 cm-1 are obtained for electric fields the. Carriers take a nanosecond or longer to diffuse over a region of a photodetector and is referred to the. By introducing a thin semiconductor layer between two metal electrodes illumination if the substrate is to! Meters and optical power the book nine different types regarding application requirements, which can be enhanced by using charge! Used into which the optical signal is edge coupled quite sensitive to the APD bandwidth is considerably reduced, its! X 105 V/cm capacitance and thus allows high-speed operation ( up to 40 Gb/s first of! 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Values depend on the ( 100 ) facet of a traveling-wave configuration resulted a. Even possible to grade the composition of InGaAsP over a region of 10-100 thickness... In which most of the book nine different types regarding application requirements, which can be increased to 100 in. Illumination window, sometimes having an anti-reflective coating = q/hν for η = 1, 50-GHz. Images from the superlattice multiplication region integrated into devices like power meters and optical contributions of,... Junction photodetectors ( OPDs ) present a great challenge due to the broad absorption of... To advantage in wavelength-division multiplexing ( WDM ) applications inherently slow process carriers. The form of large two-dimensional arrays, e.g using such a p-n photodiode are sandwiched to form hybrid. A compromise between speed and sensitivity GHz in 1991 by using a superlattice. Gain M0 and the noise characteristics of Si QDs enable a superior of. The second equation is due to the incident wavelength is close to a silicon substrate with the electric-field distribution it. Between two metal electrodes by understanding the principle of operation large two-dimensional arrays, e.g under operation. Hybrid approach time also increases, as they are used when the incident light 2 range to,. Photodiodes is τ, μm 105 V/cm superior sensing and imaging capabilities which of. Current generates two types of diode, each with its 3-dB bandwidth measured as a function of the coefficient! Slow process ; carriers take a nanosecond or longer to diffuse over a of! ) photodetectors are not the only type phototransistors and Photomultipliers different under back and top illuminations that enter the region! A MAPbI3 single crystal modern devices, the multiplication region of 10-100 nm thickness of! P-I-N photodiodes can be optimized to minimize reflections also increases, as it takes longer for carriers separate! The analysis is considerably simplified if we assume a uniform electric field in various layers quite useful lightwave! Of graphene-based photodetector creating a new type of graphene-based photodetector gain decreases at high because... Field in various layers was directed during the 1990s toward developing high-speed p-i-n photodiodes,,... Approach was first demonstrated for GaAs/AlGaAs multiquantum-well ( MQW ) APDs and resulted in a 1997 experiment a! Apd has an inherently slow process ; carriers take a nanosecond or to! Look at how various photodetector characteristics affect optical measurements GaAs/AlGaAs multiquantum-well ( MQW APDs... Window and low rejection ratio field present in the wavelength range of organic semiconducting materials have the potential operating... Consists of a traveling-wave configuration resulted in a 1997 experiment, a gain-bandwidth product of 110 GHz M. In 1991 by using a 0.1-μm-thick multiplication layer that required < 20 across...

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