Enhanced carrier velocity to early impact ionization

Abstract

Numerical simulations predict that carriers which impact ionize shortly after their dead space travel to their ionization event with an average velocity much greater than their saturated drift velocity. The effect is argued to result from the fortuitously reduced phonon scattering which leads to this early ionization. Analytical models constructed using pictures of impact ionization similar to those of both Wolff and Shockley give good agreement with Monte Carlo simulations of the effect.

Exponential Time Response in Analogue and Geiger Mode Avalanche Photodiodes

“…We have shown [14] that this speed enhancement can be explained in terms of the reduction in scattering which causes carriers to ionise at short distances. We have also argued [14], using simple models, that the mean speed, to ionization at a distance after the carrier is injected cold can be written approximately as , where is the limiting value of at long ionization path lengths.…”

“…We have also argued [14], using simple models, that the mean speed, to ionization at a distance after the carrier is injected cold can be written approximately as , where is the limiting value of at long ionization path lengths. Indeed, this simple expression describes very well the behavior which we observe in our numerical simulations.…”

Gain-bandwidth product optimization of heterostructure avalanche photodiodes

“…The dead space is the minimum distance a carrier must travel in the high-field multiplication region before reaching the ionization threshold energy [13], [14]. This spatial inhibition between successive ionizations, which is brought about by the dead space, turns out to increase the buildup time, as was initially shown analytically by Hayat and Saleh [15] and subsequently confirmed by others [16], [17]. However, dead space is not the only factor that affects the speed in thin multiplication layers.…”

“…Interestingly, the above effect was shown earlier to reduce the excess noise factor as well in a host of low-noise APDs (termed impact-ionization-engineered, , APDs) developed at the University of Texas. The low-noise characteristics of these bandgap engineered devices where shown to be a result of the initial energy effect using both analytical techniques [20], [21] as well as Monte Carlo simulation [16], [22]. We show in this paper that by carefully selecting the width of the energy buildup layer, the gain-bandwidth product can also be improved and optimized.…”

“…However, dead space is not the only factor that affects the speed in thin multiplication layers. Monte Carlo simulations have shown that carriers experiencing the high electric field, which is required in thin multiplication region, have the tendency to ionize at early stages of their sojourn in the multiplication region [16], [18]. Moreover, those carriers that impact ionize early on are found to possess speeds well above the saturated drift velocities [18].…”