Staircase avalanche photodiode

DETAILED DESCRIPTION It is the principal object of the present invention to solve the above-mentioned problems and to provide a staircase APD of lower noise and faster response.
A staircase APD according to the present invention comprises a photoabsorption layer and an avalanche multiplication semiconductor layer on a semiconductor substrate wherein the avalanche multiplication semiconductor layer is a periodic multilayer structure graded in composition from InAlAs to InGa.
sub.
x Al.
sub.
(1-x) As (0.
1<x<1).
Another APD according to the present invention comprises, on a semiconductor substrate, a photoabsorption layer, an avalanche multiplication semiconductor layer, and an electric-field relaxation layer sandwiched between the photoabsorption layer and the avalanche multiplication semiconductor layer wherein the bandgap of the electric-field relaxation layer is wider than the bandgap of the photoabsorption layer.
A further APD according to the present invention wherein the electronic-field relaxation layer having a bandgap wider than the bandgap of the photoabsorption layer has a structure with the highly-doped layer sandwiched between the lightly-doped layers.