Effect of impact ionization in the InGaAs absorber on excess noise of avalanche photodiodes

The effects of impact ionization in the InGaAs absorption layer on the multiplication, excess noise and breakdown voltage are modeled for avalanche photodiodes (APDs), both with InP and with InAlAs multiplication regions. The calculations allow for dead space effects and for the low field electron ionization observed in InGaAs. The results confirm that impact ionization in the InGaAs absorption layer increases the excess noise in InP APDs and that the effect imposes tight constraints on the doping of the charge control layer if avalanche noise is to be minimized. However, the excess noise of InAlAs APDs is predicted to be reduced by impact ionization in the InGaAs layer. Furthermore the breakdown voltage of InAlAs APDs is less sensitive to ionization in the InGaAs layer and these results increase tolerance to doping variations in the field control layer

The merits and limitations of local impact ionization theory

Multiplication measurements on GaAs p+-i-n+s with i-region thicknesses, w, between 1 μm and 0.025 μm and Monte Carlo (MC) calculations of the avalanche process are used to investigate the applicability of the local ionization theory. The local expressions for multiplication are able to predict the measured values surprisingly well in p+-i-n+s with i-region thicknesses, w, as thin as 0.2 μm before the effect of dead-space, where carriers have insufficient energy to ionize, causes significant errors. Moreover, only a very simple correction to the local expressions is needed to predict the multiplication accurately where the field varies rapidly in abrupt one-sided p+-n junctions doped up to 1018 cm-3. However, MC modeling also shows that complex dead-space effects cause the local ionization coefficients to be increasingly unrepresentative of the position dependent values in the device as w is reduced below 1 μm. The success of the local model in predicting multiplication is therefore attributed to the dead-space information already being contained within the experimentally determined values of local coefficients. It is suggested that these should therefore be thought of as effective coefficients which, despite the presence of dead-space effects, can be still be used with the existing local theory for efficiently quantifying multiplication and breakdown voltages

Avalanche noise characteristics of single Al/sub x/Ga/sub 1-x/As(0.3

Avalanche multiplication and excess noise have been measured on a series of Al/sub x/Ga/sub 1-x/As-GaAs and GaAs-Al/sub x/Ga/sub 1-x/As (x=0.3,0.45, and 0.6) single heterojunction p/sup +/-i-n/sup +/ diodes. In some devices excess noise is lower than in equivalent homojunction devices with avalanche regions composed of either of the constituent materials, the heterojunction with x=0.3 showing the greatest improvement. Excess noise deteriorates with higher values of x because of the associated increase in hole ionization in the Al/sub x/Ga/sub 1-x/As layer. It also depends critically upon the carrier injection conditions and Monte Carlo simulations show that this dependence results from the variation in the degree of noisy feedback processes on the position of the injected carriers

Field dependence of impact ionization coefficients in In0.53Ga0.47As

Electron and hole ionization coefficients in In/sub 0.53/Ga/sub 0.47/As are deduced from mixed carrier avalanche photomultiplication measurements on a series of p-i-n diode layers, eliminating other effects that can lead to an increase in photocurrent with reverse bias. Low field ionization is observed for electrons but not for holes, resulting in a larger ratio of ionization coefficients, even at moderately high electric fields than previously reported. The measured ionization coefficients are marginally lower than those of GaAs for fields above 250 kVcm/sup -1/, supporting reports of slightly higher avalanche breakdown voltages in In/sub 0.53/Ga/sub 0.47/As than in GaAs p-i-n diodes

The effects of nonlocal impact ionization on the speed of avalanche photodiodes

The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions

Low multiplication noise thin Al0.6Ga0.4As avalanche photodiodes

Avalanche multiplication and excess noise were measured on a series of Al0.6Ga0.4As p+in+ and n+ip+ diodes, with avalanche region thickness, w ranging from 0.026 μm to 0.85 μm. The results show that the ionization coefficient for electrons is slightly higher than for holes in thick, bulk material. At fixed multiplication values the excess noise factor was found to decrease with decreasing w, irrespective of injected carrier type. Owing to the wide Al0.6Ga0.4As bandgap extremely thin devices can sustain very high electric fields, giving rise to very low excess noise factors, of around F~3.3 at a multiplication factor of M~15.5 in the structure with w=0.026 μm. This is the lowest reported excess noise at this value of multiplication for devices grown on GaAs substrates. Recursion equation modeling, using both a hard threshold dead space model and one which incorporates the detailed history of the ionizing carriers, is used to model the nonlocal nature of impact ionization giving rise to the reduction in excess noise with decreasing w. Although the hard threshold dead space model could reproduce qualitatively the experimental results, better agreement was obtained from the history-dependent mode

Avalanche multiplication in AlxGa1-xAs (x=0to0.60)

Electron and hole multiplication characteristics, Me and Mh, have been measured in AlxGa1-xAs (x=0-0.60) homojunction p+-i-n+ diodes with i-region thicknesses, w, from 1 μm to 0.025 μm and analyzed using a Monte Carlo model (MC). The effect of the composition on both the macroscopic multiplication characteristics and microscopic behavior is therefore shown for the first time. Increasing the alloy fraction causes the multiplication curves to be shifted to higher voltages such that the multiplication curves at any given thickness are practically parallel for different x. The Me/Mh ratio also decreases as x increases, varying from ~2 to ~1 as x increases from 0 to 0.60 in a w=1 μm p+-i-n+. The Monte-Carlo model is also used to extract ionization coefficients and dead-space distances from the measured results which cover electric field ranges from ~250 kV/cm-1200 kV/cm in each composition. These parameters can be used to calculate the nonlocal multiplication process by solving recurrence equations. Limitations to the applicability of field-dependent ionization coefficients are shown to arise however when the electric-field profile becomes highly nonunifor

Optimization of InP APDs for high-speed lightwave systems

Calculations based on a rigorous analytical model are carried out to optimize the width of the indium phosphide avalanche region in high-speed direct-detection avalanche photodiode-based optical receivers. The model includes the effects of intersymbol interference (ISI), tunneling current, avalanche noise, and its correlation with the stochastic avalanche duration, as well as dead space. A minimum receiver sensitivity of -28 dBm is predicted at an optimal width of 0.18 mu m and an optimal gain of approximately 13, for a 10 Gb/s communication system, assuming a Johnson noise level of 629 noise electrons per bit. The interplay among the factors controlling the optimum sensitivity is confirmed. Results show that for a given transmission speed, as the device width decreases below an optimum value, increased tunneling current outweighs avalanche noise reduction due to dead space, resulting in an increase in receiver sensitivity. As the device width increases above its optimum value, the receiver sensitivity increases as device bandwidth decreases, causing ISI to dominate avalanche noise and tunneling current shot noise

Multiplication and excess noise characteristics of thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of thin 4H-SiC avalanche photodiodes with an i-region width of 0.1 µm have been investigated. The diodes are found to exhibit multiplication characteristics which change significantly when the wavelength of the illuminating light changes from 230 to 365 nm. These multiplication characteristics show unambiguously that β > α in 4H-SiC and that the β/α ratio remains large even in thin 4H-SiC diodes. Low excess noise, corresponding to k=0.1 in the local model where k=α/β for hole injection, was measured using 325-nm light. The results indicate that 4H-SiC is a suitable material for realizing low-noise UV avalanche photodiodes requiring good visible-blind performance

Excess noise measurement in In0.53Ga0.47As

The excess noise due to impact ionization has been measured explicitly for the first time in In/sub 0.53/Ga/sub 0.47/As. By using a phase sensitive detection technique, the noise due to avalanche current was determined even in the presence of high tunneling currents. The excess noise due to pure electron injection measured on a series of thick In/sub 0.53/Ga/sub 0.47/As p/sup +/-i-n/sup +/ diodes suggests large electron to hole ionization coefficient ratio between 3.7 at electric field of 310 kV/spl middot/cm/sup -1/ to 5.3 at 260 kV/spl middot/cm/sup -1/. Excess noise was also measured at fields as low as 155 kV/spl middot/cm/sup -1/ suggesting that significant impact ionization occurs at these low fields. The multiplication and excess noise calculated using published ionization coefficients and ignoring dead space effects, gave good agreement with the experimental data for mixed and pure electron injection