on an n - type substrate 11 , the following layers are deposited by a gas - source mbe ( molecular beam epitaxy ) method in due order , which include , an n - type buffer layer 12 of a thickness of about 1 μm ; a non - doped inalgaas / inalas superlattice multiplication layer 13 of a thickness of less than 0 . 5 μm ; a p + - type inp field buffer layer 14 of a thickness of 30 to 100 nm ; a p - type depleted ingasas light absorbing layer 15 of a thickness of 10 nm ; a p - type non - depleted ingaas light absorbing layer 16 of a thickness less than 2 . 0 μm ; a p + - type inp cap layer 17 of a thickness of 0 . 5 μm ; a p + - type ingaas layer 18 at a thickness of 0 . 1 μm . subsequently , a circular light receiving mesa - region 19 with a diameter in the range of 20 to 50 μm is formed by etching . a passivation film 110 is formed by sin or by polyimide , p / n electrodes 111 / 112 are formed , and , after the substrate 11 is mirror polished to approximately 150 μm , the ar coat 113 is formed on the rear surface of the substrate . the first mesa - type avalanche photodiode which receives light at the rear surface is made by the above processes . the first avalanche photodiode of the present invention is proved to have a high speed response characteristic ( the gb ( gain - bandwidth ) product is 70 to 80 ghz ) at a low dark current such as a multiplication dark current of less than 10 na , and the first avalanche photodiode of the present invention is recognized to have sufficient reliability because the dark current does not change after aging for 1000 hours at 150 ° c . first , the following layers are deposited on an n - type substrate 21 by the gas - source mbe method in due order , which include an n - type buffer layer 22 of a thickness of approximately 1 μm , a non - dope inalgaas / inalas superlattice multiplication layer 23 of a thickness of less than 2 . 0 μm , a p + - type inp field buffer layer 24 of a thickness in a rage of 30 to 100 nm , a p - type depleted ingaas light absorbing layer 25 of a thickness of 10 nm , a p - type non - depleted ingaas light absorbing layer 26 of a thickness less than 2 . 0 μm , a p + - type inp cap layer 27 of a thickness of 0 . 5 nm , and a p + - ingaas layer 28 at a thickness of 0 . 1 μm . subsequently , a rectangular light receiving mesa region 29 in the width of 10 nm and the length of 50 to 100 μm is formed by etching . the passivation film 210 is formed using sin , the p / n electrodes 211 / 212 are then formed , and the substrate is mirror polished into approximately 150 μm . in addition , a polyimide layer is applied for relieving and reinforcing the step excepting the light receiving area in the rectangular mesa region . the mesa - type waveguide structured avalanche photodiode according the second embodiment of the present invention is made as described above . the second avalanche photodiode is recognized to have a high speed response ( the gb product is 70 to 80 ghz ) at a low dark current such as a low multiplication dark current of 10 na and the second element of the present invention is recognized to have sufficient reliability because the dark current does not change after aging for 1000 hours at 150 ° c . first , the following layers are deposited on a si - inp substrate 31 by the gas - source mbe method in due order , which include a p - type buffer layer 32 of a thickness of approximately 1 μm , a p - type non - depleted ingaas light absorbing layer 33 of a thickness of less than 2 . 0 μm , a p - type depleted ingaas light absorbing layer 34 at a thickness of 10 nm , a p + - type inp field buffer layer 35 of a thickness in a range of 30 to 100 nm , a non - dope inalgaas / inalas superlattice multiplication layer 36 of a thickness of less than 0 . 5 μm , a non - doped inp etching stop layer 37 of a thickness of 10 nm , a n + - type inalas cap layer 38 of a thickness of 0 . 5 μm , and a n + - type ingaas layer 39 of a thickness of 0 . 1 μm . either the p - type or n - type inp substrate may be used . subsequently , an annular shaped isolation trench 312 with a width of 2 to 20 μm is formed by a selective etching method using the etching stop layer 37 at an outside area of the circular light receiving region 310 having a diameter of 25 μm . subsequently , a p - type region 311 is formed selectively at an outside area of the annular shaped separation groove 312 by diffusion of zn or by an be ion implantation , leaving the circular region 310 inside the annular region . in the case of be ion implantation , a two inches full wafer may be subjected to processing . the passivation film 313 is formed by use of silicon nitride or polyimide , the p / n electrodes 314 / 315 are formed , and the ar coat 316 is formed on the rear side of the substrate after the substrate 31 is mirror polished to approximately 150 μm . the third element , which is the back - illumination structure planar - type avalanche photodiode according to the third embodiment of the present invention is formed by the above processes . the third avalanche photodiode is recognized to have a high speed response ( the gb product is 70 to 80 ghz ) at a low dark current such as a low multiplication dark current of 10 na , and the third element of the present invention is recognized to have sufficient reliability because the dark current does not change after aging for 1000 hours at 150 ° c . first , the following layers are deposited on a si - inp substrate 41 by the gas - source mbe method in due order , which include a p - type buffer layer 42 of a thickness of approximately 1 μm , a p - type non - depleted ingaas light absorbing layer 43 of a thickness of less than 2 . 0 μm , a p - type depleted ingaas light absorbing layer 44 of a thickness of 10 nm , a p + - type inp field buffer layer 45 of a thickness in the range of 30 to 100 nm , a non - doped inalgaas / inalas superlattice multiplication layer 46 of a thickness of less than 0 . 5 μm , a non - dope inp etching stop layer 47 of a thickness of 10 nm , a n + - type inalas cap layer 48 of a thickness of 0 . 5 μm , and a n + - type ingaas layer 49 of a thickness of 0 . 1 μm . the p - type or n - type inp substrate may be used similarly . at the outside periphery of a light receiving region 410 which is formed in the form of an elongated rectangular waveguide structure so as to connect two circular portions with a diameter of 15 μm disposed on both ends of the mesa region , an annular shaped isolation trench 412 having a width of 2 to 10 μm is formed around the rectangular waveguide structure by selective etching using the above etching stop layer 47 . subsequently , at the outside periphery of the annular separation groove 412 , a p - type region 411 is formed by diffusion of zn or by ion implantation of be . in the case of ion implantation of be ions , a two inch full wafer may be processed . subsequently , a rectangular waveguide mesa is formed by a mesa - etching process , and a passivation film 413 is deposited using silicone nitride , and after forming p / n electrodes 414 / 415 , the substrate 41 is mirror polished to approximately 150 μm . the passivation film acts as a nonreflective film ( ar ) 416 at the light incident edge . the planar waveguide - type avalanche photodiode according to the fourth embodiment of the present invention is made by the above manufacturing processes . the fourth avalanche photodiode is recognized to have a high speed response ( the gb product is 70 to 80 ghz ) at a dark current such as a low multiplication dark current of 10 na and the fourth diode of the present invention is recognized to have sufficient reliability because the dark current does not change after aging for 1000 hours at 150 ° c . it is also recognized that the present photodiode is easily used as an element for passive alignment mounting with other light waveguide devices ( plc etc .) an element in which a high reflective film is formed on the back edge surface of the waveguide structure is recognized to be capable of providing a short waveguide length , that is , a low element capacitance and a high quantum efficiency . it is to be understood that modifications may by included within the scope of the present invention . that is , use of an etching stop layer is shown in the third and fourth embodiments , but a similar elemental structure may be formed by dry etching without degrading the depthwise accuracy , so that dry etching is included in the scope of the present invention . furthermore , in the waveguide structured avalanche photodiodes according to the second and fourth embodiments , it is possible to improve the external quantum efficiency by transforming the waveguide mode to a multi - mode by inserting a non - depleted intermediate refractive index layer in the waveguide structure of the apd , in order to reduce the coupling loss between a light incidence surface of the apd and an optical fiber . in each of the above described embodiments from the first to the fourth embodiments , the description is given for the case that the inalas / inalgaas super lattice layer is used as the multiplication layer , and the inp is used as the field buffer layer . however , it is possible to adopt modified elemental structures in which an inalas / ingaasp superlattice layer , an inalas / ingaas super lattice layer , or a semiconductor layer including al ( such as inalas , inalasp ) as the multiplication layer , and an inalas layer or an ingaasp layer as the field buffer layer are used in combinations . in the above described embodiments from the first to the fourth embodiment , the carrier concentration of the non - depleted light absorbing layer is assumed to be homogeneous . however , it is possible to adopt modified element structures in which the carrier concentration changes stepwise or continuously toward the depleted light absorbing layer . in the above described third and fourth embodiments , it is possible to adopt p - type inp or n - type inp for the substrate . as described above in detail , the avalanche photodiode of the present invention provides a high sensitivity light receiving element for use in next generation subscriber &# 39 ; s optical communication system , and the light receiving element of the present invention is simple in structure , and reliable and has a giga - bit response characteristic .