Impact Ionization Coefficients of Digital Alloy and Random Alloy Al0.85Ga0.15As0.56Sb0.44 in a Wide Electric Field Range

Digital alloy and random alloy Al
_0.85
Ga
_0.15
As
_0.56
Sb
_0.44
avalanche photodiodes (APDs) exhibit low excess noise, comparable to Si APDs. Consequently, this material is a promising multiplication layer candidate for separate absorption, charge, and multiplication structure APDs with high gain-bandwidth product. Characterization of the impact ionization coefficients of electrons (
α
) and holes (
β
) plays an important role in the simulation of avalanche photodiodes. The multiplication gain curves of eight p
⁺
-i-n
⁺
and n
⁺
-i-p
⁺
APDs covering a wide range of avalanche widths have been used to determine the electric field dependence of the impact ionization coefficients of Al
_0.85
Ga
_0.15
As
_0.56
Sb
_0.44
. A large impact ionization coefficient ratio between that of electrons to holes was seen across a wide electric field range. Simulations of the avalanche multiplication in these structures using a random path length (RPL) model gave good agreement with experimental results over almost three orders of magnitude, and a mixed injection method was employed to verify the extracted impact ionization coefficients. Interestingly, no difference in the impact ionization coefficients was seen between digital alloy and random alloy Al
_0.85
Ga
_0.15
As
_0.56
Sb
_0.44
. This knowledge of impact ionization coefficients is beneficial for the future utilization of the Al
_x
Ga
_1-x
As
_y
Sb
_1-y
material system.