Semiconductor light receiving element

A semiconductor light detector for detecting multiple wavelengths of light includes a semiconductor substrate, a plurality of first conductivity type semiconductor layers successively disposed on the semiconductor substrate with increasing energy band gap from the substrate towards a light incident surface of the detector, a first second conductivity type semiconductor region extending from the light incident surface and reaching the first conductivity type layer closest to the surface, and one or more additional second semiconductor regions successively surrounding the first second semiconductor region, reaching the surface and respective first conductivity type semiconductor layers.

FIELD OF THE INVENTION 
The present invention relates to a semiconductor light detector and, more 
particularly, to an improvement in a multiple wavelength light detector. 
BACKGROUND OF THE INVENTION 
FIG. 4 is a cross-sectional view of a prior art multiple wavelength 
semiconductor light detector. 
Reference numeral 1 designates incident light of wavelength .lambda.=1.3 
microns. Reference numeral 2 designates incident light of wavelength 
.lambda.=1.55 microns. Reference numeral 8 designates an n-type InP 
substrate. Disposed on the n-type InP substrate 8 is an n-type InGaAsP 
layer 17 having an energy band gap corresponding to a wavelength of 1.55 
microns and a p-type InGaAsP layer 16 having an energy band gap 
corresponding to a wavelength of 1.55 microns is disposed thereon. A 
p-type InP window layer 15 is disposed on the p-type InGaAsP layer 16. 
Disposed on part of the p-type InP window layer 15 is a p-type InGaAsP 
layer 14 having an energy band gap corresponding to a wavelength of 1.3 
microns and an n-type InP window layer 12. A first n side electrode 18 is 
disposed on the n-type InP window layer 12. A p side electrode 19 disposed 
on the p-type InP window layer 15 is grounded. A second n side electrode 
20 is disposed on the rear surface of the n-type InP substrate 8. 
In this structure, n-type and p-type InGaAsP layers 17 and 16, p-type InP 
window layer 15, p-type and n-type InGaAsP layers 14 and 13, and n-type 
InP window layer 12 are successively epitaxially grown on the n-type InP 
substrate 8. Thereafter, the n-type InP window layer 12, n-type InGaAsP 
layer 13, and p-type InGaAsP layer 14 are etched to produce a mesa-type 
structure. Two n side electrodes 20 and 18 are respectively produced at 
the rear surface of the substrate 8 and the surface of the n-type InP 
window layer 12, respectively, and a p side electrode 19 is produced on 
the surface of the p-type InP window layer 15. 
Light including components at a wavelength .lambda.=1.3 microns and a 
wavelength .lambda.=1.55 microns is incident on the surface. The p side 
electrode 19 is grounded and the first n side electrode 18 and the second 
n side electrode 20 are respectively biased at positive voltages. The 
light of wavelength .lambda.=1.3 microns transits the n-type InP window 
layer 12 and, thereafter, is absorbed by the InGaAsP layers 13 and 14, 
thereby generating charge carriers. In the pn junction between the InGaAsP 
layers 13 and 14, there is an electric field generated by a voltage 
applied from the outside. Therefore, the charge carriers generated by the 
irradiation of light are collected and generate an electromotive force 
between the first n side electrode 18 and the p side electrode 19. This 
electromotive force is extracted from the terminal OUT1 as an electrical 
output signal. 
On the other hand, the light of wavelength .lambda.=1.55 microns transits 
the n side InP window layers 12 and 15 and InGaAsP layers 13 and 14 and is 
absorbed by the InGaAsp layers 16 and 17, thereby generating charge 
carriers. The charge carriers are output to the external terminal OUT2 as 
an electrical signal by the same process as described above. Herein, when 
the InGaAsP layers 13 and 14 are made sufficiently thick, the light of 
wavelength .lambda.=1.3 microns does not pass beyond the respective layers 
13 and 14, and only the signal produced by the light of wavelength 
.lambda.=1.3 microns is output from the terminal OUTI and only the signal 
produced by the light of wavelength .lambda.=1.55 microns is output from 
the terminal OUT2. Therefore, this element functions as a multiple 
wavelength light detector. 
In the prior art detector, however, the p side electrode 19 has to be 
connected to an internal layer in a multiple layer structure, the 
structure is thus complicated, and its production is difficult. 
Furthermore, matching of the light detector with other elements is more 
difficult than with a planar detector. In addition, since a crystalline 
surface is exposed, a surface leakage current flows, thereby reducing 
reliability. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor light 
detector having a simple structure and high reliability. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. It should be understood, 
however, that the detailed description and specific embodiments are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent from the 
detailed description. 
According to an aspect of the present invention, a semiconductor light 
detector includes a first conductivity type first layer of a first 
semiconductor disposed on a semiconductor substrate, a first conductivity 
type second layer of a second semiconductor having a larger energy band 
gap than that of the first semiconductor disposed on the first layer, a 
second conductivity type first region disposed in the second layer at the 
surface on which light is incident, and a second conductivity type second 
region surrounding the first region, not including the first region, and 
reaching the first layer through the second layer from the surface. 
In such a structure, light of different wavelengths is respectively 
absorbed by the layers having different energy band gaps and by the 
heterojunction barrier produced between the layers, preventing crosstalk 
of the generated carriers. Therefore, light of different wavelengths 
absorbed by the respective layers can be effectively detected. Further, 
the element structure is planar, resulting in simplified production and 
good matching with other elements. 
According to another aspect of the present invention, a semiconductor light 
detector is produced by successively growing a first conductivity type 
first layer of a first semiconductor and a first conductivity type second 
layer of a second semiconductor having a larger energy band gap than that 
of the first semiconductor on a semiconductor substrate, producing a 
second conductivity type first region in the second layer at a surface 
where light is incident and a second conductivity type annular region 
spaced a predetermined distance from the first region and penetrating to 
the second layer from the surface through the first semiconductor layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the structure of a semiconductor light detector according to a 
first embodiment of the present invention. In FIG. 1, reference numeral 1 
designates incident light of wavelength .lambda.=1.3 microns. Reference 
numeral 2 designates incident light of wavelength .lambda.=1.55 microns. 
Reference numeral 8 designates an n-type InP substrate. Disposed on the 
n-type InP substrate 8 is an n-type InGaAsP layer 7 is having an energy 
band gap corresponding to a wavelength .lambda..sub.9 =1.55 microns. 
Disposed on the n-type InGaAsP layer 7 is an n-type InGaAsP layer 6 having 
an energy band corresponding to a wavelength .lambda..sub.g =1.3 microns. 
An n-type InP window layer 5 is disposed on the n-type InGaAsP layer 6. A 
first p-type region 10 is disposed in a region of the n-type InP window 
layer and penetrates to the n-type InGaAsP layer 6. A second p-type region 
11 is disposed in the n-type InGaAsP layer 6 and the n-type InP window 
layer 5 and reaches the n-type InGaAsP layer 7. A first p side electrode 3 
is produced on the first p-type region 10 and a second p side electrode 4 
is produced on the second p-type region 11. An n side electrode 9 is 
produced on the rear surface of the n-type InP substrate 8. 
On the n-type InP substrate 8, an undoped n-type In.sub.x Ga.sub.1-x 
As.sub.y P.sub.1-y layer 7, having an energy band gap corresponding to a 
wavelength .lambda..sub.g =1.55 microns is grown to a thickness of about 2 
microns by liquid phase epitaxy or metal organic chemical vapor 
deposition. Thereafter, by a similar method, an n-type 
In.sub.x',GA.sub.1-x',AS.sub.y',P.sub.1-y', layer 6 having an energy band 
gap corresponding to a wavelength .lambda..sub.g =1.3 microns is grown to 
a thickness of about 2 microns, and an n-type InP layer 5 as a window 
layer is grown to a thickness of about 1 micron. 
Surrounding the region where the light is incident, zinc is diffused into 
an annular region which is spaced apart by about 10 to 20 microns from the 
light collecting region, thereby producing a second p-type region 11 
reaching the n-type InGaAsP layer 7 through the n-type InGaAsP layer 6 
from the surface. 
A first p-type region 10 is produced by diffusing zinc into the light 
incident region surrounded by the p-type region 11 by a similar method. 
Finally, an n side electrode 9 comprising Ag/Ge is produced at the rear 
surface of substrate 8, and p side electrodes 3 and 4 comprising Au/Zn are 
produced at the surfaces of the first p-type region 10 and the second 
p-type region 11 to a thickness of several thousand angstroms, thereby 
completing the light detector. 
As described for the prior art device, incident light includes components 
at wavelengths .lambda.=1.3 microns and .lambda.=1.55 microns. The n 
electrode 9 of the light detector is grounded and the p side electrodes 3 
and 4 are biased to negative voltages. The light of wavelength 
.lambda.=1.3 microns transits the n-type InP layer 5 and is absorbed by 
the n-type In.sub.x',GA.sub.1-x',AS.sub.y',P.sub.1-y', layer 6, thereby 
producing charge carriers. Since an electric field is generated by the 
bias applied to the pn junction produced in the InGaAsP layer 6, the 
charge carriers flow and are collected to generate an electromotive force 
between the first p side electrode 3 and the n side electrode 9. Since the 
first p-type region 10 and the second p-type region 11 are separated by 
about 20 microns while the layer thickness of the n-type 
In.sub.x',Ga.sub.1-x',As.sub.y',P.sub.1-y', layer 6 is about 2 microns, if 
the incident light is sufficiently collimated on the first p-type region 
10, all the generated holes are captured in the first p-type region 10, so 
that there is no leakage to the second p-type region 11. 
The light of wavelength .lambda.=1.55 microns transits the n-type InP layer 
5 and n-type In.sub.x',Ga.sub.1-x',As.sub.y',P.sub.1-y', layer 6 and is 
absorbed by the n-type In.sub.x',Ga.sub.1-x',As.sub.y',P.sub.1-y', layer 
7, thereby generating charge carriers. The depletion layer extending from 
the first p-type region 10 is expanded into the InGaAsP layer 6 by 
adjusting the external bias but it does not reach the lower InGaAsP layer 
7. As an electric field is not applied to the region where carriers are 
generated in the InGaAsP layer 7, the carriers move by diffusion. On the 
other hand, since the InGaAsP layer 6 has a larger energy band gap than 
that of the InGaAsP layer 7, a barrier is produced in the energy band 
structure at the interface between the InGaAsP layer 6 and the InGaAsP 
layer 7. The carriers generated in the InGaAsP layer 7 cannot flow to the 
upper InGaAsP layer 6 because of this barrier. Therefore, holes generated 
in the InGaAsP layer 7 reach the second p-type region 11 by diffusion. If 
the carrier concentration in the InGaAsP layer 7 is sufficiently low (for 
example, about 1.times.10.sup.15 cm.sup.-3), the diffusion length of holes 
in the transverse direction is several tens of microns and almost all the 
holes can reach the second p-type region 11. In this way, the holes, as 
minority carriers generated in the InGaAsP layer 7, are almost all 
captured by the p-type region 11, thereby generating an electromotive 
force between electrodes 4 and 9. 
Based on the above-described operation, the signal corresponding to 
incident light of wavelength .lambda.=1.3 microns is output to the output 
terminal OUT1, and the signal corresponding to incident light of 
wavelength .lambda.=1.55 microns is output to the output terminal OUT2. 
Thereby, this element operates as a multiple wavelength light detector. 
FIG. 2 shows a structure of a semiconductor light detector according to a 
second embodiment of the present invention which can exploit the 
advantages of the above-described embodiment even more. 
In FIG. 2, the same reference numerals designate the same or corresponding 
elements as those shown in FIG. 1. An n-type InP layer 21 having a larger 
energy band gap than those of the InGaAsP layer 6 and the InGaAsP layer 7 
is disposed between layers 6 and 7. By the introduction of this layer 21, 
the barrier in the energy band structure between the InGaAsp layer 6 and 
the InGaAsP layer 7 is further increased and mutual intrusion of carriers 
generated in the InGaAsP layer 6 and in the InGaAsP layer 7 can be 
effectively prevented. 
In the above-described first and second embodiments, a semiconductor light 
detecting element for detecting two wavelengths of light is described. The 
present invention, however, is not restricted thereto, and a structure for 
detecting one wavelength of light mixed in a plurality of wavelengths can 
also be provided. 
FIG. 3 shows an element for detecting three wavelengths. In the embodiment 
of FIG. 3, the same reference numerals designate the same or corresponding 
elements as those shown in FIG. 1. Reference numeral 22 designates 
incident light having a wavelength longer than 1.55 microns. Reference 
numeral 25 designates an n-type In.sub.x",Ga.sub.1-x",As.sub.y", 
P.sub.1-y" layer having a smaller energy band gap than the n-type 
In.sub.x,Ga.sub.1-x,As.sub.y,P.sub.1-y layer 7. Reference numeral 24 
designates a p-type region that reaches the n-type 
In.sub.x",Ga.sub.1-x",As.sub.y",P.sub.1-y", layer 25 from the surface of 
the structure. Reference numeral 23 designates a p side electrode disposed 
at the surface of the p-type region 24. The operation of this construction 
is the same as that of the above-described embodiment. The n side 
electrode 9 of the light detector is grounded, the p-type electrodes 3, 4, 
and 23 are biased to negative voltages, and the depletion layer extending 
from the first p-type region 10 is limited to the InGaAsP layer 6 by 
adjusting the external bias. In this state, when incident light comprising 
three wavelengths of light is considered, the light of wavelength 
.lambda.=1.3 microns transits the n-type InP layer and is absorbed by the 
InGaAsP layer 6, thereby producing charge carriers and generating an 
electromotive force between the first p side electrode 3 and the n side 
electrode 9. Furthermore, the light of wavelength .lambda.=1.55 microns 
transits the InP layer 5 and the InGaAsP layer 6, is absorbed by the 
InGaAsP layer 7, and the generated holes reach the second p-type region 11 
by diffusion, thereby generating an electromotive force between the 
electrodes 4 and 9. The light having wavelengths longer than the 1.55 
microns transits the InP window layer 5, the InGaAsP layer 6, and the 
InGaAsP layer 7 and is absorbed by the InGaAsP layer 25. The holes 
generated there reach the third p-type region 23 by diffusion without 
reaching the upper InGaAsP layer 7, because they are obstructed by the 
heterojunction barrier at the interface of the InGaAsP layer 7 and the 
InGaAsP layer 25, thereby generating an electromotive force only between 
the electrodes 23 and 9. 
When light comprising three wavelengths is incident as described above, 
first, second, and third n-type InGaAsP layers are successively arranged 
so that the larger energy band gap layers are positioned closer to the 
light incident surface and separate p-type regions reach the third, 
second, and first semiconductor layers from the surface, spaced at 
constant intervals. The respective light wavelengths are absorbed and 
detected by the first, second, and third n-type InGaAsP layers 
successively in the order of increasing wavelength. Therefore, even when a 
plurality of wavelengths are present in incident light, the respective 
light wavelengths can be easily detected. Moreover, the production of the 
detector is simplified because the detector structure is planar. 
In the semiconductor light detectors according to the first and third 
embodiments, the materials are InGaAsP mixed crystals. However, the 
present invention can be applied to other materials used in a light 
detector, such as AlGaAs, AlGaSb, and HgCdTe. 
As is evident from the foregoing description, according to the present 
invention, a multiple wavelength light detector includes a plurality of 
first conductivity type semiconductor layers having different energy band 
gaps arranged from a light incident surface in decreasing energy band gap 
order. A first second conductivity type region is disposed at the surface 
reaching the shallowest first conductivity type semiconductor layer, and 
one or more second conductivity type regions are disposed successively 
surrounding the first second conductivity type region at intervals. 
Therefore, respective light wavelengths can be easily detected from light 
including a plurality of wavelengths. Matching of the detector with the 
other elements is improved because the detector is planar. The production 
of detectors is also simplified. Furthermore, since carrier crosstalk 
between the respective layers is suppressed by a heterojunction barrier 
between layers, a highly reliable multiple wavelength light detector is 
obtained.