A PIN photodiode comprising: a first p-type semiconductor layer; an n-type semiconductor layer; a second p-type semiconductor layer disposed between the first p-type semiconductor layer and the n-type semiconductor layer such that the second p-type semiconductor is directly adjacent to the n-type semiconductor, the second p-type semiconductor layer having a graded doping concentration; a substrate, the n-type semiconductor layer being grown on the substrate; an anode layer for collecting holes; a cathode layer for collecting electrons; wherein the graded doping concentration defines a first concentration adjacent to the first p-type semiconductor layer and a second concentration adjacent to the n-type semiconductor layer, and further wherein the first concentration is greater than the second concentration; wherein incident light is absorbed in the second p-type semiconductor layer, the incident light that is absorbed in the second concentration part of the second p-type semiconductor layer produces electrons and holes which drift to the anode and cathode; wherein electrons generated in the second concentration part of the second p-type semiconductor layer reach the cathode with their saturation velocity and are collected, the holes generated in the second concentration part of the second p-type semiconductor layer travel to the anode, wherein the incident light that is absorbed in the first concentration part of the second p-type semiconductor layer produces electrons and holes, wherein the holes are collected in the first concentration part; and wherein said electrons in the first concentration part are subject to a pseudo field capable of providing them with an overshoot electron velocity, the pseudo field being governed by the following equation: E = − kT q dp dx 1 p, where k is Boltzman's constant, T is the temperature, q is the charge of an electron, and the value d p d x is the doping concentration gradient, and wherein the first concentration is located at a position x o and defines a concentration p o, and further wherein the graded doping concentration is governed by the following equation: p = p o e − x D over the depth D of the second p-type semiconductor layer for all x and for D greater than zero. a first p-type semiconductor layer; an n-type semiconductor layer; a second p-type semiconductor layer disposed between the first p-type semiconductor layer and the n-type semiconductor layer such that the second p-type semiconductor is directly adjacent to the n-type semiconductor, the second p-type semiconductor layer having a graded doping concentration; a substrate, the n-type semiconductor layer being grown on the substrate; an anode layer for collecting holes; a cathode layer for collecting electrons; wherein the graded doping concentration defines a first concentration adjacent to the first p-type semiconductor layer and a second concentration adjacent to the n-type semiconductor layer, and further wherein the first concentration is greater than the second concentration; wherein incident light is absorbed in the second p-type semiconductor layer, the incident light that is absorbed in the second concentration part of the second p-type semiconductor layer produces electrons and holes which drift to the anode and cathode; wherein electrons generated in the second concentration part of the second p-type semiconductor layer reach the cathode with their saturation velocity and are collected, the holes generated in the second concentration part of the second p-type semiconductor layer travel to the anode, wherein the incident light that is absorbed in the first concentration part of the second p-type semiconductor layer produces electrons and holes, wherein the holes are collected in the first concentration part; and wherein said electrons in the first concentration part are subject to a pseudo field capable of providing them with an overshoot electron velocity, the pseudo field being governed by the following equation: E = − kT q dp dx 1 p, where k is Boltzman's constant, T is the temperature, q is the charge of an electron, and the value d p d x is the doping concentration gradient, and wherein the first concentration is located at a position x o and defines a concentration p o, and further wherein the graded doping concentration is governed by the following equation: p = p o e − x D over the depth D of the second p-type semiconductor layer for all x and for D greater than zero.