Photoelectric converting device

Abstract: A photoelectric converting device comprises a first semiconductor area of a first conductive type, a second semiconductor area of a second conductive type, and a third semiconductor area of the first conductive type. A charge is photoelectrically excited by light incident on second semiconductor area, and is derived from the first semiconductor area after amplification. A fourth semiconductor area of the first conductive type is formed in contact with the second semiconductor area and so positioned as to oppose to the third semiconductor area. During an operation of the device, a depletion layer extending from the interface between the second and fourth semiconductor areas reaches a depletion layer extending from the interface between the third and second semiconductor areas.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a photoelectric converting device. Related Background Art In the prior art of the photoelectric converting device, there is for example known a device of bipolar transistor type. Fig. 8 shows, in a schematic cross-sectional view, an example of the known bipolar photoelectric converting device, in which there are provided a silicon substrate 1 of n or p type by doping of an impurity such as phosphorus (P), antimony (Sb) or arsenic (As); an n + area 2 formed for example by an epitaxial process; an n - area 3 of lower impurity concentration serving as a collector region; a p-area 4 serving as a light receiving base area for accumulating photogenerated carriers; an n + -area 6 serving as an emitter; a p + area 7 constituting the drain of an MOS transistor; and an n + -area 8 serving as a channel stop or connected with the collector. However, such known photoelectric converting device has been associated with a drawback that the photosensitivity thereof is not always sufficient, and such drawback is more conspicuous in a miniaturised photoelectric converting device. In the known bipolar photoelectric converting device, the photosensitivity can be approximately represented as follows: Se = i p · A s · t s C bc wherein i p is the photoinduced current density per unit area; A s is the aperture area; t s is the accumulation time; and C bc is the base-collector capacitance. As will be apparent from the equation (1), the photosensitivity Se decreases with the increase of the base-collector capacitance C bc . US-A-4649409 shows a photoelectric transducer element comprising a bipolar transistor in which carriers are accumulated in the base. An additional region, of the same conductivity type as the collector and the emitter, is formed between the base and the light-receiving surface, and is electrically floating. The additional region is intended to increase switching speed by absorbing photo-generated carriers when the device is shielded from light. SUMMARY OF THE INVENTION According to the present invention there is provided a photoelectric conversion device as set out in claim 1 or claim 2 and a method of operating the device as set out in claim 11 or claim 12. The remaining claims set out optional features. The meeting of the depletion layers tends to reduce the base-collector capacitance C bc in the above-explained equation (1), thereby improving the photosensitivity of the photoelectric converting device. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a schematic plan view of a first embodiment of the photoelectric converting device of the present invention; Fig. 1(b) is a schematic cross-sectional view of said first embodiment; Fig. 2(a) is a magnified view of a portion B-B' in Fig. 1(b); Fig. 2(b) is a potential chart in the direction of depth of Fig. 2(a); Fig. 3 is a chart showing an example of impurity distribution in the areas shown in Figs. 2(a) and 2(b); Fig. 4 is a chart showing the relation between the carrier life time and the diffusion length of positive holes in the n-type area; Fig. 5 is a chart showing the relation of V R and W in silicon, with N A as parameter. Fig. 6(a) is a schematic plan view of a second embodiment of the photoelectric converting device of the present invention; Fig. 6(b) is a schematic cross-sectional view of said second embodiment; Fig. 7(a) is a schematic plan view of a third embodiment of the photoelectric converting device of the present invention, Fig. 7(b) is a schematic cross-sectional view of said third embodiment; and Fig. 8 is a schematic cross-sectional view of an example of the known bipolar photoelectric converting device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, the further semiconductor region is present and is provided in such a manner that, in combination with the biassing means, the depletion layer extending from the interface between the base region and the further semiconductor region reaches the depletion layer extending from the interface between the collector region and the base region, whereby the capacitance of the base area can be diminished to improve the photosensitivity of the device. In a preferred embodiment, the loss in sensitivity to blue light, resulting from the presence of said further semiconductor layer, can be reduced by selecting the thickness of said further semiconductor layer in such a manner that said thickness is smaller than the penetration length of blue light and is equal to or less than 1/2 of the diffusion length of minority carriers in said further semiconductor layer. Also in a preferred embodiment, the presence of the area of the second conductivity type of a high impurity concentration allows to determine the impurity concentration of said base region without affecting the characteristics of the bipolar transistor. The preferred embodiments will now be described with reference to the accompanying drawings. [First embodiment] Figs. 1(a) and 1(b) schematically illustrate a first embodiment of the photoelectric converting device of the present invention, respectively in a plan view of a photoelectric converting cell, and a vertical cross-sectional view along a line A-A' in Fig. 1(a). As shown in Figs. 1(a) and 1(b), said photoelectric converting cell comprises: a substrate 1 formed as n-type by doping of an impurity such as phosphorus (P), antimony (Sb) or arsenic (As), or as p type by doping of an impurity such as boron (B) or aluminum (Al); an embedded n + -area 2 formed on said substrate 1; an n - -area 3 of a low impurity concentration, formed for example by an epitaxial process on the embedded area 2 and serving as a collector area; a p - -area 4 doped with an impurity such as boron (B), serving as a light-receiving area and a base area, and formed on the n - -area 3 for example by impurity diffusion, ion implantation or epitaxial growth; an n + -area 5 formed at the surface of the p - -area 4; an n + -area 6 constituting the emitter of a bipolar transistor; a p + -area 7 constituting the drain of a MOS transistor; an n + -area 8 serving as a channel stop and also connected to the collector; an n + -area 9 for reducing the collector resistance of the bipolar transistor; an electrode 101 composed of polysilicon or metal and serving as the gate of the MOS transistor; a wiring 108 connected to said electrode 101; wirings 102, 103, 104 composed of polysilicon or metal and connected to the emitter of the bipolar transistor; a wiring 109 connected to the drain of the MOS transistor; an electrode 110 connected to the n + -area 9; and insulation films 105, 106, 107 for separating the electrodes, wirings and device elements. For the purpose of simplicity, the insulation films 105, 106, 107 and the wiring 104 are omitted in Fig. 1(a). As is shown in Fig. 1(b), an intervening n + -area joins the n +- area 5 to the collector n + -area 3. Biassing means is provided to bias the n + -area 5 and the collector n - -area 3 so that a depletion layer formed in the base area 4 due to the n + -area 5 meets a depletion layer formed in the base area 4 due to the collector area 3. Fig. 2(a) is a magnified view of a portion B-B' in Fig. 1(b), and Fig. 2(b) is a potential chart in the direction of depth of Fig. 2(a). In the conventional photoelectric converting device, the depletion layer does not completely cover the base area 4, because of the absence of the n + -area 5 and because the impurity concentrations at the pn junction between the collector (n - ) area 3 and the base (p) area 4 have a relation |p| > |n|. Consequently the surface of the base area 4 is not depleted but contains a neutral area. In practice the complete depletion to the surface is difficult to realize if the area 4 is used as the base. The complete depletion to the surface, if achieved by a special structure, induces carrier generation at the interface with an insulating material, thus increasing dark current which is a major drawback in the photoelectric converting device. In the present embodiment, a surface-side n + -area 5 is provided to form an n + p junction, thereby extending the depletion layer also from the surface side, and thus depleting the p-area completely in the direction of depth thereof. Fig. 3 is a chart showing an example of impurity distribution in the areas shown in Figs. 2(a) and 2(b). As the p-area 4 is formed by ion implantation followed by thermal treatment or thermal diffusion, the impurity concentration is highest at the surface and gradually decreases with the depth. Consequently, the depletion layer extending from the n - -area enters the p-area 4 but cannot easily reach the upper part of the p-area 4 because the extension has to be done in the increasing direction of impurity concentration. On the other hand, the depletion layer extending from the n + -area 5 can easily reach the pn - junction because the extension is in the decreasing direction of impurity concentration, so that the p-area 4 can be readily and completely depleted in the entire depth thereof. This phenomenon can explain the fact that the emitter-collector breakdown voltage is high in a normal bipolar transistor but is low in an inverted bipolar transistor in which the emitter and the collector are inverted. The expansion of the depletion layer from the side of n + -area is easy, and complete depletion of the p-area can be easily achieved with a voltage not exceeding 5 V. The depletion of the base area 4 under the n + -area 5 drastically reduces the base-collector electrostatic capacitance C bc . Although the entire base area 4 contributes to the capacitance C bc in the conventional structure, the presence of the n + -area 5 excludes the capacitance of the p-area 4 corresponding to the area of said n + -area 5 from the capacitance C bc , so that said capacitance C bc is almost solely governed by the capacitance around the emitter (see Fig. 1(b)). Said capacitance C bc can therefore be easily reduced to 1/2 - 1/10 of the conventional value, depending on the pattern design. Besides the dark current decreases in comparison with that in the conventional structure, because the photogenerated positive holes are not accumulated on the semiconductor surface but in the body of the semiconductor material and accumulated by drifting in C bc and C be around the emitter as the result of potential distribution shown in Fig. 2(b), and also because the presence of the n + -area at the semiconductor surface avoids the carrier formation at the interface with the insulating material. However, the depletion layer extending from the n + -area has to reach the depletion layer extending from the pn - junction, since, otherwise, the base-collector capacitance becomes larger than in the conventional structure (almost doubled in area). In the following there will be explained the spectral sensitivity of the photoelectric converting device of the present embodiment. Said spectral photosensitivity can be approximately represented as follows: S (λ) = λ 1.24 exp (-α · x d ) × {1- exp(- α · W) ·T} [A/W] wherein λ is the wavelength of light; α is the absorption coefficient of light (cm -1 ); x d is thickness of the insensitive area (n + -area 5); W is width of the high sensitivity area (depletion layer); and T is the proportion of light entering the semiconductor (transmittance). As will be understood from the equation (2), the thickness x d should preferably be as small as possible, because the spectral sensitivity is strongly affected by x d . In case of silicon, the absorption coefficients for blue (λ = 0.45 µm), green (λ = 0.53 µm) and red (λ = 0.65 µm) are respectively about 2 x 10 4 cm -1 , 7.5 x 10 3 cm -1 and 3 x 10 3 cm -1 . In consideration of a half-peak width of about 0.05 µm for each color, the light absorption in silicon takes place sufficiently at a depth of about 1 µm for blue color, 2 µm for green color and 5 µm for red color. Consequently the blue light is most strongly influenced by the thickness x d of the neutral area at the semiconductor surface, so that the sensitivity for blue color is deteriorated. However, even the n + -area is not without light absorption. Nevertheless, the probability of transition is modified to a certain extent because free carriers are already present in the conduction band, and the efficiency is lowered to a certain extent because the minority carriers generated by light absorption partly recombine with the majority carriers in the course of diffusion. Fig. 4 shows the result of calculation on the relation between the carrier life time and the diffusion length for the positive holes in the n-area. For example, for N = 10 19 cm -3 , there is obtained L p of about 5 µm, which is considerably long for the diffusion length. Therefore, since the penetration distance (1/α) for the blue light in question is about 0.5 µm, the difficulty can be mostly prevented by selecting the thickness of said n + -area 5 less than said diffusion distance and not exceeding 1/2 of L p , namely so as to satisfy a condition: x d < ( 1 α ) (Blue) , 1 2 L p (n + ) Besides the problem of sensitivity to blue light is alleviated since the photosensitivity is generally improved in comparison with that in the conventional photoelectric converting device, as explained above. The width of the depletion layer at the n + p junction can be represented in the following manner, in a step junction approximation: W = { 2 · ε s q · N A +N D N A ·N D · (V bi +V R )} ½ V Bi = kT q · 1n N A ·N D n i 2 wherein W is the width of depletion layer; N A is the impurity density of p -area; N D is the impurity density of n + -area; ε s is the dielectric constant; n i is the true carrier density; and V r is the reverse bias voltage. Since N D >> N A in an n + p junction, the above-mentioned equation (4) can be approximated as follows: W ≃ { 2 · ε s q · 1 N A · (V bi +V R )} ½ Fig. 5 is a chart showing the relation between V R and W in silicon, with N A as a parameter. In the ordinary photoelectric converting device in which 5 V is applied to the collector, the effect of the present embodiment can be obtained by selecting W, corresponding to N A for V R of about 3 V, larger than the thickness of the p-area 4. Stated differently, the thickness of the base area should be selected smaller than W corresponding to the base impurity concentration N A , for a given value of V R . Or, the thickness x d of the p-area 4 positioned between the n + -area 5 and the n - -area 3 should be selected smaller than W determined by N A and V R . [Second embodiment] Figs. 6(a) and 6(b) illustrate a second embodiment of the photoelectric converting device of the present invention, respectively in a plan view and in a vertical cross-sectional view along a line A-A' in Fig. 6(a). Different from the device of the foregoing first embodiment, the device of the present embodiment is provided with a p + -area 20 only below the emitter area 6. The presence of said p + -area 20 provides an advantage that the impurity concentration of the p-area can be arbitrarily determined, without being influenced by the characteristics of the bipolar transistor. Such photoelectric converting device showed, in an operation test, a remarkable improvement in the photosensitivity, in comparison with the prior art device. [Third Embodiment] Figs. 7(a) and 7(b) illustrate a third embodiment of the photoelectric converting device of the present invention, respectively in a plan view of a photoelectric converting cell and in a vertical cross-sectional view along a line A-A' in Fig. 7(a). In the structure shown in Figs 7(a) and 7(b), the lead wire from the emitter electrode 103 is taken out directly, without the polysilicon layer 102. Also a barrier metal or the like may be made present between the layers 103 and 6. Such photoelectric converting device showed, in an operation test, a remarkably improved photosensitivity, in comparison with the prior art device. The foregoing first to third embodiments have been limited to the application of the present invention to a line sensor, but similar effect can naturally be obtained in the applications to other photoelectric conversion devices, for example area sensors. As explained in detail above, the present embodiments of the invention allow the base-collector capacitance C bc of the bipolar transistor to be drastically reduced, thereby remarkably improving the photosensitivity of the photoelectric converting device. Also in the illustrated embodiments the presence of a surface-side n + -area of high impurity concentration reduces the current generated at the interface between SiO 2 and Si, whereby the dark current is reduced to decrease noise and to significantly improve the S/N ratio. Also the dynamic range is widened so that the signal can be obtained down to a low irradiation level. The absorption of blue light is maintained by making the n + layer 5 to be thin. Blue light typically has, for example, a wavelength no greater than 530 nm, and preferably light absorption is also made good for light of wavelength 450 nm or even less.

Claims: A photoelectric conversion device in combination with biassing means, the device comprising: an emitter region (6) of a first conductivity type; a base region (4) of a second conductivity type; and a collector region (3) of the first conductivity type, charges entering or being retained in the base region (4) in response to light energy received through a light-receiving surface of the device, the device further comprising a further semiconductor region (5) of the first conductivity type formed at a light-receiving surface of the base region (4) and spaced from the emitter region (6), and the base region (4), the collector region (3) and the further region (5) being arranged in a vertical structure so that the further region (5) is between the base region (4) and the said light-receiving surface of the device, and the base region (4) is between the collector region (3) and the further region (5), the biassing means being arranged to bias the collector region (3) and the further region (5) so that a depletion layer formed in the base region (4) due to the collector region (3) meets a depletion layer formed in the base region (4) due to the further region (5). A photoelectric conversion device in combination with biassing means, the device comprising: an emitter region (6) of a first conductivity type; a base region (4) of a second conductivity type; and a collector region (3) of the first conductivity type, charges entering or being retained in the region (4) in response to light energy received through a light-receiving surface of the device, the device further comprising a further semiconductor region (5) of the first conductivity type formed at a light-receiving surface of the base region (4) and spaced from the emitter region (6), and the base region (4), the collector region (3) and the further region (5) being arranged in a vertical structure so that the further region (5) is between the base region (4) and the said light-receiving surface of the device, and the base region (4) is between the collector region (3) and the further region (5), the collector region (3) and the further region (5) being electrically connected to each other through a semiconductor connecting region of the first conductivity type, the connecting region being more heavily doped than the collector region (3), and the biassing means being arranged to bias the collector region (3) and the further region (5) so that a depletion layer formed in the base region (4) due to the collector region (3) meets a depletion layer formed in the base region (4) due to the further region (5). A photoelectric conversion device according to claim 1 or claim 2 in which the said depletion layers meet if a reverse bias of 3 volts relative to the base region is applied to the collector region (3) and the further region (5). A photoelectric conversion device according to any one of the preceding claims, wherein the thickness of said further semiconductor region (5) is smaller than the penetration distance of blue light and is equal to or less than the diffusion length of minority carriers in said further semiconductor region (5). A photoelectric conversion device according to claim 4 wherein the thickness of the further semiconductor region (5) is equal to or less than half the diffusion length of minority carriers in the further semiconductor region (5). A photoelectric conversion device according to any one of claims 1 to 3 wherein the thickness of the further semiconductor region (5) is smaller than the penetration distance of light of a wavelength not greater than 530nm. A photoelectric conversion device according to any one of the claims 1 to 3 wherein the thickness of the further semiconductor region (5) is smaller than the penetration distance of light of a wavelength not greater than 450nm. A photoelectric conversion device according to any one of the preceding claims, comprising a second base semiconductor region (20) of the second conductivity type, formed on said base region (4) and having a higher impurity concentration than said base region (4), wherein said emitter region (6) is formed on said second base semiconductor region (20). A photoelectric conversion device according to any one of the preceding claims comprising an electrode (103, 104) connected to the emitter region (6) with a metal-semiconductor junction. A photoelectric conversion device according to any one of the preceding claims comprising an additional semiconductor region (7) of the second conductivity type and a gate (101), forming a MOS transistor with a region extending from the base region (4). A method of operating a photoelectric conversion device, wherein the device comprises: an emitter region (6) of a first conductivity type; a base region (4) of a second conductivity type; and a collector region (3) of the first conductivity type, charges entering or being retained in the base region (4) in response to light energy received through a light-receiving surface of the device, the device further comprising a further semiconductor region (5) of the first conductivity type formed at a light-receiving surface of the base region (4) and spaced from the emitter region (6), and the base region (4), the collector region (3) and the further region (5) being arranged in a vertical structure so that the further region (5) is between the base region (4) and the said light-receiving surface of the device, and the base region (4) is between the collector region (3) and the further region (5), and the method comprises biassing the collector region (3) and the further region (5) so that a depletion layer formed in the base region (4) due to the collector region (3) meets a depletion layer formed in the base region (4) due to the further region (5). A method of operating a photoelectric conversion device, wherein the device comprises: an emitter region (6) of a first conductivity type; a base region (4) of a second conductivity type; and a collector region (3) of the first conductivity type, charges entering or being retained in the region (4) in response to light energy received through a light-receiving surface of the device, the device further comprising a further semiconductor region (5) of the first conductivity type formed at a light-receiving surface of the base region (4) and spaced from the emitter region (6), and the base region (4), the collector region (3) and the further region (5) being arranged in a vertical structure so that the further region (5) is between the base region (4) and the said light-receiving surface of the device, and the base region (4) is between the collector region (3) and the further region (5), the collector region (3) and the further region (5) being electrically connected to each other through a semiconductor connecting region of the first conductivity type, the connecting region being more heavily doped than the collector region (3), and the method comprises biassing the collector region (3) and the further region (5) so that a depletion layer formed in the base region (4) due to the collector region (3) meets a depletion layer formed in the base region (4) due to the further region (5). A method according to claim 11 or claim 12 which comprises biassing the collector region (3) and the further region (5) with a reverse bias voltage, relative to the base region (4), of about 3 volts.

Inventors: MORISHITA MASAKAZU, MORISHITA, MASAKAZU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H01L31
IPC codes: H01L31, H01L21, H01L29