Negative resistance and filamentary currents in avalanching silicon p+-i-n+junctions

Space-charge effects in avalanching p⁺-i-n⁺diodes give rise to a current-controlled bulk negative resistance. It is shown that this negative resistance gives rise to an instability which tends to lead to the formation of current filaments. A steady state can be found in which the generation of carriers in the filament by impact ionization is balanced by radial diffusion of carriers. We present the results of approximate numerical calculations of filament current-density profiles and total filament current as a function of applied voltage. The total filament current is a decreasing function of the applied voltage; thus, the diode exhibits a quasistatic negative external resistance. It is suggested that this negative resistance may be used to interpret observed sub-transit-time oscillations of p⁺-i-n⁺structures.     

H2+和D2+谐波强度与波长的关系

为了了解H₂+及其同位素分子谐波光谱效率与激光波长之间的关系,采用求解2维薛定谔方程的方法,理论研究了600nm~1600nm激光波长下H₂+和D₂+谐波光谱强度随波长的变化关系。结果表明,光谱强度随波长增大而减小;在短波长区间,H₂+光谱强度减小的倍率要大于D₂+,在长波长区间,H₂+光谱强度减小的倍率要小于D₂+;此外,在弱光强下,H₂+光谱强度总是大于D₂+, 在强光强下,H₂+光谱强度在短波长区间小于D₂+, 而其在长波长区间大于D₂+; 核间距延伸和电荷共振增强电离在H₂+和D₂+谐波光谱强度变化上起到主要作用。这一结果对分子谐波调控是有帮助的。     

Stabilizing effects for the supercritical GaAs n+nn+-transferred electron amplifier

In this paper effects of importance for the stabilization of supercritical n⁺nn⁺ GaAs transferred electron devices are considered. By small-signal impedance calculations and measurements it is shown that doping- as well as temperature gradients of correct polarity reduce the device negative resistance and enhance stability. It is also found that an increasing doping density reduces the negative resistance. Finally it is demonstrated that relaxation effects have a profound influence on the impedance, and that such effects have to be included in a small-signal analysis in order to give reasonable agreement with measurements.     

Cr3+→Nd3+ energy transfer and Nd3+ laser action studies of La3Ga5SiO14:Cr3+,Nd3+ co-doped crystal

Energy transfer between Cr³⁺ and Nd³⁺ ions has been investigated in the 4.2-300-K temperature range by using steady-state and site-selective time-resolved laser spectroscopy. Radiative and nonradiative energy transfer has been studied from the time-resolved emission spectra and the donor fluorescence decays. The transfer efficiency was calculated as a function of temperature by using the Cr³⁺ lifetimes of the single doped and co-doped samples. Laser experiments were carried out in a diffusive cavity by pumping a co-doped rod 54 mm long and 5 mm in diameter with xenon flashlamps. The laser spectral emission shows a complex structure which varies as a function of pump energy. The temporal evolution of the laser spectrum is discussed in terms of a simple four-level spectral rate-equation laser model which takes into account the existence of two main broad Nd³⁺ site distributions with a large spectral overlap     

多光子电离技术在NO与O_3反应动力学研究中的应用

本文报道了把多光子电离(MPI)方法用于气体反应动力学的研究.通过NO分子中间态O~2H的多光子电离作为对NO的检测方法,在流管体系中测得的NO与O_3反应速率常数为1.l×10~(-14)cm~3·mol~(-1)·s~(-1),与已报道的数据相符.在激光波长为365.45nm时,必须考虑NO~2光解产生的NO对MPI信号的贡献,在该波长时测得的NO_2的表观解离率为50%.     

n+-p-p+structure InP solar cells grown by organometallic vapor-phase epitaxy

The fabrication of n⁺-p-p⁺InP solar cells by OMVPE has been studied. A conversion efficiency (active area) as high as 20 percent under AM1.5 illumination has been obtained. It is experimentally verified that the n⁺-p-p⁺InP solar cell has a higher resistance to radiation degradation than the n⁺-p InP solar cell. Diode characteristics and photovoltaic performances such as saturation current density, diode-ideality factor, and open-circuit voltage for the n⁺-p-p⁺cells are found to drastically deteriorate when junction depth decreases to ≤ 0.15 µm. An electron concentration dependence of a hole diffusion length in n-InP is estimated from the measurement of photoluminescence spectra. For the improvement of photovoltaic performances, a series of systematic examinations has been made on the relationship between collection efficiency and hole diffusion length from photogenerated carrier distribution analysis.     

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     

High-power punch-through avalanche-diode microwave oscillators

Two distinctive modes of oscillation are observed in specially profiled silicon avalanche diodes: transit-time mode at 3 to 7 GHz at lower bias current densities and an anomalous mode at around 1 GHz above certain threshold current levels. High pulsed power has been produced from both modes of operation. Space-charge-induced negative resistances are computed and their dependence on device parameters are discussed. Experimentally observed correlations between these two modes, including a locking behavior of the anomalous mode by the transit-time mode, and the computed and observed slight negative resistance at the anomalous-mode threshold suggest that the transit- time-mode oscillation and the space-charge-induced negative resistance are two possible pre-requisites for the initiation of the anomalous mode of oscillation.     

High-frequency oscillations of p++-n+-n-n++avalanche diodes below the transit-time cutoff

According to Read, n⁺⁺-P⁺-i-P⁺⁺diodes should oscillate at special high frequencies determined by carrier transit time in the space-charge layer. Oscillations not affected by transit time were observed with p⁺⁺-n⁺- n-n⁺⁺silicon diodes. The corresponding current-voltage characteristic revealed a negative resistance setting in at a critical current. Theoretical considerations show that one-sided avalanche injection in n⁺⁺-p⁺-p⁺⁺structures may lead to a slight negative resistance for carrier concentrations smaller than the impurity concentration and for certain widths of the depletion layer. This type of negative resistance disappears in n⁺⁺-p⁺-i-P⁺⁺structures, but with increasing injection multiplication is induced in the intrinsic layer. Therefore the carrier space-charge is reduced and a negative resistance appears at a critical current density. The onset of this second injection is an upper current limit of the Read transit-time mode. The frequency range of oscillations due to avalanche space-charge feedback generally will not be separated from the range of transit-time oscillations. Thus, it must be judged carefully which mechanism is responsible for observed high-frequency oscillations. On the other hand, space-charge feedback may give additional stability to the transit-time mode.     

Effect of unequal ionization rates on GaAs IMPATT device admittance

The effect of the recently reported unequal ionization rates in GaAs on small signal IMPATT device admittance has been calculated and is compared with equal ionization rate results at room temperature. The ionization rate effect on the static breakdown voltage has also been obtained.     

Increase of parasitic resistance in shallow p+ extensionby SiN sidewall process and its improvement by Ge preamorphization forsub-0.25-μm pMOSFET's

Anomalously high parasitic resistance is observed when SiN gate sidewall spacer is incorporated into sub-0.25-μm pMOSFET's. The parasitic resistance in p⁺ S/D extension region increases remarkably by decreasing BF₂ ion implantation energy to lower than 10 keV. It is confirmed that low activation efficiency of boron in p⁺ extension is the reason for such high parasitic resistance. The reduction of activation efficiency of boron may result from hydrogen passivation of boron acceptor; Fourier transform infrared absorption (FT-IR) measurement suggests that diffused hydrogen from SIN into p⁺ extension region forms the silicon-hydrogen-boron complex. It is also found that the activation efficiency of boron correlates well both with implantation energy of BF₂ and the amorphization rate of substrate. Therefore, in sub-0.25-μm era, the extra amorphization step is essential not only to form a shallow junction but also to enhance boron activation. Germanium preamorphization implantation (Ge PAI) is hence applied to p⁺ extension of 0.15 μm pMOSFET's. It is finally demonstrated that this Ge PAI process reduces the total parasitic resistance to improve the drain saturation current by up to 10%     

Spectral response of n+-n-p and n+-p photodiodes

Results of calculations for the quantum efficiency of three different types of n⁺-p, n⁺-n-p, and OCI-HLE diodes are reported. Exact numerical modeling of current density equations, modified to include bandgap reduction and Auger recombination is used to compute the quantum efficiency of these diodes. It is found that an optimized n⁺-p structure can result in over all spectral response comparable to the n⁺-n-p structure, although it is not as good as that of the OCI-HLE type of diodes. Further, these calculations show that one can achieve low dark current in these diodes, but at the expense of lower quantum efficiency particularly for wavelengths less than 0.4 µm.     

Forward transient characteristics of gold-doped silicon p+-n-n+diodes

The transient response of high-resistivity long-base low-lifetime p⁺-n-n⁺silicon diodes was examined experimentally. The diodes were doped with gold in order to reduce the minority carrier lifetime. Voltage oscillations were observed at different current levels. A large inductive effect was shown to exist when the diode was forward biased in a negative resistance region of the dc voltage-current characteristics.     

Avalanche multiplication noise characteristics in thin GaAs p+-i-n+ diodes

Avalanche noise measurements have been performed on a range of homojunction GaAs p⁺-i-n⁺ and n⁺-i-p ⁺ diodes with “i” region widths, ω from 2.61 to 0.05 μm. The results show that for ω⩽1 μm the dependence of excess noise factor F on multiplication does not follow the well-established continuous noise theory of McIntyre [1966]. Instead, a decreasing noise factor is observed as ω decreases for a constant multiplication. This reduction in F occurs for both electron and hole initiated multiplication in the thinner ω structures even though the ionization coefficient ratio is close to unity. The dead-space, the minimum distance a carrier must travel to gain the ionization threshold energy, becomes increasingly important in these thinner structures and largely accounts for the reduction in noise     

A study of the conversion efficiency limit of p+-i-n+silicon solar cells in concentrated sunlight

The conversion efficiency limit of p⁺-i-n⁺silicon solar cells in concentrated sunlight is explored with numerical simulations of an idealized p⁺-i-n⁺cell having field-induced junctions. Conversion efficiencies greater than 30 percent are calculated for this cell operating in sunlight concentrated 1000 times. The relative importance of bulk and surface recombination in limiting the cell conversion efficiency is illustrated for operation in 1 to 1000 suns. For surface recombination velocities below 100 cm/s, it is shown that bulk recombination losses limit the cell performance rather than recombination losses occurring in the p⁺or n⁺regions. The results show that Auger recombination in the bulk region will limit ultimately the cell conversion efficiency.     

Anomalous dispersion in Er3+ and Yb3+-dopedfibers

In experimental and theoretical study of anomalous dispersion in Er³⁺and Er³⁺-Yb³⁺-doped fibers has been developed. Anomalous time delay caused by both absorption and emission at 1.535 μm has been theoretically calculated and experimentally measured. A pump power dependence of anomalous time delay in rare-earth-doped fibers has been theoretically calculated and experimentally investigated. It has been shown that pump power fluctuations lead to propagation time jitter in Er³⁺-doped fiber amplifiers. The pulse interaction due to refractive index change caused by gain saturation is predicted. It has been shown that for Er ³⁺-doped fibers with SiO₂-GeO₂ core composition, the anomalous dispersion per 1-dB gain is twice that of fibers with SiO₂-Al₂O₃ core, which is caused by gain curve form difference. A scheme of mutual compensation of intrinsic fiber dispersion and anomalous dispersion caused by Er³⁺ in the region 1.532-1.537 μm has been suggested     

SiO$_2$ Barriers for Increasing Gain Events in Solid-State Impact-Ionization Multipliers

A solid-state impact-ionization multiplier (SIM) was designed to amplify signals from arbitrary current sources through impact ionization. A primary application is amplification of signals produced by photodiodes. Photodiodes made from any semiconductor can be wired directly to the SIM's injection node. Planar versions of the SIM suffer from nonideal impact ionization efficiency as a result of injected carriers drifting through the device's depletion region to the output electrode without passing through the highest electric field regions and undergoing ionization events. Low impact ionization efficiency can lead to an increased excess noise factor, higher temperature sensitivity, and higher voltage sensitivity (rate of gain change with respect to applied voltage). This paper describes increasing SIM ionization efficiencies by introducing an insulator between the SIM's injection and output electrodes, effectively directing the carriers into the highest electric field. This method has shown to greatly increase the impact ionization efficiency in simulation and experimental results. Ionization efficiency improvements are demonstrated primarily through decreases in voltage sensitivity.      

Carrier distribution and low-field resistance in short n+-n--n+and n+-p--n+structures

In the present paper, we calculate the potential, field, and carrier distributions in short n⁺-n^--n⁺and n⁺-p^--n⁺devices and estimate the low-field resistance. The results of the calculations present a set of universal curves which may be used to find the minimum carrier density in the sample, the barrier height, the electric field at the boundary, etc. Our calculations show that electron injection becomes very important when the doping level is smaller than 1.5 × 10¹⁴(cm^-3). (T/300 K)/ L²(µm) for GaAs diodes, whereLis the sample length. The low-field resistance of the sample is limited by the thermionic emission of the sample and by the diffusion and drift in the sample. The thermionic emission dominates at low temperatures, in short samples, and the diffusion-drift dominates in longer samples at higher temperatures. The experimental values of low-field resistance for GaAs 0.4-µm n⁺-n^--n⁺devices at 77 and 300 K are in good agreement with the predicted values. The agreement is not so good for 0.25-µm devices and for n⁺-p^--n⁺devices. In the latter case, the disagreement may be due to uncertainty in the doping level because the low-field resistance of the n⁺-p^--n⁺structure is shown to be very sensitive to the doping level of the p-region.     

Hole trapping and breakdown in thin SiO2

The field dependence of the hole generation rate, also known as the impact ionization coefficient α, in thin SiO2(< 20 nm) was characterized by measuring the negative flat-band shift due to hole trapping. In thicker oxides,alpha = alpha_{0}e^{-H/E}where H = 78 MV/cm for electric fields ranging from 7 to 14 MV/cm, which covers the field range from the onset of significant Fowler-Nordheim current to instant breakdown. The similar field dependences of α and charge-to-breakdown supports the model that hole generation and trapping leads to oxide wearout. Because of the fact that positive charge generation is observed for oxide voltage well below the SiO2bandgap, we propose that the generated holes arise from transition between band tails in the amorphous SiO2. It is also observed that α decreases rapidly when the applied oxide voltage is very low; thus α is a function of both oxide field and voltage in general. This suggests that ultra-thin oxide with low operating voltages might be a good candidate for high endurance E²PROM devices at very low oxide field.