hereinafter , the modes for carrying out the invention are exemplified in the following preferred embodiments with reference to the accompanying drawings , in which the same reference numerals indicate the identical parts , the same explanation for which is omitted to avoid redundancy . the present embodiment is explained with reference to fig1 and 2 respectively showing a sectional view of the periphery of the second mesa of the backside - illuminated type apd and a block diagram of the optical receiver module according to the invention . with reference to fig1 , the structure and production steps of the backside - illuminated type apd are explained . to note , fig1 and 3 ( described later ) show the sectional view of the principal parts of the finished apd , with reference to which the person in the art could understand the productions steps thereof . as shown in fig1 , on an n - type inp substrate 101 whose impurity concentration amounts to 1 × 10 18 / cm 3 , such layers are formed by means of molecular beam epitaxy ( mbe ) as an n - type inalas buffer layer 103 whose impurity concentration amounts to 2 × 10 18 / cm 3 and whose thickness amounts to 0 . 7 μm , an n - type inalas multiplication layer 104 whose impurity concentration amounts to 5 × 10 14 / cm 3 and whose thickness amounts to 0 . 2 μm , a p - type inalas / p - type ingaas / p - type inalas field control layer 105 whose impurity concentration amounts to 1 × 10 18 / cm 3 and whose respective thickness amounts to 0 . 02 μm , a p - type ingaas absorption layer 106 whose impurity concentration amounts to 1 × 10 15 / cm 3 and whose thickness amounts to 1 . 2 μm , a p - type inalgaas cap layer 107 whose impurity concentration amounts to 5 × 10 17 / cm 3 and whose thickness amounts to 1 . 0 μm and a p - type ingaas contact layer 108 whose impurity concentration amounts to 5 × 10 19 / cm 3 and whose thickness amounts to 0 . 1 μm . after the formation of a circular hard mask , the contact layer 108 , the cap layer 107 , the absorption layer 106 and the field control layer 105 are etched with a phosphoric acid based etching liquid . the etching operation comes to a halt in the middle of the field control layer 105 in order to prevent a pn junction ( or an interface between the field control layer 105 and the multiplication layer 104 below the same ) from being exposed . with the production steps up to here completed , a first mesa 110 is formed on the substrate 101 . then , by means of metal organic vaporphase epitaxy ( movpe ), a buried layer 111 whose impurity concentration amounts to 1 × 10 15 / cm 3 and whose thickness amounts to 1 . 6 μm and that is made from a p - type inp crystal , is grown on the substrate 101 on the periphery of the first mesa 110 . a p - type ingaas contact layer 108 a whose impurity concentration amounts to 5 × 10 19 / cm 3 and whose thickness amounts to 0 . 1 μm is formed again on the buried layer 111 with the hard mask removed , which contact layer 108 a is integrally coupled to the contact layer 108 . subsequently , a photo - resist having a circular planar pattern whose diameter is larger than that of the first mesa 110 is formed on the contact layer 108 a , with which photo - resist acted as a mask the contact layer 108 a , the buried layer 111 , the field control layer 105 , the multiplication layer 104 , the buffer layer 103 and the substrate 101 surface are etched with a br based etching liquid . with the production steps up to here completed , a second mesa 120 is formed on the substrate 101 on the periphery of the first mesa 110 . the second mesa 120 has a concentric planar pattern with regard to the first mesa 110 . with the photo - resist removed , the whole surface of the substrate 101 is coated with an insulating protective film , which film is formed by depositing a sin film 113 whose thickness amounts to 0 . 2 μm and a sio 2 film 114 whose thickness amounts to 0 . 3 μm thereon . after a part ( through hole ) of the contact layer 108 a and an unshown part of the substrate 101 are exposed with the protective film subjected to the photolithographic process , a p - type electrode 115 to be coupled to the contact layer 108 a and an unshown n - type electrode to be coupled to the substrate 101 are formed . the electrodes are formed by patterning with the photolithographic process a ti / pt / au film that is deposited on the substrate by evaporation method and whose thickness amounts to 0 . 5 μm , which film components are slashed in between them hereby to indicate the component on the left side slash is nearest to the substrate and that on the right side slash is farthest therefrom . to note , the through hole formed on the contact layer 108 a has a ring shape comprising two concentric circles . it is arranged hereby that the diameter of the first mesa 110 is equal to or smaller than that of the inner concentric circle . as a result of it , looking inside the inner concentric circle , a mirror is made up of with the transparent protective films 113 and 114 as well as the ti / pt / au metallized film formed on the contact layer 108 . the backside - illuminated type apd 200 as shown in fig1 is arranged such that the light entered from the backside surface thereof is reflected by the mirror and passed bi - directionally through the absorption layer 106 so as to generate carrier , and the multiplication layer is provided to multiply carrier as well as parasitic capacitance is reduced by making the diameter of the first mesa smaller , so that it is provided with higher light receiving sensitivity and excellent in response speed . the optical receiver module 300 as shown in fig2 comprises a backside - illuminated type apd 200 and a trans impedance amplifier ( tia ) 330 provided with a limit amplifier 333 . besides the limit amplifier 333 , this tia 330 comprises a pre - amplifier 331 and a feedback resistance 332 , which tia 330 is a negative feedback amplifier to convert input current into output voltage . the optical receiver module 300 is arranged such that it receives light signals as shown in the drawing with an arrow at the backside - illuminated type apd 200 and those signals are output as electrical signals from an out 1 terminal 310 that is a positive - phase terminal of the tia 330 and from an out 2 terminal 320 that is a negative - phase terminal thereof . as the light receiving portion of the backside - illuminated type apd 200 incorporated in the optical receiver module 300 is comparatively larger in diameter , it enables the latter to be combined with a pre - amplifier of smaller capacitance and lower input impedance , which enhances high - frequency response speed . further , the module 300 facilitates optical axis alignment and results in lower production cost and higher yield especially in such high - speed optical communication field as 2 . 5 gbits / s or more . according to the above embodiment , the through hole in contact with the metallized electrode is formed outside the first mesa , so that the smallest diameter of the first mesa can be achieved that meets the requirements for the light receiving sensitivity and the diameter of the light receiving portion of the apd , with the result that parasitic capacitance reduces so as to enhance response speed . the mirror covers the whole upper space over the first mesa , so that the planar distribution of the light receiving sensitivity becomes uniform so as to facilitate the coupling with the optical fibers . with reference to fig3 , the present embodiment is explained below . fig3 shows a sectional view of the periphery of the second mesa of the surface - illuminated type apd . in conjunction with fig3 , the structure and production steps of the surface - illuminated type apd are explained . as shown in fig3 , on an n - type inp substrate 101 whose impurity concentration amounts to 1 × 10 18 / cm 3 , such layers are formed by means of mbe as an n - type inalas buffer layer 103 whose impurity concentration amounts to 2 × 10 18 / cm 3 and whose thickness amounts to 0 . 7 μm , an n - type inalas multiplication layer 104 whose impurity concentration amounts to 5 × 10 14 / cm 3 and whose thickness amounts to 0 . 2 μm , a p - type inalas / p - type ingaas / p - type inalas field control layer 105 whose impurity concentration amounts to 1 × 10 18 / cm 3 and whose respective thickness amounts to 0 . 02 μm , a p - type ingaas absorption layer 106 whose impurity concentration amounts to 1 × 10 15 / cm 3 and whose thickness amounts to 1 . 4 μm , a p - type inalgaas cap layer 107 whose impurity concentration amounts to 5 × 10 17 / cm 3 and whose thickness amounts to 1 . 0 μm and a p - type ingaas contact layer 108 whose impurity concentration amounts to 5 × 10 19 / cm 3 and whose thickness amounts to 0 . 1 μm . after the formation of a circular hard mask , the contact layer 108 , the cap layer 107 , the absorption layer 106 and the field control layer 105 are etched with a phosphoric acid based etching liquid . the etching operation comes to a halt in the middle of the field control layer 105 in order to prevent a pn junction from being exposed . with the production steps up to here completed , a first mesa 110 is formed on the substrate 101 . then , by means of metal organic vaporphase epitaxy , a buried layer 111 whose impurity concentration amounts to 1 × 10 15 / cm 3 and whose thickness amounts to 1 . 6 μm and that is made from a p - type inp crystal , is grown on the substrate 101 on the periphery of the first mesa 110 . a p - type ingaas contact layer 108 a whose impurity concentration amounts to 5 × 10 19 / cm 3 and whose thickness amounts to 0 . 1 μm is formed again on the buried layer 111 with the hard mask removed , which contact layer 108 a is integrally coupled to the contact layer 108 . to note , the thickness of the absorption layer 106 according to the first embodiment amounts to 1 . 2 μm while that amounts to 1 . 4 μm according to the present embodiment . on the other hand , the thickness of the buried layer 111 according to the present embodiment is the same as that of the first embodiment or 1 . 6 μm . optimization is must for the thickness of the buried layer 111 , as the lateral side surfaces of the first mesa 110 are not buried to the top ends by the same if it is too thin while it protrusively heaps up if it is too thick . the inventors and the concerned experimentally do not find that applying the same thickness of the buried layer 111 as the first embodiment to the present embodiment causes any problem . subsequently , a photo - resist having a circular planar pattern whose diameter is larger than that of the first mesa 110 is formed on the contact layer 108 , with which photo - resist acting as a mask the contact layer 108 a , the buried layer 111 , the field control layer 105 , the multiplication layer 104 , the buffer layer 103 and the substrate 101 surface are etched with a br based etching liquid . with the production steps up to here completed , a second mesa 120 is formed on the substrate 101 on the periphery of the first mesa 110 . the second mesa 120 has a concentric planar pattern with regard to the first mesa 110 . with the photo - resist removed , the whole surface of the substrate 101 is coated with an insulating protective film , which film is formed by depositing a sin film 113 whose thickness amounts to 0 . 2 μm and a sio 2 film 114 whose thickness amounts to 0 . 3 μm thereon . after a part ( through hole ) of the contact layer 108 a and an unshown part of the substrate 101 are exposed with the protective film subjected to the photolithographic process . to note , the through hole formed on the contact layer 108 a has a ring shape comprising two concentric circles . it is arranged hereby that the diameter of the first mesa 110 is equal to or smaller than that of the inner concentric circle . the sio 2 film 114 coated on the inside of the through hole formed on the contact layer 108 a is etched again with the photolithographic process . as a result of it , the sin film 113 remained on the inside of the through hole acts as an anti - reflective film . subsequently , a p - type electrode 115 to be coupled to the contact layer 108 a and an unshown n - type electrode to be coupled to the substrate 101 are formed , which electrodes are formed by patterning a ti / pt / au film that is deposited on the substrate by evaporation method and whose thickness amounts to 0 . 5 μm with the photolithographic process . to note , the p - type electrode 115 has a ring shape , the inside of which corresponds to a light receiving portion . fig3 shows only a sectional view of the electrode . the surface - illuminated type apd 200 a as shown in fig3 is arranged such that the light entered from the surface is unidirectionally passed through the absorption layer 106 that is thicker than that of the first embodiment so as to generate carrier and the multiplication layer is provided to multiply carrier as well as parasitic capacitance is reduced , so that it enhances light receiving sensitivity and response speed . with the surface - illuminated type apd 200 a as shown in fig3 incorporated in the optical receiver module 300 as shown in fig2 , it enables the latter to be combined with a pre - amplifier of smaller capacitance and lower input impedance , which enhances high - frequency response speed . further , the module 300 facilitates optical axis alignment and results in lower production cost and higher yield especially in such high - speed optical communication field as 2 . 5 gbits / s or more . according to the above embodiment , a ring - like electrode is formed outside the first mesa , so that the smallest diameter of the first mesa can be achieved that meets the requirements for the diameter of the light receiving portion of the apd , with the result that parasitic capacitance reduces so as to enhance response speed . according to the invention , it enables parasitic capacitance to reduce without deteriorating the light receiving sensitivity of the apd and reducing the diameter of the receiving portion thereof .