III-V alloy heterostructure high speed avalanche photodiodes

Heterostructure avalanche photodiodes have been successfully fabricated in several III-V alloy systems: GaAlAs/GaAs, GaAlSb/GaSb, GaAlAsSb/GaAlSb, and InGaAsP/InP. These diodes cover optical wavelengths from0.4 to 1.8 mum. Early stages of development show very encouraging results. High speed response of <35 ps and high quantum efficiency >95 percent have been obtained. The dark currents and the excess avalanche noise will also be discussed. A direct comparison of GaAlSb, GaAlAsSb, and InGaAsP avalanche photodiodes is given.     

Buried-mesa avalanche photodiodes

We have developed a low-cost buried-mesa avalanche photodiode (APD) primarily targeted for 2.5-Gb/s lightwave applications. These APDs are made by a simple batch process that produces a robust and reliable device with potentially high yield and thus low cost. The entire base structure of our InGaAs-InP APD is grown in one epitaxial step and the remaining process consists of four simple steps including a mesa etch, one epitaxial overgrowth, isolation, and metallization. Buried-mesa APDs fabricated in this way show high uniform gain that rises smoothly to breakdown with increasing reverse bias. When biased to operate at a gain of 10, these unoptimized devices show dark current less than 20 nA, excess noise factor less than 5, and a 3-dB bandwidth of about 4 GHz. With a 1550-nm laser modulated at 2488 Mb/s, a maximum sensitivity of -327 dBm was obtained with an optical receiver using one such APD, without antireflection coatings. These APD's not only demonstrate excellent device characteristics but also high reliability under rigorous stress testing. No degradation was observed even after being biased near breakdown for over 2000 h at 200°C     

HgCdTe electron avalanche photodiodes

Exponential-gain values well in excess of 1,000 have been obtained in HgCdTe high-density, vertically integrated photodiode (HDVIP) avalanche photodiodes (APDs) with essentially zero excess noise. This phenomenon has been observed at temperatures in the range of 77–260 K for a variety of cutoff wavelengths in the mid-wavelength infrared (MWIR) band, with evidence of similar behavior in other IR bands. A theory for electron avalanche multiplication has been developed using density of states and electron-interaction matrix elements associated with the unique band structure of HgCdTe, with allowances being made for the relevant scattering mechanisms of both electrons and holes at these temperatures. This theory is used to develop an empirical model to fit the experimental data obtained at DRS Infrared Technologies. The functional dependence of gain on applied bias voltage is obtained by the use of one adjustable parameter relating electron energy to applied voltage. A more quantitative physical theory requires the use of Monte Carlo techniques incorporating the preceding scattering rates and ionization probabilities. This has been performed at the University of Texas at Austin, and preliminary data indicate good agreement with DRS models for both avalanche gain and excess noise as a function of applied bias. These data are discussed with a view to applications at a variety of wavelengths.     

Improved germanium avalanche photodiodes

New kinds of germanium avalanche photodiodes with n+-n-p and p+-n structures were devised for improved excess noise and high quantum efficiency performance. Multiplication noise, quantum efficiency, and pulse response were studied and compared with those of the conventional n+-p structure diode. Multiplication noise of the new type of diodes were measured in the wavelength range between 0.63 and 1.52 μm. The effective ionization coefficient ratio of the p+-n diode was lower than unity at a wavelength longer than 1.1 μm and 0.6-0.7 at 1.52 μm, and that of the n+-n-p diode was 0.6-0.7 in the whole sensitive wavelength region. Response times were evaluated by using a mode-locked Nd:YAG laser beam and a frequency bandwidth wider than 1 GHz was estimated. Receiving optical power levels were compared with each other using parameters measured in this study.     

Multi-element reachthrough avalanche photodiodes

Two approaches to making multi-element arrays of p⁺-π-p-n⁺reachthrough avalanche photodiodes are reported. In the first approach a single common avalanche region (p-layer) for all elements is used, with the segmentation between elements being on the p⁺layer. This approach has the advantage of having zero dead space between adjacent elements, but is difficult to fabricate, and has a very narrow range of operation in which it is neither noisy due to injection nor suffers from poor element-to-element isolation. In a second approach, the p⁺contact is common and separate avalanche regions are used. The problem for this case is the width of the dead space between adjacent elements which, because of field-fringing effects, is considerably wider than the actual physical distance between elements. A self-aligning technique is described for fabricating arrays by the second approach and the technique demonstrated with a 25-element linear array on 300-µm centers. The measured dead space is in the 60-80 µm range, depending on the gain. The array can be used at an average gain of 100 or more, has excellent element-to-element isolation, and NEP's below 2 × 10¹⁵W/Hz^1/2at 800-900 nm and below 10^-14W/ Hz^1/2over the whole spectral range from 400 to 1060 nm.     

Spatial nonuniformity of 4H-SiC avalanche photodiodes at high gain

We report spatial nonuniformity of responsivity of 4H-SiC avalanche photodiodes at high gain (M > 1000) that results from variation in the doping density. Two-dimensional raster scans show a steady decline laterally across the device. The direction in which the spatial response decreases is the same as that of increasing breakdown voltage on the wafer.     

Signal and noise response of high speed germanium avalanche photodiodes

Germanium avalanche photodiodes, providing gain at microwave frequencies, have been fabricated and tested. The diodes employ a guard ring structure to achieve a uniform, microplasma-free, multiplying region with an active diameter of 40 microns. Low-frequency chopped light current gains of greater than 200, and small-signal 6 GHz current gains of greater than 10 have been obtained at room temperature for a carrier wavelength of 1.15 microns. In the normal operating range, the signal output power is found to vary as the square of the multiplication, and the noise is found to vary as the cube of the multiplication. This limits the maximum useful multiplication of the diode to that level which gives a diode noise equal to the receiver noise. A small-signal equivalent circuit with lumped elements corresponding to the physical processes occurring within the diode, is introduced to describe the small signal behavior. The model is valid over the entire multiplication range, up to frequencies of about 10 GHz.     

PIN avalanche photodiodes model for circuit simulation

A circuit model of PIN avalanche photodiodes (APD's) based on the carrier rate equations for circuit simulation is presented. This model is for dc, ac, and transient analysis. As an example, an In_0.53 Ga_0.47As-InP PIN APD is simulated     

Breakdown probabilities for thin heterostructure avalanche photodiodes

The recurrence theory for the breakdown probability in avalanche photodiodes (APDs) is generalized to heterostructure APDs that may have multiple multiplication layers. The generalization addresses layer-boundary effects such as the initial energy of injected carriers as well as the layer-dependent profile of the dead space in the multiplication region. Reducing the width of the multiplication layer serves to both downshift and sharpen the breakdown probability curve as a function of the applied reverse-bias voltage. In structures where the injected carriers have an initial energy that is comparable to the ionization threshold energy, the transition from linear mode to Geiger-mode is more abrupt than in structures in which such initial energy is negligible. The theory is applied to two recently fabricated Al/sub 0.6/Ga/sub 0.4/As-GaAs heterostructure APDs and to other homostructure thin GaAs APDs and the predictions of the breakdown-voltage thresholds are verified.     

Pulse response of avalanche photodiodes

A study has been made of the time response of heterostructure avalanche photodiodes for InGaAs and InP/InGaAs material systems. A transfer/scattering matrix method, where the matrix parameters are related to the ionization coefficients, has been used. A time domain study has been carried out to find the time variation of electron and hole number densities and currents     

Silicon Geiger mode avalanche photodiodes

In this letter we present the results regarding the electrical and optical characterization of Geiger mode silicon avalanche photodiodes(GMAP) fabricated by silicon standard planar technology. Low dark count rates,negligible afterpulsing effects,good timing resolution and high quantum detection efficiency in all the visible range have been measured. The very good electro-optical performances of our photodiodes make them attractive for the fabrication of arrays with a large number of GMAP to be used both in the commercial and the scientific fields,as telecommunications and nuclear medical imaging.     

Double epitaxial silicon avalanche photodiodes for optical-fibre communications

A new type of planar silicon avalanche photodiodes have been fabricated with a high-low impurity profile with a wide avalanche region by double epitaxy. The a.p.d. characteristics of low noise, high speed, high quantum efficiency and relatively low operating voltage make them particularly suitable for optical-fibre communication systems.     

Novel edge suppression technique for planar avalanche photodiodes

We present an advanced drift diffusion simulation of the joint opening effect (JOE) avalanche photodiode (APD). The joint opening effect APD is a new design for achieving edge breakdown suppression in planar avalanche photodiodes. It is a single growth process that achieves center breakdown dominance without the use of guard rings, partial charge sheets, or surface etches. The JOE APD only requires the diffusion of the primary well. Edge breakdown suppression is achieved by partially insulating the electric field growth in the active region from the geometry of the primary well     

Simple planar structure for high-performance AlInAs avalanche photodiodes

We demonstrate a high-performance AlInAs avalanche photodiode (APD) based on a novel planar diode concept. The APD features a simple planar structure without a guardring, which simplifies production making it more like a PIN photodiode process. Measured device characteristics designed for 10-Gb/s use were a dark current of 0.16 /spl mu/A, responsivity of 0.88 A/W, and a gain-bandwidth product of 120 GHz. Reliability was guaranteed by an aging test exceeding 2400 h, whose conditions were a reverse dark current of 100 /spl mu/A at 175/spl deg/C. These features and performance indicate that the AlInAs APD is highly practical.     

Epitaxial structures for InGaAs/InP avalanche photodiodes

The influence of parameters of the MOS hydride epitaxy on structural and electrophysical characteristics of InGaAs/InP heterostructures is studied experimentally. The chosen parameters are used to grow device structures and fabricate planar avalanche photodiodes based on them. The results of measuring of their photoelectrical properties suggest that the developed structures are suitable for fabrication of commercial planar avalanche photodiodes.     

Arrays of Geiger mode avalanche photodiodes

Following our previous work which has led us to fabricate single pixels of geiger mode avalanche photodiodes (GMAPs), we present in this letter the results regarding the fabrication and characterization of a bidimensional array of GMAPs. Low dark count rates and very good uniformity over the sensor are reported. High quantum efficiency in the visible range has been measured. Measurements indicate that not all the nominal active area is effectively sensitive. We have some preliminary evidence that no crosstalk effects are present in our device. Notwithstanding this, in view of a near future shrinking of all dead regions, an optical trench process has been developed and is illustrated here. Possible future trends are highlighted.     

InP/InGaAs buried-structure avalanche photodiodes

Planar InP/InGaAs avalanche photodiodes with a new guardring structure have been designed and fabricated. The diodes had a buried n-InP layer and an n?-InP multiplication region under p-n junctions. A successful guardring effect was obtained. The diode exhibited a uniform multiplication over the active region, a maximum multiplication factor of 30, low dark currents of around 20 nA at 90% of breakdown voltage and a flat frequency response up to 1 GHz. Multiplication noise was measured up to a multiplication factor of 17.