Enhanced performance of AlGaN-based ultraviolet light-emitting diodes with linearly graded AlGaN inserting layer in electron blocking layer

The conventional stationary Al content Al GaN electron blocking layer(EBL) in ultraviolet light-emitting diode(UV LED) is optimized by employing a linearly graded Al Ga N inserting layer which is 2.0 nm Al_(0.3) Ga_(0.7) N/5.0 nm Alx Ga_(1-x) N/8.0 nm Al_(0.3) Ga_(0.7) N with decreasing value of x. The results indicate that the internal quantum efficiency is significantly improved and the efficiency droop is mitigated by using the proposed structure. These improvements are attributed to the increase of the effective barrier height for electrons and the reduction of the effective barrier height for holes,which result in an increased hole injection efficiency and a decreased electron leakage into the p-type region. In addition,the linearly graded AlGaN inserting layer can generate more holes in EBL due to the polarization-induced hole doping and a tunneling effect probably occurs to enhance the hole transportation to the active regions, which will be beneficial to the radiative recombination.     

High-speed waveguide-integrated Ge/Si avalanche photodetector

Waveguide-integrated Ge/Si heterostructure avalanche photodetectors(APDs) were designed and fabricated using a CMOS-compatible process on 8-inch SOI substrate. The structure of the APD was designed as separate-absorption-chargemultiplication(SACM) using germanium and silicon as absorption region and multiplication region, respectively. The breakdown voltage(V_b) of such a device is 19 V at reverse bias and dark current appears to be 0.71 μA at 90% of the V_b. The device with a 10-μm length and 7-μm width of Ge layer shows a maximum 3-dB bandwidth of 17.8 GHz at the wavelength of 1550 nm. For the device with a 30-μm-length Ge region, gain-bandwidth product achieves 325 GHz.     

Polarization and p-type doping effects on photoresponse of separate absorption and multiplication AlGaN solar-blind avalanche photodiodes

The photoresponse characteristics of separate absorption and multiplication (SAM) AlGaN solar-blind avalanche photodiodes (APDs) were investigated in detail. The p-i-n-i-n avalanche photodiodes were examined using the newly designed model of avalanche photodiodes in AlGaN. The research results showed that the dark current density was about 3.51 × 10^?8 A/cm², the light current density was 5.86 × 10^?5 A/cm² under near-zero bias, and the avalanche breakdown occurred at about 135.0 V under reverse bias, which were all consistent with the experimental data. To investigate the effects influencing the photoresponse characteristics of the APDs, their photo responsivity spectra under different biases were simulated. The APD featured a window region over the wavelength range from 260 to 280 nm with a high rejection ratio on the short-wavelength side. Meanwhile, the dependence of APD responsivity on the polarization charge revealed that the negative polarization charges strongly affected the responsivity. Increased negative polarization charges at the Al_0.4Ga_0.6N/Al_0.6Ga_0.4N interface markedly lowered the responsivity, whereas charges of the same polarity at the GaN/Al_0.4Ga_0.6N interface enhanced the responsivity. Furthermore, the dependence of responsivity on p-type doping was analyzed by comparison with the effects of negative polarization charges on the conduction band of the APDs. Finally, the inversion layer models are used to interpret the effects of these on the APD responsivity. This research is useful for exploring polarization and p-type doping effects in SAM AlGaN structures and realization of high responsivity solar-blind APDs.     

Cascaded homojunction avalanche photodiodes

The structure and the performance of a cascaded avalanche photodiode (APD) based on cascaded home-junctions are discussed. Similar to the hetero-structure superlattice APD, such a device has a lower excess noise and a higher gain at lower bias voltages than conventional APDs. In addition to the ionization ratio, several parameters also affect the device's performance.     

AlGaN barrier thickness dependent surface and interface trapping characteristics of AlGaN/GaN heterostructure

The aluminium gallium nitride (AlGaN) barrier thickness dependent trapping characteristic of AlGaN/GaN heterostructure is investigated in detail by frequency dependent conductance measurements. The conductance measurementsin the depletion region biases (−4.8 V to −3.2 V) shows that the Al_0.3Ga_0.7N(18 nm)/GaN structure suffers from both the surface (the metal/AlGaN interface of the gate region) and interface (the AlGaN/GaN interface of the channel region) trapping states, whereas the AlGaN/GaN structure with a thicker AlGaN barrier (25 nm) layer suffers from only interface (the channel region of AlGaN/GaN) trap energy states in the bias region (−6 V to −4.2). The two extracted time constants of the trap levels are (2.6–4.59) μs (surface) and (113.4–33.8) μs (interface) for the Al_0.3Ga_0.7N(18 nm)/GaN structure in the depletion region of biases (−4.8 V to −3.2 V), whereas the Al_0.3Ga_0.7N (25 nm)/GaN structure yields only interface trap states with time constants of (86.8–33.3) μs in the voltage bias range of −6.0 V to −4.2 V. The extracted surface trapping time constants are found to be very muchless in the Al_0.3Ga_0.7N(18 nm)/GaN heterostructure compared to that of the interface trap states. The higher electric field formation across the AlGaN barrier causes de-trapping of the surface trapped electron through a tunnelling process for the Al_0.3Ga_0.7N(18 nm)/GaN structure, and hence the time constants of the surface trap are less.     

Monte Carlo simulation of excess noise in heterojunction avalanche photodetector

A Monte Carlo (MC) simulation of excess noise in heterojunction avalanche photodetector (APD), made up of InP/InGaAs, is made. The simulation is based on the hard threshold dead space consideration in the displaced exponential model of the distribution of ionization path lengths. Impact ionization and multiplication of electrons as function of ionizing electric field are also studied. The multiplication and noise are seen to be reduced compared to those in component materials. The simulated results are seen to agree well with the reports of other theoretical predictions and experimental results published in literatures.     

Fang–Howard wave function modelling of electron mobility in AlInGaN/AlN/InGaN/GaN double heterostructures

To study the electron transport properties in InGaN channel-based heterostructures,a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier.Various scattering mechanisms,such as dislocation impurity(DIS) scattering,polar optical phonon(POP) scattering,piezoelectric field(PE) scattering,interface roughness(IFR) scattering,deformation potential(DP) scattering,alloy disorder(ADO) scattering from InGaN channel layer,and temperature-dependent energy bandgaps are considered in the calculation model.A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure(DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure(SH) is made and analyzed with a full range of barrier alloy composition.The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is 0.04 for higher mobility in DH with Al_(0.4)In_(0.07)Ga_(0.53)N barrier.Finally,the temperature-dependent two-dimensional electron gas(2 DEG) density and mobility in InGaN channel-based DH with Al_(0.83)In_(0.13)Ga_(0.0)4 N and Al_(0.4)In_(0.07)Ga_(0.53)N barrier are investigated.Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.     

Random response time of thin avalanche photodiodes

The avalanche built-up time using random response time model for avalanche photodiode (APD) is presented. A random response time model considers the randomness of times at which the primary and secondary carriers exit the multiplication region. The dead-space effect is included in our model to demonstrate its effect on response time of APDs especially for the thin devices. Our results show that feedback impact ionisation process and dead-space prolong the response time in APDs. The time response of homojunction InP p⁺-i-n⁺ diodes with the multiplication region of 0.281, 0.582 and 1.243 m are calculated.     

Short-gate AlGaN/GaN high-electron mobility transistors with BGaN buffer

Using the semi-insulating property and small lattice constant a of wurtzite BGaN alloy, we propose a BGaN buffer with a B-content of 1% to enhance two-dimensional electron gas(2 DEG) confinement in a short-gate AlGaN/GaN highelectron mobility transistor(HEMT). Based on the two-dimensional TCAD simulation, the direct current(DC) and radio frequency(RF) characteristics of the AlGaN/GaN/B_(0.01)Ga_(0.99)N structure HEMTs are theoretically studied. Our results show that the BGaN buffer device achieves good pinch-off quality and improves RF performance compared with GaN buffer device. The BGaN buffer device can allow a good immunity to shift of threshold voltage for the aspect ratio(LG/d)down to 6, which is much lower than that the GaN buffer device with L_G/d=11 can reach. Furthermore, due to a similar manner of enhancing 2 DEG confinement, the B_(0.01)Ga_(0.99)N buffer device has similar DC and RF characteristics to those the AlGaN buffer device possesses, and its ability to control short-channel effects(SCEs) is comparable to that of an Al_(0.03)Ga_(0.97)N buffer. Therefore, this BGaN buffer with very small B-content promises to be a new method to suppress SCEs in GaN HEMTs.     

光通信用雪崩光电二极管(APD)频率响应特性研究

吸收层、电荷层和倍增层分离结构雪崩光电二极管(SACM-APD),包括InP/InGaAs、InAlAs/InGaAs和Si/Ge APD是光通信领域近年来研究的热点. 本文基于电路模型,系统比较了不同外延层厚度、不同材料以及不同结构APD的频率响应特性,重点探讨Si/Ge APD吸收层厚度、光敏面大小、寄生参数等各项参数对带宽的影响,仿真结果与实际器件实验数据相符合. 本文的研究成果对SACM-APD的优化设计具有指导意义. 关键词： SACM-APD 电路模型 频率响应     

High temperature and wavelength dependence of avalanche gain of AlAsSb avalanche photodiodes

The evolution of the dark currents and breakdown at elevated temperatures of up to 450 K are studied using thin AlAsSb avalanche regions. While the dark currents increase rapidly as the temperature is increased, the avalanche gain is shown to only have a weak temperature dependence. Temperature coefficients of breakdown voltage of 0.93 and 1.93 mV/K were obtained from the diodes of 80 and 230 nm avalanche regions (i-regions), respectively. These values are significantly lower than for other available avalanche materials at these temperatures. The wavelength dependence of multiplication characteristics of AlAsSb p-i-n diodes has also been investigated, and it was found that the ionization coefficients for electrons and holes are comparable within the electric field and wavelength ranges measured.     

Noise characteristics of a superlattice avalanche photodiode

The noise generated due to randomness of multiplication process in the avalanche region of an Al _x Ga_1–x As/GaAs quantum well p⁺-i-n⁺ structure has been studied. The paper presents a quantitative evaluation of the noise performance of the superlattice APD which has not been done so far. Further, useful design data for low noise structure is given.     

Temperature-dependent characteristics of a reach-through avalanche photodiode (RAPD) in Ge,Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs reach-through avalanche photodiodes (RAPD). The temperature coefficient of avalanche breakdown voltage in a depletion region is studied. The response time of a reach-through APD in these materials is also discussed. Finally a comparison of the characteristics between PIN APD and RAPD is presented. The theoretical data have also been substantiated experimentally by Kanedaet al. Supported by National Science Council, the Republic of China     

Performance comparisons between inverted and conventional avalanche photodiodes for larger capacity applications

Optical Review - In this paper, two types of avalanche photodiode (APD) are compared, namely, a P-down APD using an inverted structure and a P-up APD using a conventional structure. Comparison...     

HgCdTe avalanche photodiodes: A review

This paper presents a comprehensive review of fundamental issues, device architectures, technology development and applications of HgCdTe based avalanche photodiodes (APD). High gain, above 5×10³, a low excess noise factor close to unity, THz gain-bandwidth product, and fast response in the range of pico-seconds has been achieved by electron-initiated avalanche multiplication for SWIR, MWIR, and LWIR detector applications involving low optical signals. Detector arrays with good element-to-element uniformity have been fabricated paving the way for fabrication of HgCdTe-APD FPAs.     

Analysis of avalanche mechanisms in short-pulses laser-induced damage

A theory based on the rate equation of free electron is used to analyze the process of free electron multiplication in optical materials under the laser irradiation, specially researching on the effect of avalanche ionization on the electron multiplication and material damage threshold. Numerical investigation with SiO₂ is processed using this theoretical model, and damage thresholds under different avalanche models are analyzed. The result shows that during research on the energy charge between electron and electromagnetic field, the probability of avalanche ionization should be considered. Under this assumption, the numerical threshold gets well with the experimental result.     

Weak avalanche multiplication in SiGe heterojunction bipolar transistors on thin film silicon-on-insulator

In this paper, we propose an analytical avalanche multiplication model for the next generation of SiGe silicon-on-insulator (SOI) heterojunction bipolar transistors (HBTs) and consider their vertical and lateral impact ionizations for the first time. Supported by experimental data, the analytical model predicts that the avalanche multiplication governed by impact ionization shows kinks and the impact ionization effect is small compared with that of the bulk HBT, resulting in a larger base-collector breakdown voltage. The model presented in the paper is significant and has useful applications in the design and simulation of the next generation of SiGe SOI BiCMOS technology.     

Temperature dependent characteristics of a PIN avalanche photodiode (APD) in Ge,Si and GaAs

A theoretical assessment is presented based on a modification of Baraff's theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs PIN avalanche photodiodes. The temperature coefficient of avalanche breakdown voltage in a high field region is studied. Finally the response time of a PIN APD in these materials is discussed.This paper is supported by the National Science Council, the Republic of China.     

Ultraviolet emissions realized in ZnO via an avalanche multiplication process

Au/MgO/ZnO/MgO/Au structures have been designed and constructed in this study. Under a bias voltage, a carrier avalanche multiplication will occur via an impact ionization process in the MgO layer. The generated holes will be drifted into the ZnO layer, and recombine radiatively with the electrons in the ZnO layer. Thus obvious emissions at around 387 nm coming from the near-band-edge emission of ZnO will be observed. The reported results demonstrate the ultraviolet (UV) emission realized via a carrier multiplication process, and so may provide an alternative route to efficient UV emissions by bypassing the challenging p-type doping issue of ZnO.