Industrial manufacturing of low loss optical fiber cables and their specific splicing toolings, together with recent developments of optoelectronic components will lead to real systems installations in the 1980’s. The requirements for these new systems are related to some of the fiber characteristics: 


protection against electromagnetic perturbations


radiation leakage


small size


low weight


electrical isolation


high bandwidth


low attenuation


low cost expectation



Understanding fiber optic transmission systems requires a knowledge of the characteristics of optoelectronic transmitters and receivers, which do not differ much from traditional copper pairs (choice of analog to digital modulation, total attenuation, etc.). Therefore, throughout this course we will limit our effort to descriptions of specific elements, essentially those whose parameters are directly connected to system performance.

Fiber Optic Communication Systems

Monte Carlo simulations of electron transport in seven semiconductors of the diamond and zinc-blende structure (Ge, Si, GaAs, InP, AlAs, InAs, GaP) and some of their alloys (Al/sub x/Ga/sub 1-x/As, In/sub x/Ga/sub 1-x/As, Ga/sub x/In/sub 1-x/P) and hole transport in Si were performed at two lattice temperatures (77 and 300 K). The model uses band structures obtained from local empirical pseudopotential calculations and particle-lattice scattering rates computed from the Fermi golden rule to account for band-structure effects. Intervalley deformation potentials significantly lower than those which have been previously reported are needed to reproduce available experimental data. This is attributed to the more complicated band structures, particularly around the L- and X-symmetry points in most materials. Satisfactory agreement is obtained between Monte Carlo results and some experiments. >

Monte Carlo simulation of transport in technologically significant semiconductors of the diamond and zinc-blende structures. I. Homogeneous transport

A key element in the integration of microprocessor chips with optical communications circuits is a photodetector to mediate the optical and electronic signals. Germanium photodetectors are very attractive in this regard because they are compatible with conventional silicon circuitry, but they suffer from noise that limits their performance. Assefa et al. now show how the poor intrinsic noise characteristics of germanium can be overcome through the careful engineering of optical and electrical fields at the nanometre scale. The result is a compact and efficient photodetector that could enable a range of optoelectronic applications. To integrate microchips with optical communications a photodetector is required to mediate the optical and electronic signals. Although germanium photodetectors are compatible with silicon their performance is impaired by poor intrinsic noise. Here the noise is reduced by nanometre engineering of optical and electrical fields to produce a compact and efficient photodetector. Integration of optical communication circuits directly into high-performance microprocessor chips can enable extremely powerful computer systems1. A germanium photodetector that can be monolithically integrated with silicon transistor technology2,3,4,5,6,7,8 is viewed as a key element in connecting chip components with infrared optical signals. Such a device should have the capability to detect very-low-power optical signals at very high speed. Although germanium avalanche photodetectors9,10 (APD) using charge amplification close to avalanche breakdown can achieve high gain and thus detect low-power optical signals, they are universally considered to suffer from an intolerably high amplification noise characteristic of germanium11. High gain with low excess noise has been demonstrated using a germanium layer only for detection of light signals, with amplification taking place in a separate silicon layer12. However, the relatively thick semiconductor layers that are required in such structures limit APD speeds to about 10 GHz, and require excessively high bias voltages of around 25 V (ref. 12). Here we show how nanophotonic and nanoelectronic engineering aimed at shaping optical and electrical fields on the nanometre scale within a germanium amplification layer can overcome the otherwise intrinsically poor noise characteristics, achieving a dramatic reduction of amplification noise by over 70 per cent. By generating strongly non-uniform electric fields, the region of impact ionization in germanium is reduced to just 30 nm, allowing the device to benefit from the noise reduction effects13,14,15 that arise at these small distances. Furthermore, the smallness of the APDs means that a bias voltage of only 1.5 V is required to achieve an avalanche gain of over 10 dB with operational speeds exceeding 30 GHz. Monolithic integration of such a device into computer chips might enable applications beyond computer optical interconnects1—in telecommunications16, secure quantum key distribution17, and subthreshold ultralow-power transistors18.

Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects

One of the "fundamental" problems in the continued scaling of MOSFETs is the 60 mV/decade room temperature limit in subthreshold slope. In this paper, we report initial studies on a new kind of transistor, the I-MOS. The I-MOS uses modulation of the breakdown voltage of a gated p-i-n structure in order to switch from the OFF to the ON state and vice versa. Since impact-ionization is an abrupt function of the electric field (or the carrier energy), simulations show that the device has a subthreshold slope much lower than kT/q. Simulations also show that it is indeed possible to make complementary circuits with switching speeds comparable to or exceeding CMOS. Experimental results on a silicon based prototype verify the basic concept and show very steep subthreshold slopes with high speed turn-on and turn-off. Lower bandgap materials are also being investigated to reduce the value of the breakdown voltage and permit lower voltage operation.

I-MOS: a novel semiconductor device with a subthreshold slope lower than kT/q

The first superlattice avalanche photodiode (APD) is reported. The high field region of this p‐i‐n structure consists of 50 alternating Al0.45Ga0.55As (550 A) and GaAs (450 A) layers. A large ionization rate ratio has been measured in the field range (2.1–2.7)×105 V/cm, with α/β≃10 at a gain of 10 giving a McIntyre noise factor Fn = 3. The ionization rate ratio enhancement with respect to bulk GaAs and AlGaAs is attributed to the large difference in the band edge discontinuities for electrons and holes at the heterojunction interfaces. The superlattice APD is a new device concept which can be used to develop low noise APD’s in a variety of III‐V materials including long wavelength 1.3–1.6‐μm semiconductors.

Enhancement of electron impact ionization in a superlattice: A new avalanche photodiode with a large ionization rate ratio

The arsenic composition dependences of electron and hole ionization coefficients, α and β, in

Impact ionization coefficients of electrons and holes in

Dark current, quantum efficiency, multiplication noise, and pulse response of germanium avalanche photodiodes with n+-p junction were studied to find an optimum structure. The dark current can be separated by graphical means into a leakage current component and a multiplied component which flows through the junction. The dark current components are also evaluated by using diodes with various diameters. The quantum efficiency and the multiplication noise are shown to be strongly affected by the n+ layer thickness. An n+ layer thickness optimized for signal-to-noise ratio is estimated from experimental and calculated results, using a figure of merit for avalanche photodiodes. The response waveform for mode-locked Nd:YAG laser shows a rise time of 100 ps and a half pulsewidth of less than 200 ps.

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 &#181;m

Breakdown voltage and dark current density in p+n and n+p In0.53Ga0.47As diodes are compared with theoretical values taking the backward tunneling current into account. Predominant origin of dark current in an InGaAs diode is attributed to the tunneling current. Using these results, optimum design of an InGaAs/InP avalanche photodiode (APD) to obtain low dark current, high multiplication gain, high quantum efficiency and fast response is also discussed.

https://iopscience.iop.org/article/10.1143/JJAP.19.L277/pdf

Tunneling Current in InGaAs and Optimum Design for InGaAs/InP Avalanche Photodiode

Ionization rates in GaAs are determined from the measurement of photocarrier multiplication. Pure electron and hole initiations are achieved by using the novel crater mesa structure and appropriate optical‐injection wavelengths. The ionization rates for holes are greater than that for electrons except at highest fields. This agrees with the studies of Stillman et al., except for the individual values. The ionization rates for electrons and holes are expressed as α=5.6×106 exp(−2.41×106/E) and β=1.5×106 exp(−1.57×106/E), respectively.

Ionization rates for electrons and holes in GaAs

Ionization rates in 〈111〉 and 〈100〉 germanium are determined experimentally. The ionization rates obtained are expressed as α=2.72×106 exp(−1.1×106/E), β=1.72×106 exp(−9.37×105/E) for 〈111〉 and α=8.04×106 exp(−1.4×106/E), β=6.39×106 exp(−1.27×106 /E) cm−1 for 〈100〉 where α and β are ionization rates for electrons and holes, respectively, and E is the electric field. Hole‐ to electron‐ionization‐rate ratios of 〈100〉 Ge are found to be greater than those of 〈111〉 Ge. The multiplication noise power of Ge avalanche photodiodes calculated by using the ionization rates obtained shows good agreement with experimental results.

Takao Kaneda

Papers

Impact ionization coefficients of electrons and holes in

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 µm

Tunneling Current in InGaAs and Optimum Design for InGaAs/InP Avalanche Photodiode

Ionization rates for electrons and holes in GaAs

Crystal orientation dependence of ionization rates in germanium

Takao Kaneda

Papers

Impact ionization coefficients of electrons and holes in

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 &#181;m

Tunneling Current in InGaAs and Optimum Design for InGaAs/InP Avalanche Photodiode

Ionization rates for electrons and holes in GaAs

Crystal orientation dependence of ionization rates in germanium

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 µm