Conventional Photodiodes Research Articles

Performance comparisons between PIN and APD photodetectors for use in optical communication systems

In this report, a performance comparison of the conventional PIN photodiode with the Avalanche Photodiode (APD) in an optical communication system is presented. The effects of bandwidth, gain, extinction ratio, shot noise and thermal noise are compared and studied in detail. It was shown that the Q factor produced by each detector is heavily affected by the thermal noise in the PIN device, and by both the thermal and shot noise in the APD. It was also found that the APD's gain plays a significant role, and the shot noise has to be carefully dealt with. Additionally, the relationship of receiver sensitivity with thermal and shot noise was investigated and compared.

Comparative study of various pixel photodiodes for digital radiography: Junction structure, corner shape and noble window opening

Recently, large-size 3-transistors (3-Tr) active pixel complementary metal-oxide silicon (CMOS) image sensors have been being used for medium-size digital X-ray radiography, such as dental computed tomography (CT), mammography and nondestructive testing (NDT) for consumer products. We designed and fabricated 50 µm × 50 µm 3-Tr test pixels having a pixel photodiode with various structures and shapes by using the TSMC 0.25-m standard CMOS process to compare their optical characteristics. The pixel photodiode output was continuously sampled while a test pixel was continuously illuminated by using 550-nm light at a constant intensity. The measurement was repeated 300 times for each test pixel to obtain reliable results on the mean and the variance of the pixel output at each sampling time. The sampling rate was 50 kHz, and the reset period was 200 msec. To estimate the conversion gain, we used the mean-variance method. From the measured results, the n-well/p-substrate photodiode, among 3 photodiode structures available in a standard CMOS process, showed the best performance at a low illumination equivalent to the typical X-ray signal range. The quantum efficiencies of the n+/p-well, n-well/p-substrate, and n+/p-substrate photodiodes were 18.5%, 62.1%, and 51.5%, respectively. From a comparison of pixels with rounded and rectangular corners, we found that a rounded corner structure could reduce the dark current in large-size pixels. A pixel with four rounded corners showed a reduced dark current of about 200fA compared to a pixel with four rectangular corners in our pixel sample size. Photodiodes with round p-implant openings showed about 5% higher dark current, but about 34% higher sensitivities, than the conventional photodiodes.

High speed two-dimensional optical beam position detector

Disclosed is the design of a high speed two-dimensional optical beam position detector which outputs the X and Y displacement and total intensity linearly. The experimental detector measures the displacement from DC to 123 MHz and the intensity of an optical spot in a similar way as a conventional quadrant photodiode detector. The design uses four discrete photodiodes and simple dedicated optics for the position decomposition which enables higher spatial accuracy and faster electronic processing than conventional detectors. Measurements of the frequency response and the spatial sensitivity demonstrate high suitability for atomic force microscopy, scanning probe data storage applications, and wideband wavefront sensing. The operation principle allows for position measurements up to 20 GHz and more in bandwidth.

XB<italic>n</italic> barrier photodetectors based on InAsSb with high operating temperatures

We demonstrate the suppression of the bulk generation- recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark cur- rents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn + ). This results in a significant shortening of the device cut- off wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10 −7 A/cm 2 at 150 K. Such a device with a4 -μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have madenBnnfocalplane arraydetectors(FPAs)with a 320 ×256 formatand a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns. C 2011 Society of Photo-Optical Instrumentation Engineers

Solar-blind photodiodes composed of a Au Schottky contact and a β-Ga2O3 single crystal with a high resistivity cap layer

We fabricated Ga2O3 photodiodes composed of a Au Schottky contact and a β-Ga2O3 single-crystal substrate with a sol–gel prepared high resistivity cap layer. The photodiodes with the cap layer showed solar-blind photosensitivity under both forward and reverse biases in contrast to conventional Schottky photodiodes. Finally, we proposed energy band diagram of the i-n junction to determine the photodetection mechanism of our photodiodes. The photoconductive device model explained the high responsivity of over 1 A/W at forward bias. In this model, the cap layer behaves like a photoconductor, and the substrate behaves like an electrode that replenishes electrons.

Linear-Cascade Near-Ballistic Unitraveling-Carrier Photodiodes With an Extremely High Saturation Current–Bandwidth Product

Saturation current-bandwidth product (SCBP), the key of figure of merit in high-speed and high-power photodiodes (PDs), is mainly limited by the tradeoff between carrier drift time in depletion layer and RC-limited bandwidth of conventional PDs. Here, we present a revolutionary photodiode structure: linear-cascade photodiodes (LCPDs), designed to further improve the SCBP performance. Our demonstrated LCPD structure can greatly increase the SCBP without using a complex distributed structure of the traveling-wave PD or reducing the load resistance (output RF power). Two flip-chip bonding packaged near-ballistic unitraveling-carrier photodiode (NBUTC-PD) units are employed in our LCPD structure. It exhibits a great improvement in SCBP compared to that of the control device with a single NBUTC-PD. A two-port equivalent-circuit model is established for the LCPDs and the modeling results clearly indicate that the increase in SCBP can be attributed to the significant reduction in its total capacitance due to the serial connection. Furthermore, we find that only when each PD unit in the LCPD structure has the same amount of injected optical power and modulated frequency of optical signal, the whole structure exhibits a carrier transit time as short as that of a single PD. Under the proper optical excitation, we can achieve a record high SCBP (7500 mA·GHz and 100 GHz) for two-element LCPDs under a 50 Ω load.

Heterojunction bandgap engineered photodetector based on type-II InAs/GaSb superlattice for single color and bicolor infrared detection

We report a heterostructure bandgap engineered strained layer superlattice photodetector design for performance improvement for longwave infrared (LWIR) detection. At 77 K, the dark current density of the reported device was at least two orders of magnitude lower than that of the conventional PIN superlattice photodiode. We have obtained a shot-noise limited detectivity of 8.7 × 10 10 cm Hz 1/2 W −1 ( λ c = 10.8 μm), responsivity of 1.8 A/W, 23% QE at 250 mV of applied reverse bias. A three contact heterojunction bandgap engineered dual color detector was demonstrated with simultaneous detection of midwave and longwave infrared radiation. The design showed midwave responsivity of 0.93 A/W and detectivity of 1.0 × 10 11 cm Hz 1/2 W −1 (at 4 μm) at 77 K for V b = −70 mV. Longwave responsivity and detectivity of 1.5 A/W and 2.42 × 10 10 cm Hz 1/2 W −1 (at 10 μm) were observed at −100 mV of applied bias at 77 K.

Nanometer-Scale Oxide Thin Film Transistor with Potential for High-Density Image Sensor Applications

The integration of electronically active oxide components onto silicon circuits represents an innovative approach to improving the functionality of novel devices. Like most semiconductor devices, complementary-metal-oxide-semiconductor image sensors (CISs) have physical limitations when progressively scaled down to extremely small dimensions. In this paper, we propose a novel hybrid CIS architecture that is based on the combination of nanometer-scale amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) and a conventional Si photo diode (PD). With this approach, we aim to overcome the loss of quantum efficiency and image quality due to the continuous miniaturization of PDs. Specifically, the a-IGZO TFT with 180 nm gate length is probed to exhibit remarkable performance including low 1/f noise and high output gain, despite fabrication temperatures as low as 200 °C. In particular, excellent device performance is achieved using a double-layer gate dielectric (Al₂O₃/SiO₂) combined with a trapezoidal active region formed by a tailored etching process. A self-aligned top gate structure is adopted to ensure low parasitic capacitance. Lastly, three-dimensional (3D) process simulation tools are employed to optimize the four-pixel CIS structure. The results demonstrate how our stacked hybrid device could be the starting point for new device strategies in image sensor architectures. Furthermore, we expect the proposed approach to be applicable to a wide range of micro- and nanoelectronic devices and systems.

Performance improvement of large area GaN MSM photodiode with thin AlGaN surface layer

PurposeThe purpose of this paper is to propose a new structure of GaN based metal‐semiconductor‐metal (MSM) photodiode, in which a thin unintentionally doped n‐type AlGaN layer is added on the conventional GaN on Si(111) device structure.Design/methodology/approachA thin Al0.50Ga0.50N cap layer of 100 nm was incorporated in GaN MSM photodiode to enhance the effective Schottky barrier height and reduce the dark current. When the incident light with photon energy higher than the band edge of GaN but lower than the bandgap of AlGaN illuminates the front face of photodiode, the light can be transparent in the top AlGaN layer and is only absorbed by the GaN layer. As a result, the photogenerated carriers in the GaN layer would be influenced by the interface states of AlGaN/GaN. It is known that the density of the interface states is normally lower than that of surface states, so the recombination of photogenerated electron‐hole pairs will be reduced. A barrier height of 0.54 eV for normal GaN MSM photodiode was increased to the effective barrier height of 0.60 eV.FindingsThe resulting MSM photodiode shows a dark current of as low as 8.0×10−4 A at 5 V bias, which is about two orders of magnitude lower than that of normal GaN (1.0×10−2 A at 5 V bias) MSM photodiode.Originality/valueThe paper reports on barrier enhanced GaN Schottky MSM photodiode using a thin AlGaN cap layer. AlGaN cap layers were found to effectively suppress the leakage current of the GaN Schottky MSM photodiode, resulting in improved device characteristics. The dark current for the Schottky contact with the AlGaN cap layer was shown to be about about two orders of magnitude smaller than that of conventional GaN Schottky MSM photodiode.

The effect of photodiode shape on charge transfer in CMOS image sensors

The output signal of a CMOS image sensor is derived from the electrons generated by light incident on the pinned photodiode in each pixel. The output voltage depends on the transfer of the signal electrons from the pinned photodiode to the readout node. In large photodiodes it becomes difficult to fully extract the generated electrons because of the reduction in lateral electric field that pushes the electrons to the transfer transistor. To enhance the transfer, a triangular shaped photodiode was proposed which increases the lateral electric field applied to the photo generated electrons. Compared with a conventional rectangular shaped pixel, the electrons are extracted more easily in a triangular shape since the narrow tail end forms an increased potential gradient. A VGA type image sensor chip showed that the triangular photodiode had 50% higher output voltage than a conventional rectangular photodiode. It was verified that the output voltage is enhanced more by the electric force than the light receiving area. The noise due to the residual electrons was not seen in the image acquired from image sensor having triangular photodiode.

Use of epitaxial unipolar barriers to block surface leakage currents in photodetectors

AbstractSurface leakage currents are one of the many causes of dark current exhibited by semiconductor devices and are a common limiting factor to device performance for many types of devices. Traditionally, these surface leakage currents are controlled through the post‐epitaxial insertion of a surface passivation layer, but surface passivation is often expensive, time consuming, and not entirely effective. Unipolar barriers may be incorporated into electronic devices as a less expensive and more effective alternative way to control surface leakage currents. Unipolar barrier devices are demonstrated here using MBE‐grown InAs nBn detectors and unipolar barrier photodiodes. A commercial InAs photodiode becomes surface leakage current limited at 220 K while the nBn detector shows no surface leakage current down to at least 135 K. The unipolar barrier photodiode shows no detectable surface leakage down to at least 130 K while a matching conventional photodiode is limited by surface conduction at a temperature of 150 K. This indicates the greatly improved performance of the unipolar barrier devices (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Linear External Optical Gain Photodetector at Few-Photon Level

Here we show that by using an external optical gain instead of electronic gain, a conventional photodiode was able to respond linearly to few-photon pulses. Few-photon pulses could be detected after amplification to ~ 104-photon pulses by stimulated emission in erbium-doped fiber amplifiers. The voltage amplitude of the peak output signal from the photodiode was proportional to the incident photon numbers and no saturation effects were observed. Such a linear external optical gain photodetector was implemented in an ultralow light interference experiment, showing its prospective potential application as a simple but robust solution for the ultralow optical detection.

A humidity sensor based on a hetero-core optical fiber

A new type of fiber optic sensor has been developed for sensing relative humidity (RH). This RH sensor was fabricated by using a hetero-core optical fiber structure that was coated with hygroscopic polymer layers by a layer-by-layer technique, producing a [poly-glutamic acid/poly-lysine] nanostructured overlay. The refractive index of the polymer layer varies with RH around the sensor, so that the light intensity transmitted in the fiber is found to be affected by the ambient RH because the layer condition influences the light leakage from the fiber. It is found that the intensity change of 0.26 dB is observed at a wavelength of 1310 nm in the range of 50–92.9%RH, which is appreciably large to conventional photo diode detection. Additionally, discussion is made in detail on the operational mechanism of RH sensor from the view point of layers hygroscopic property. The time response of the developed sensor is also investigated by experiments using the flow meter bulb action of a humid air flow system and by a preliminary blow experiment due to human breathing, resulting in a time response of approximately 400 ms.

Suppression of surface leakage currents using molecular beam epitaxy-grown unipolar barriers

The performance of many electronic devices and materials is limited by surface leakage. These currents are conventionally inhibited by post-epitaxy deposition of a surface passivation layer. This work presents the use of epitaxial, unipolar barriers to limit surface leakage currents, which are a more effective and less expensive approach to control surface current than surface passivation. The concept is illustrated with molecular beam epitaxy-grown InAs photodiodes. The dark current in a conventional InAs photodiode is dominated by surface conduction at temperatures below 150 K, whereas the unipolar barrier InAs photodiode exhibits no detectable surface leakage.

Interband-cascade infrared photodetectors with superlattice absorbers

Interband-cascade infrared photodetectors (ICIPs), composed of discrete superlattice absorbers, are demonstrated at temperatures up to 350 K with a cutoff wavelength near 5 μm at 80 K to beyond 7 μm above room temperature. The peak responsivity exceeds 200 mA/W, higher than the values reported from early interband cascade laser structures, suggesting a significantly enhanced quantum efficiency of the superlattice absorbers. A theoretical model, originally developed for quantum well infrared photodetectors (QWIPs), is applied to ICIPs to analyze their device performance. The Johnson-limited and background-limited detectivities are extracted and indicate that background-limited performance temperatures for two ICIP structures are 126 and 105 K at 5 μm. It is expected that optimized ICIPs will provide improved performance by combining the advantages of conventional photodiodes and the discrete nature of QWIPs and IC lasers.

AlGaN/GaN two-dimensional electron gas Schottky barrier photodiodes with multiple MgxNy/GaN layers

In this study, AlGaN/GaN two-dimensional electron gas (2DEG) heterostructures were grown by metal organic chemical vapor deposition (MOCVD). It was found that we could reduce reverse leakage current and provide high-voltage operation by introducing multiple MgxNy/GaN layers into the conventional Schottky barrier photodiodes (SBPD). An atomic force microscopy (AFM) scan image showed that surface pits of TD terminations were hardly observed as the multiple MgxNy/GaN layers were grown before subsequently depositing a high-temperature (HT) AlGaN/GaN epitaxial layer. A larger Schottky barrier height (ΦB) and smaller ideality factor (n) extracted from the current–voltage (I–V) curve for SBPD with multiple MgxNy/GaN layers also suggested that better crystal quality and rectifying properties were achieved. The larger value of ΦB might be explained by the reduction of the TD density and interface state (IS) density.

Integrated Silicon PIN Photodiodes Using Deep N-Well in a Standard 0.18-$\mu$m CMOS Technology

This paper studies integrated silicon photodiodes (PDs) implemented in standard CMOS technologies. A new PIN PD structure utilizing deep n-well is presented, and compared with conventional vertical and lateral PIN PDs at 850-nm wavelength and different bias conditions. Prototype PDs were fabricated in a 0.18-mum standard CMOS technology, and their DC, impulse and frequency responses were characterized. A 70 times 70 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> PD with the new structure achieved a 3-dB bandwidth of 2.2 GHz in small signal at 5-V bias, whereas conventional lateral and vertical PIN PDs could only operate up to 0.94 GHz and 1.15 GHz, respectively. At 5-V bias, the impulse response of the new PD exhibited a full-width at half-maximum pulsewidth of 127 ps, versus 175 and 150 ps for the conventional lateral and vertical ones, respectively. At 15.5-V bias, the bandwidth of this new PD reached 3.13 GHz, with an impulse response pulsewidth of 102 ps. The responsivity of all prototype PDs was measured at approximately 0.14 A/W up to 10-V bias, which corresponded to a quantum efficiency of 20%. The responsivity of the new PD could be further increased to 0.4 A/W or 58% quantum efficiency, when operating in the avalanche region at 16.2-V bias.

Equivalent Circuit Modeling of Separate Absorption Grading Charge Multiplication Avalanche Photodiode

In this paper, a novel equivalent circuit model for the frequency performance of separate absorption grading charge multiplication (SAGCM) avalanche photodiode (APD) is developed. This model includes effects of carrier transit time, avalanche buildup time, and parasitic RC elements. Based on the equivalent circuit model, frequency and bandwidth characteristics of SAGCM APD can be simulated in advance to device fabrication, and the simulation results are in good agreement with experimental data. Conventional pin photodiodes can also be simulated as a special case when M=1. In addition, the frequency response of SAGCM APDs and pin photodiodes with different illumination directions are investigated.

Defect-selective photothermal imaging: a sensitive tool for damage mapping in optical coatings

A defect-selective photothermal imaging system for the diagnostics of optical coatings is demonstrated. The instrument has been optimized for pump and probe parameters, detector performance, and signal processing algorithm. The imager is capable of mapping purely optical or thermal defects efficiently in coatings of low damage threshold and low absorbance. Detailed mapping of minor inhomogeneities at low pump power has been achieved through the simultaneous action of a low-noise fiber optic photothermal beam defection sensor and a common-mode-rejection demodulation (CMRD) technique. The linearity and sensitivity of the sensor have been examined theoretically and experimentally, and the signal to noise ratio improvement factor is found to be about 110 compared to a conventional bicell photodiode. The scanner is so designed that mapping of static or shock sensitive samples is possible. In the case of a sample with absolute absorptance of 3.8 × 10−4, a change in absorptance of about 0.005 × 10−4 has been detected without ambiguity, ensuring a contrast parameter of 760. This is about 1085% improvement over the conventional approach containing a bicell photodiode, at the same pump power. The merits of the system have been demonstrated by mapping two intentionally created damage sites in a MgF2 coating on fused silica at different excitation powers. Amplitude and phase maps were recorded for thermally thin and thick cases, and the results are compared to demonstrate a case which, in conventional imaging, would lead to a deceptive conclusion regarding the type and location of the damage. Also, a residual damage profile created by long term irradiation with high pump power density has been depicted.

Epitaxial Lead Chalcogenides on Si for Mid-IR Detectors and Emitters Including Cavities

Lead chalcogenide (IV–VI narrow-gap semiconductor) layers on Si or BaF2(111) substrates are employed to realize two mid-infrared optoelectronic devices for the first time. A tunable resonant cavity enhanced detector is realized by employing a movable mirror. Tuning is across the 4 μm to 5.5 μm wavelength range, and linewidth is <0.1 μm. Due to the thin (0.3 μm) PbTe photodiode inside the cavity, a higher sensitivity at higher operating temperatures was achieved as compared to conventional thick photodiodes. The second device is an optically pumped vertical external-cavity surface-emitting laser with PbTe-based gain layers. It emits at ∼5 μm wavelength and with output power up to 50 mW pulsed, or 3 mW continuous wave at 100 K.