InGaAs/InAlAs multiquantum-well waveguided pin photodiodes withwide tunability and avalanche multiplication

A new high-speed, waveguided InP based on the InGaAs/InAlAs multiquantum-well pin photodiode with gain and fabricated by metal organic vapour phase epitaxy is reported. The quantum-confined Stark effect can be used to tune this diode over a 250 nm range in the wavelength region around 1.55 μm. A 3 dB bandwidth of more than 12 GHz and avalanche multiplication have been observed     

Monte Carlo simulations of double-gate MOSFETs

A fullband Monte Carlo simulator has been used to analyze the performance of scaled n-channel double-gate (DG) MOSFETs. Size quantization in the channel is accounted for by using a quantum correction based on Schrodinger equation. Scattering induced by the oxide interface is included with a model calibrated with measurements for bulk devices. The detailed self-consistent treatment of quantum effects leads to several interesting observations. We observe that the sheet charge in DG devices does not decrease as much as expected in bulk devices when quantum-mechanical effects are included. The average carrier velocity in the channel is also somewhat reduced by quantum effects, as a second-order effect. In the test cases studied here, application of quantum effects causes a reduction in simulated current not exceeding 15%. In a DG structure, quantum effects tend to concentrate the charge density in the center of the channel, where transverse fields are lower. Because of this, interface scattering appears to be less pronounced when quantum effects are included.     

Monte Carlo simulations of InGaAs nano-MOSFETs

The potential performance of sub-50 nm n-type implant free III-V MOSFETs with an In_0.75Ga_0.25As channel is studied using Monte Carlo (MC) device simulations. The simulated I_D-V_G characteristics of the In_0.75Ga_0.25As implant free MOSFETs are compared with equivalent In_0.3Ga_0.7As implant free MOSFETs and with a state-of-the-art silicon CMOS transistors. The study is based on careful calibration of the MC simulator against experimental data obtained from a δ-doped In_0.52Ga_0.48As/ In_0.53Ga_0.47As/In_0.75Ga_0.25As heterostructure with a high-κ gate dielectric. At 0.8 V supply voltage, the 30 nm gate length In_0.75Ga_0.25As implant free III-V MOSFET delivers a drive current of 1730 μA/μm as compared to the 1550 μA/μm obtained in the equivalent In_0.3Ga_0.7As implant free MOSFET. When this high indium channel transistor is scaled to 20 and 15 nm gate lengths the drive current at 0.8 V supply voltage increases to 2465 and 2745 μA/μm, respectively, making it a good candidate for high performance, low power digital applications at the 22 nm technology generation and beyond.     

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     

An InGaAs/InAlAs superlattice avalanche photodiode with a gain bandwidth product of 90 GHz

The authors describe the fabrication of an InGaAs/InAlAs superlattice avalanche photodiode with a gain-bandwidth product of 90 GHz. The device is composed of an InGaAs/InAlAs superlattice multiplication region and an InGaAs photoabsorption layer. The effect of the superlattice multiplication region thickness on the gain-bandwidth product was studied. A gain-bandwidth product of 90 GHz was obtained for the device with a multiplication region thickness of 0.52 mu m. Low noise performance is compatible with the high gain-bandwidth product due to improvement of the ionization rate ratio made by introducing a superlattice structure into the multiplication region.<>     

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.     

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.     

Monte Carlo simulations of high-speed InSb-InAlSb FETs

Self consistent Monte Carlo simulations which include impact ionization are used to study the high-speed potential of InSb field-effect transistors. It is found that the impact ionization has a strong influence on the performance of InSb for high speed. The ionization leads to a high electron drift velocity and substrate bias can be used to extract the holes which are generated in the channel. Residual hole density within the channel, however, limits the maximum speed. The substrate bias and buffer doping are critical for extracting holes from the channel without inducing excess ionization. Simulations yield a peak cutoff frequency of 820 GHz with a 0.03125-/spl mu/m gate, a source to drain voltage of 0.58, and a sheet doping density of 1.7/spl times/10/sup 12/ cm/sup -2/.     

A Monte Carlo investigation of multiplication noise in thin p+-i-n+ GaAs avalanche photodiodes

A Monte Carlo (MC) model has been used to estimate the excess noise factor in thin p⁺-i-n⁺ GaAs avalanche photodiodes (APD's). Multiplication initiated both by pure electron and hole injection is studied for different lengths of multiplication region and for a range of electric fields. In each ease a reduction in excess noise factor is observed as the multiplication length decreases, in good agreement with recent experimental measurements. This low noise behavior results from the higher operating electric field needed in short devices, which causes the probability distribution function for both electron and hole ionization path lengths to change from the conventionally assumed exponential shape and to exhibit a strong dead space effect. In turn this reduces the probability of higher order ionization events and narrows the probability distribution for multiplication. In addition, our simulations suggest that fur a given overall multiplication, electron initiated multiplication in short devices has inherently reduced noise, despite the higher feedback from hole ionization, compared to long devices     

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.     

Monte Carlo simulations for tilted-channel electron multipliers

Microchannel electron multipliers with tilted structures are simulated using the Monte Carlo method. Gains of secondary electrons are calculated for different structures of the electron multiplier. For a short tilted cylindrical channel of the electron multiplier, a maximum gain is achieved greater than 10⁴ at a tilt angle near 25°. The maximum gain is about 10³ times larger than that of the nontilted channel. An explanation for the improvement of gain in tilted channel is suggested     

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     

500 /spl mu/m diameter CMOS compatible avalanche photodiodes with 500 MHz bandwidth

Novel silicon-on-insulator, large area (500 /spl mu/m diameter), CMOS avalanche photodiodes for use with plastic optical fibre are presented. Patterns have been formed on the devices to reduce junction capacitance. Measurements on the patterned devices, at 650 nm and 26 V reverse bias, revealed bandwidths of >500 MHz.     

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.     

Monte Carlo simulation of threshold bandwidth for high-order harmonic extraction

The feasibility of bulk semiconductors subjected to strong periodic electric fields for terahertz radiation generation due to the high-order harmonic extraction is analyzed by using Monte Carlo simulations. The high-order harmonic intensity and the spectral density of velocity fluctuations are calculated for GaAs, InP, and InN. By comparing the harmonic intensity with the noise level the threshold bandwidth for high-order harmonic extraction determined by their ratio is introduced and evaluated for the above materials. The results show that semiconductor materials with a high value of the threshold field for the Gunn-effect are characterized by a high value of the threshold bandwidth under high-order harmonic generation and, hence, they are promising materials for microwave generation in the THz frequency range by high-order harmonic extraction.     

InGaAsP/InAlAs superlattice avalanche photodiode

The fabrication of an InGaAsP-InAlAs superlattice avalanche photodiode using a gas source molecular beam epitaxy is discussed. A quaternal alloy of InGaAsP was used for the well layers in order to suppress the dark current due to the tunneling effect. With this structure, the valance band discontinuity almost vanishes and a gain bandwidth of 110 GHz was obtained     

Absolute noise characterisation of avalanche photodiodes

Excess noise in four types of commercially obtained avalanche photodiodes (a.p.d.s) has been measured absolutely, by comparing avalanche noise from the a.p.d. with shot noise from an illuminated p-i-n diode. The method used yields directly the noise-current spectral density, simplifies the deduction of the quantum efficiency keff and hence the true value of the multiplication factor, and ultimately yields a measured value of the noise parameter x.     

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.     

Numerical simulation of avalanche breakdown within InP-InGaAs SAGCMstandoff avalanche photodiodes

The breakdown location within a planar InP/In_0.53Ga_0.47As (InGaAs) separate absorption, grading, charge sheet, and multiplication (SAGCM) avalanche photodiode (APD), using the standoff breakdown suppression design to replace guard rings, depends on the two-dimensional (2-D) geometry of the Zn diffused well. Since the geometry of this p⁺ diffusion is dependent upon the surface etch, the effects of varying the etch depth (t_standoff) and length of the sloped etch edge (w_slope ) are studied using a two-dimensional drift-diffusion simulator. It is determined that the etch depth brackets a region where center breakdown dominance is possible. To ensure center breakdown within this region it is concluded that there is a maximum value that the slope of the etch walls must not exceed