(a) Field of the Invention
The present invention relates to a waveguide type semiconductor photodetector and, more particular, to a wide-band semiconductor photodetector having a waveguide and exhibiting a high optical sensitivity and a low output distortion.
(b) Description of the Related Art
A conventional semiconductor photodetector incorporating therein a waveguide for guiding incident light to an optical absorption layer (hereinafter called waveguide type semiconductor photodetector or simply photodetector) has a layer structure wherein p- and n-type semiconductor layers sandwich therebetween a lightly-doped optical absorption layer or core layer to form a p-n junction. This type of semiconductor photodetector is generally applied with a reverse bias voltage between the p- and n-type semiconductor layers to deplete the optical absorption layer of carriers, and takes advantage of the high electric field generated in the depleted layer in the optical absorption layer to effect a photo-electric conversion of a signal light received through the incidence facet of the waveguide. In this process, the semiconductor photodetector receives the signal light through the incidence facet to guide the same to the optical absorption layer, and detects excited carriers generated by the incident light in the depletion layer as a photo-current. The excited carriers generated and drifting in the depletion layer are separated by the high electric field in the depletion layer as separate holes and electrons. The separated electrons reach p-type cladding layer and the separated holes reach n-type cladding layer, thereby contributing the generation of the photo-current.
The waveguide type semiconductor photodetector has several advantages including selection ability by the wavelength of the signal light, a high operational speed, and a wide-band characteristic. It also has the advantage of having a profile and a structure similar to those of a semiconductor laser etc. to facilitate integration therewith.
However, it is difficult to adapt the mode field diameter of the conventional photodetector to the core diameter (or mode field diameter) of an optical fiber to be coupled with the photodetector for optical transmission, thereby raising a problem of coupling loss therebetween.
The optical adaptation in the mode field diameter between two systems, such as between the optical fiber and photodetector is discussed herein in view of the importance in reduction of the optical loss. Assuming that axial deviation between the two systems is in one direction, i.e., either horizontal or vertical direction, the coupling factor .eta. between the two systems can be represented by the following equation: EQU .eta.=2.multidot.W.sub.1 .multidot.W.sub.2 /{(W.sub.1.sup.2 +W.sub.2.sup.2).multidot.exp [-2.delta..sup.2 /(W.sub.1.sup.2 +W.sub.2.sup.2)]} (1)
wherein W1 and W2 represent mode field diameters of both the systems, and .delta. represents the deviation or offset amount between the optical axes of both the system. The equation (1) is obtained from overlapping integral of electric field in the one direction.
The coupling loss between the two systems can be calculated by the above equation (1) from a specified mode field diameter ratio (or relative mode field diameter) between the two systems: if the ratio is 2, then the calculated coupling loss is 1 dB; and if the ratio is 3, then the calculated coupling loss is 2.5 dB.
In the case where the internal quantum effect is assumed 100% and if the mode filed diameter ratio between the two systems is 2, the optical sensitivity of the photodetector for a signal light having a 1.3 .mu.m wavelength is approximately 0.85 A/W, and if the ratio is 3 in a similar condition, the optical sensitivity is approximately 0.6 A/W.
It is to be noted the mode field diameter of the photodetector should be equal to that of the optical fiber or quartz waveguide to be coupled therewith in order to obtain a satisfactory coupling efficiency or to reduce the coupling loss. For this purpose, the optical absorption layer of the photodetector should be grown to have a sufficient thickness, for example, 4 .mu.m or above to be adapted with the diameter of the light emitted from the optical fiber or quartz waveguide.
However, it is difficult to obtain a thickness of 4 .mu.m or above for an optical absorption layer in a photodetector wherein the layer structures including the optical absorption layer is epitaxially grown on a substrate. Namely, the adaptation of the mode field diameter by increasing the thickness of the epitaxial optical absorption layer is not practical in the photodetector.
JP-A-4(1992)-241272 proposes a photodetector wherein the optical absorption be 0.15 .mu.m thick to reduce the confinement efficiency thereof, thereby increasing the effective mode field diameter of the optical absorption layer. This proposal, however, has a problem in which a high electric field generated within the optical absorption layer causes a zener break-down due to a tunnel current.