GB1597538A

GB1597538A - Photovoltaic semiconductor device having increased detectivity and decreased capacitance

Info

Publication number: GB1597538A
Application number: GB896478A
Authority: GB
Original assignee: Ford Motor Co Ltd; Ford Motor Co
Current assignee: Ford Motor Co Ltd; Ford Motor Co
Priority date: 1977-03-31
Filing date: 1978-03-07
Publication date: 1981-09-09
Also published as: DE2813671A1; JPS53123092A

Description

(54) PHOTOVOLTAIC SEMICONDUCTOR DEVICE HAVING INCREASED DETECTIVITY AND DECREASED CAPACITANCE (71) We, FORD MOTOR COMPANY LIMITED, of Eagle Way, Brentwood, Essex CM13 3BW,-a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a photovoltaic semiconductor device for detecting electromagnetic radiation having a photon energy greater than or equal to the energy bandgap value of the semiconductor material in which absorption of the radiation occurs. The semiconductor device of the invention, which in the preferred form is a diode semiconductor device, is particularly suitable as a detector of infrared radiation. Infrared detectors are used in imaging arrays and in heat-seeking devices. Other applications for such devices also are known.

A semiconductor photovoltaic detector is a device having a p-n junction and a surface which is exposed to incident electromagnetic radiation. If the incident radiation has a photon energy greater than the energy bandgap value of the semiconductor material, an electron-hole pair is generated. The photogenerated minority carrier in the semiconductor material is swept across the p-n junction and this produces a detectable response.

The conventional semiconductor detector of electromagnetic radiation is a bulk crystal device having within it a p-n junction. The p-n junction may be obtained by suitable doping techniques or by the placement of a Schottky barrier metal on the surface of the bulk crystal to induce a p-n junction within the semiconductor material in the well-known manner. Also, it is known to form photovoltaic diode semiconductor detectors of electromagnetic radiation from a transparent barium fluoride substrate on which an epitaxial thin film of a group IV-VI semiconductor material is grown. A Schottky barrier metal is deposited on the epitaxial thin film to form a Schottky barrier diode.

In the past, the utility of photovoltaic detectors for small-signal applications has been limited by the attainable signal to noise ratio (S/N). For a linear response, the performance of the various detectors may be compared with one another in terms of their respective signal to noise ratios normalized to unit incident power and unit noise bandwidth. In a wide range of detectors this normalized S/N is proportional to As112, where A is the detector area. This leads to the definition of a figure of merit, called the detectivity D*, for photovoltaic detectors: So '/2 D*- N where S and N are normalized to unit incident power and unit bandwidth, respectively. This provides a measure of quality that is independent of the device area. For a particular application, the largest S/N is obtained by using the smallest detector that is compatible with the limitations of the optical system employed to supply the incident electromagnetic radiation.

In a conventional photovoltaic detector which is operated short-circuited, the current signal S for unit incident power at a particular photon energy E is: #q S= E where 77 is the quantum efficiency In the absence of a large photon flux from the background, the current noise is the Johnson noise of the diode resistance R, N=(4kT/R)"2 in unit bandwidth. Thus,

For an ideal diode, the product RA is independent of A, and D* is an areaindependent figure of merit. The same relationship is obtained if the photovoltaic detector is considered in the open-circuit mode of operation.

As the Johnson-noise-limited detectivity D* of photovoltaic detectors is increased, the noise becomes dominated by fluctuations in the photon flux from the background. This typically occurs at 3000K with some field of view that is defined by the optics of the system in which the photovoltaic detector is employed. Beyond this point, the detectivity D* remains constant at a background-limited value.

However, many photovoltaic detectors must be cooled to attain a large enough value of the produce RA to permit background-limited performance.

Consequently, it is desirable to increase the Johnson - noise - limited detectivity D*, thereby, to increase the operating temperature at which background-limited performance is obtained.

The capacitance of photovoltaic detectors of electromagnetic radiation qualitatively may be regarded as arising from the p and n regions in the semiconductor material acting as parallel plates that are separated by a dielectric which consists of the diode depletion region. The capacitance degrades the signal-to-noise ratio at high frequencies because it interacts with preamplifiers. The preamplifier imposes a frequency limitation on the photovoltaic detector system; a critical frequency exists above which the attainable signal-to-noise ratio decreases sharply. This decrease in S/N is largely due to the contribution made by the p-n junction capacitance of the photovoltaic detector. A reduction of this capacitance thus is very desirable.

The present invention provides a semiconductor device comprising a semiconductor material having a surface area to be exposed to incident radiation to be detected, the semiconductor material having within it a p-n junction area substantially parallel to such surface area.

The p-n junction area is a network either in the sense of its being formed by strips of interconnected p-n junctions or in the sense of its consisting of a plurality of discrete p-n junctions that are electrically (ohmically) connected in parallel with one another.

thereby, to form an effective single p-n junction area. The shortest imaginary closed-path boundary that could be drawn on the aforementioned semiconductor surface area to enclose all of the p-n junction area Iying within the semiconductor material enclosed a total area on the semiconductor device that is greater than twice the p-n junction area substantially parallel to the semiconductor surface area. Any imaginary line perpendicular to the semiconductor surface area and within the imaginary closed-path boundary but outside the p-n junction area is less than 100 micrometers from at least one imaginary line perpendicular to and passing through the p-n junction area.

Moreover, the device has a barrier for confining the photogenerated minority carriers to a region within the semiconductor material on the side of the barrier nearer the p-n junction area. The barrier is located less than 50 micrometers from the p-n junction area and is measured in a direction perpendicular to the p-n junction area.

The barrier may be formed by the provision within the semiconductor material of first and second regions of the same conductivity type, but with differing majority carrier concentrations, or by the use of two semiconductor materials, one of which contains the p-n junction area and the other of which has a larger energy bandgap value than the first-mentioned semiconductor material, thereby, to produce a heterojunction between the materials. Also, the barrier may be formed by the use of an insulating substrate on which the semiconductor material is positioned, such as by epitaxial growth thereon.

In the preferred form of the invention, semiconductor devices useful as photovoltaic detectors of infrared radiation are formed as Schottky barrier diodes using lead or indium as a barrier metal deposited on an epitaxial layer of group IV-VI semiconductor material epitaxially grown on a substrate of either barium fluoride (BaF2) or strontium fluoride (SrF2).

The invention may be better understood by reference to the detailed description of several embodiments thereof which follows and to the drawings, which demonstrate the embodiments and in which: Figure la is a mesa structure photovoltaic detector having n-type strips of semiconductor material the remainder of which is p-type material; Figure lb is a similar configuration, but is of the planar type of construction wherein the n-type material is flush with the surface of the p-type material in the device; and Figure lc is a Schottky barrier photovoltaic detector of design similar to that of Figures la and lb; Figure 2a is a mesa type structure having crossed and perpendicular strips of n-type material in a semiconductor material the remainder of which is of p-type material; Figure 2b is similar to Figure 2a in configuration, but is of the planar configuration; and Figure 2c is a Schottky barrier diode similar in configuration to the structures of Figures 2a and 2b; Figure 3a is a mesa type configuration wherein discrete circular areas of n-type material, essentially in a hexagonal array, are located on the surface of a semiconductor material the remainding portions of which are of p-type conductivity; Figure 3b is similar to Figure 3a, but illustrates a planar configuration; and Figure 3c is a Schottky barrier diode similar in configuration to the devices of Figures 3a and 3b; Figures 4a, 4b and 4c are sectional views of any of the planar device structures of Figures lb, 2b, and 2c; these sectional views illustrate the various ways in which a barrier may be provided to limit the prevelant minority carriers to a region of the semiconductor device on the side of the barrier nearer the p-n junction area.

Figure 5a is a graph of semiconductor energy level versus distance from the device surface and illustrates a potential barrier formed by the provision in the semiconductor material of regions of differing majority carrier concentrations; and Figure Sb is a similar energy diagram but illustrates a potential barrier formed by the use of two semiconductor materials in the device, as shown in section in Figure 4b, the semiconductor materials having different energy bandgap values, thereby, to produce a heterojunction.

Figure 6a is a plan view of an array of photovoltaic detectors in the general geometric configuration illustrated in Figurs la, lh and lc; Figure 6b is a plan view of an array of photovoltaic detectors having the general configuration illustrated in Figures 2a, 2b and 2c; and Figure 6c is a plan view of an array of photovoltaic detectors having the general configuration illustrated in Figures 3a, 3b and 3c.

Generally, thermal-imaging systems which utilize photovoltaic detectors of infrared radiation require backgroundlimited detectors. This imposes a lower limit on the Johnson-noise-limited detectivity of the detector. This, in turn, sets an upper limit on the operating temperature for the detectors because the detector resistance is exponentially dependent on reciprocal temperature such that increases in temperature result in a decrease in resistance and a consequential decrease in detectivity D*. In prior art detectors, attempts to improve performance appear to approach limiting values; for example, typical upper temperature limits for background-limited performance for lead telluride detectors is l0l 500 K.

The underlying concept of the present invention is the increase of the detector resistance by the replacement of the conventional p-n junction with a p-n junction area that resembles a network or grid having a great deal of nonjunction area within its confines or which resembles a network or pattern of discrete p-n junction areas electrically (ohmically) connected together to form a single p-n junction area.

With such structures, the p-n junction capacitance can be kept very small and the detector resistance kept high, whereas the area component A in the detectivity equation previously given is quite properly the total network or grid area, including the nonjunction areas and, in most forms of the inventive device, also the p-n junction area exposed to the incident electromagnetic radiation to be detected. However, a further aspect of the invention involves the use of a barrier to confine the prevalant photogenerated minority carriers to a region within the semiconductor on the side of the barrier nearer the p-n junction area, thereby, to retain the efficiency of the p-n junction area in collecting photogenerated minority carriers.

With reference now to the drawings, there are shown various configurations of photovoltaic semiconductor devices for detecting electromagnetic radiation of a photon energy greater than or equal to the energy bandgap value of the semiconductor materials employed in the devices. All of the devices shown are formed from a semiconductor material the bulk of which is of p-type conductivity having p-n junctions formed by smaller volumes of n-type conductivity. The prevelant minority carriers in these devices are minority electrons. Of course, the regions of n-type conductivity have holes as the minority carrier types. It should be understood that the devices of the invention may be made with semiconductor material the bulk of which is n-type conductivity and wherein the p-n junction regions are formed by smaller volumes of p-type conductivity material. In such case, the prevelant minority carriers would be holes, rather than electrons.

Figures 1, 2 and 3 illustrate sectional pictorial views of semiconductor devices of the present invention. In Figure la the device comprises a semiconductor material 10 of p-type conductivity except for strips 12 and 14 of n-type conductivity. The strips of n-type conductivity material may be formed in the mesa structure shown for this device by the use of uniform alloying of, diffusion in, ion implantation in, or growth of epitaxy over the surface of the p-type material to produce a surface layer of ntype conductivity material. This is followed by etching of the semiconductor surface to produce the strips 12 and 14 of n-type conductivity material. Thus, p-n junctions, indicated by the broken line planes 16 and 18, are formed, respectively, beneath the strips 12 and 14 at the boundary formed between them and the p-type conductivity material. These p-n junctions are substantially parallel to the surface of the device upon which the electromagnetic radiation to be detected is incident. This surface of radiation incidents may be the upper surface, as viewed in Figure la, or may be the underside of the device if the thickness of the semiconductor material 10, that is the vertical dimension of the device as viewed in Figure la, is less than one diffusion length L of the prevelant minority carrier electrons in the device. This also applies to the other devices illustrated in the drawings.

The strips of n-type conductivity semiconductor material 12 and 14 are ohmically connected together by suitable conductive means 20. Thus, in effect, a single p-n junction area equal to the combined area of the p-n junctions 16 and 18 is formed. According to the invention, the shortest imaginary closed-path boundary which could be drawn on the surface of the semiconductor material 10 to be exposed to incident electromagnetic radiation to be detected, whether the upper surface or the underside surface, to enclose the p-n junction areas 16 and 18 also encloses a total area on that surface to be exposed to incident radiation which is greater than twice the area of the enclosed pn junction area substantially parallel to such surface. Also, within such shortest closed-path boundary enclosing the p-n junction area, any imaginary line perpendicular to the semiconductor device surface area to be exposed to radiation to be detected is less than 100 micrometers from at least one imaginary line perpendicular to the p-n junction area. In Figure la and the other Figures as well, the dimension "x" illustrates the greatest distance within the abovespecified shortest closed-path boundary from a nonjunction area to the nearest point in a junction area. This dimension must be less than 100 micrometers. The dimension x for the device of Figure la and all of the other device structures shown in the drawings should be less than two diffusion lengths L of the prevelant photogenerated minority carriers in the semiconductor material from which the device is formed. Some of the devices hereinafter described are comprised of two semiconductor materials, one of which has the p-n junction area formed within it. In such case, it is the diffusion length of the prevelant photogenerated minority carriers in the semiconductor material containing the p-n junction area that is of importance. Preferably, the dimension x is about one diffusion length L of the prevelant photogenerated minority carriers.

In each of the devices shown in Figures 1, 2 and 3, there is a barrier plane 22 parallel to the p-n junction area which itself is parallel to the surface of the semiconductor device which is to be exposed to incident electromagnetic radiation to be detected.

The barrier plane 22 functions to confine the prevelant photogenerated minority carriers to the region within the device on the side of the barrier nearer the p-n junction area. In the drawings, this barrier is located a distance "y" from the surface of the semiconductor device. The barrier plane must be located within a distance, measured in a direction perpendicular to the p-n junction area that is parallel to the surface area to be exposed to incident radiation, of less than 50 micrometers from such p-n junction area. Preferably, the barrier 22 is located less than one diffusion length L of the prevelant photogenerated minority carriers from such p-n junction area measured in a direction perpendicular to it. More specifically, it is desirable that the barrier be located appoximately onetenth of a diffusion length L from such p-n junction area.

The minority carrier diffusion length L for semiconductor materials is given by the equation

where, when the minority carriers are electrons, k is Boltzman's constant, q is the charge of an electron, T is the semiconductor absolute temperature, y is the electron mobility and T is the electron lifetime. The electron mobility z for semiconductor materials is relatively easy to determine, but the electron lifetime for many semiconductor materials is not precisely known and often is very difficult to determine experimentally. For this reason, the dimension x for the devices herein described and the location of the barrier 22 for the devices is specified as being less than 100 micrometers for the former and 50 micrometers for the latter.

The vast majority of semiconductor materials are known to have diffusion lengths L less than 50 micrometers.

However, where the diffusion length L for a particular semiconductor material is reasonably well known, the dimension x may be chosen to be equal to one diffusion length and the perpendicular distance from the p-n junction area to the barrier 22 may be chosen to be substantially less (on the order of one-tenth, of such diffusion length).

With reference now to Figure lb, there is shown a device very similar to that illustrated in Figure la except that it has a planar configuration rather than the mesa structure illustrated in Figure la. The planar structure has discrete volumes 24 and 26 of n-type conductivity material in a semiconductor material which is otherwise of p-type conductivity. The n-type material 24 and 26 is in the form of strips ohmically connected together by a conductive lead 28.

The planar upper surface of the device of Figure lh may be obtained by placing a masking material on the flat surface of a ptype semiconductor material having open areas corresponding to the dimensions of the strips 24 and 26 and then by alloying, diffusing, ion implanting or epitaxially forming n-type conductivity material in the otherwise p-type conductivity semiconductor. As a result of this planar technique, a strip of n-type material is formed within the p-type material, this strip having a width w as indicated in the Figure Ib. A p-n junction is formed at the boundary of the p-type conductivity material and the strips of n-type conductivity material. This p-n junction will have an area substantially parallel to the strip-side of the semiconductor material and perhaps also to the underlying side. Although either side of the device may be exposed to incident radiation to be detected if the device thickness is limited to less than one diffusion length L as previously described, it will be assumed that the device of Figure lb is to be exposed to incident radiation from its upper surface. In such case, the area of the p-n junction which is substantially parallel to the surface to be exposed to incident radiation is equal to the width w of each of the strips 24 and 26 multiplied by their respective lengths, and, because the strips are electrically connected in parallel, these products of width times strip length are added together to give the total p-n junction area. Again, the shortest closed path boundary that can be drawn on the radiation exposed surface to enclose the total p-n junction area encloses a total device surface area greater than twice the total p-n junction area.

With reference now to Figure lc, there is shown a Schottky barrier semiconductor device in accordance with the invention. It includes strips 30 and 32 of a suitable barrier metal in contact with the surface of a semiconductor material 34. The strips 30 and 32 are connected together ohmically by conductive means 36. The positioning of the barrier metal strips 30 and 32 on the semiconductor forms beneath each a region 38 in which carrier type conversion occurs and also a diode depletion region 40. In the Figure Ic device, the semiconductor material is of p-type conductivity and the carrier type conversion produces n-type conductivity material in the region 38. Thus, p-n junction areas are induced within the semiconductor material 34. If the incident radiation to be detected impinges on the device from the upper surface in Figure lc, then the induced p-n junction, which is formed at the boundary between the carrier type conversion area 38 and the diode depletion region 40, has the width w that is substantially parallel to the surface exposed to incident radiation and, of course, has an area equal to the width w multiplied by the length of the barrier metal strip. Since the conductive means 36 connects the p-n junction areas in parallel, the total p-n junction area substantially parallel to the surface exposed to incident radiation to equal to the sum of the products obtained by the multiplication of the width of each of the strips 30 and 32 by their respective lengths.

With reference to Figure 2a, there is shown a semiconductor device of the mesa structure, in this respect similar to the device of Figure la, but the strips of n-type conductivity material cross one another to form a connected network or grid of n-type conductivity material in a device otherwise formed of p-type conductivity material.

Beneath the n-type conductivity material, there is a network or grid of p-n junction area corresponding to the configuration of the n-type conductivity material. Although this p-n junction area has much nonjunction area within it, nevertheless it is a single p-n junction area continuous in the sense that the individual strips forming the grid of p-n junction area are in communication with one another. In the device structures of Figures la, lb, and lc, conductive means 20, 28 and 36, respectively, are required to provide parallel electrical connection of the individual strips of n-type conductivity material. In the devices of Figures 2a, 2b and 2c, such conductive means are not required because the network or grid consists of crossing and interconnected strips of n-type conductivity material.

Figure 2b is similar to the device of Figure 2a, except that a semiconductor device having a planar configuration is illustrated.

Figure 2c illustrates a semiconductor device having a barrier metal 42 of a network or grid-like configuration located on the surface of a semiconductor material 44. Induced carrier-type conversion and diode depletion regions beneath the barrier metal grid structure 42 forms a p-n junction area substantially parallel to the upper surface of the semiconductor device.

In Figure 3a, there is illustrated a semiconductor device having a mesa structure wherein the individual mesas are discrete circles of n-type conductivity material on a semiconductor material the bulk of which is of p-type conductivity. P-n junctions are formed at the boundary between the n-type conductivity mesas and the underlying p-type conductivity material.

These p-n junctions are in the form of discrete circular areas spaced from one another. The areas of n-type conductivity material are to be understood as ohmically connected to one another by means not shown in Figure 3a, thereby, to place the p-n junctions in the device in parallel with one another to form in effect a single p-n junction area substantially parallel to the surface of the device to be exposed to electromagnetic radiation to be detected.

The n-type conductivity circular mesas in Figure 3a are in a hexagonal pattern. The shortest closed-path boundary which could be drawn around the total p-n junction area (the sum of the individual p-n junction areas formed by the discrete circular areas of ntype conductivity material) enclosed a total area on the surface to be exposed to incident radiation greater than twice the total p-n junction area. Within this area enclosed by the shortest closed-path boundary the dimension x between any imaginary line perpendicular to such surface area and at least one imaginary line perpendicular to the p-n junction area substantially parallel to such surface is less than 100 micrometers and, preferably, is on the order of one diffusion length L of prevelant photogenerated minority carriers.

In Figure 3b, there is shown a planar structure similar to that shown in Figure 3a.

Figure 3c shows a Schottky barrier semiconductor device in which discrete circular areas of a barrier metal 46, connected together by conductive means not shown, are located on a semiconductor material 48. Induced p-n junctions beneath the barrier metal circles 46 are formed within the semiconductor material, these p-n junction areas being connected in parallel in effect to form a single p-n junction area substantially parallel to the upper surface of the device, which may be exposed to incident radiation.

In all of the semiconductor devices previously described, electromagnetic radiation, incident upon the device from its top side or from its underside if the thickness of the semiconductor material is less than one diffusion length, having a photon energy greater than the energy bandgap value of the semiconductor material from which the device is formed results in photogeneration of minority carriers in the semiconductor material. The minority carriers may be generated in both the n-type conductivity material and the ptype conductivity material, and the minority carrier generation is not limited to the semiconductor material directly above or below the p-n junction area. Rather, the minority carriers also may be generated in the semiconductor material between the strips, grid or circular areas of n-type conductivity material. In other words, the surface area of the semiconductor device active in photogeneration of minority carriers is not limited to those areas immediately above or below a p-n junction area. Minority carriers instead may be collected by the p-n junctions even though they are generated in a region of the semiconductor material adjacent a p-n junction area. However, to be efficient in the collection of photogenerated minority carriers in the regions of the device adjacent the p-n junction areas, the limitation on the dimension x to less than 100 micrometers, and preferably to about one diffusion length, is of considerable importance. Also, the barrier 22 which confines the photogenerated minority carriers to the region of the semiconductor device nearer the p-n junction area is of great importance for efficient collection of the photogenerated minority carriers.

There are several ways in which the barrier 22 may be formed to confine the minority carriers to the side of the barrier nearer the p-n junction area.

Figure 4a illustrates the formation of a barrier 22 as the boundary between a first region 50 and a second region 52 within the same semiconductor material. The first region 50 is of p-type conductivity, and the second region 52 is of p+ conductivity but is the same semiconductor material as used in the region 50. The boundary 22 formed between the p-type and p±type regions of the material forms a potential barrier to the minority carriers nearer the p-n junction areas formed at the boundary between regions of n-type conductivity material 54 and 56 and the p-type first region 50. In Figure 5a, there is shown a graph of energy versus distance from the device surface for a device having the structure shown in Figure 4a. The dimension y illustrates the distance from the device surface to the barrier 22. It may be seen that at the distance y from the surface there is an increase in the energy level at the upper edge of the valence band as well as an upward shift in energy level of the lower edge of the conduction band. Although the energy bandgap value remains substantially constant, there is an energy increase at the boundary at which the majority carrier concentration increases. To confine the prevelant photogenerated minority carriers to the p-type conductivity region, the shift in the conduction band edge should be greater than or equal to kT, and preferably greater than or equal to 3kT, where k is Boltzman's constant and T is absolute temperature.

In Figure 4b, there is shown a barrier 22 formed as the boundary between a first semiconductor material 58 and a second semiconductor material 60 having a larger energy bandgap value than the first semiconductor material. The first semiconductor material 58 contains p-n junction areas formed at the boundary between it and regions 62 and 64 of n-type conductivity material. Figure 5b is a graph of energy versus distance from the device surface for a device having the structure of Figure 4b. From this, it may be seen that the p-type conductivity material has an energy gap E (1) This increase should be greater than kT and preferably greater than or equal to 3kT. The energy level of the upper edge of the valence band is shown in Figure Sb as a straight line, but this need not be the case. However, the increase in the energy level of the conduction band at the barrier 22 is important since it results in confinement of the prevelant photogenerated minority carriers to the first semiconductor material having the energy bandgap value E,(1), the region of the device to the left of the barrier 22 as shown in Figure 5b.

In Figure 4c, the barrier 22 is formed by the termination of the semiconductor material at a distance y from the upper surface of the device. An insulator is placed beneath the semiconductor material to form the barrier 22. The p-type semiconductor material 66 in this structure preferably is epitaxially grown on the insulator material 68. If the insulator material 68 is transparent, as are barium fluoride (BaF2) and strontium fluoride (SrF2) previously found by the inventor to be particularly satisfactory for epitaxial growth of certain group IV-VI semiconductor materials, then the incident radiation to be detected conveniently can be incident upon the semiconductor material 66 from the underside of the device.

With reference now to Figure 6a, there is shown a plan view of an array of five photovoltaic detectors having the general configuration pictorially illustrated in Figures la, lb and lc. The five detectors 70, 72, 74, 76 and 78 are formed of strips of ntype conductivity material of Schottky barrier metal 80 on or in a p-type conductivity substrate 82. The detectors 70, 72, 74 and 76 are identical and consist of four of the strips 80 which are interconnected by conductive means 84.

The broken line 86 designates the shortest closed-path boundary which can be drawn on the surface of the detector 72 that encloses the p-n junction area underlying the strips 80 of n-type conductivity material.

The dots 88 and 90, respectively, designate a line perpendicular to the p-n junction underlying one of the strips 80 and any line perpendicular to the surface to be exposed to incident radiation and within the boundary 86. Within the closed-path boundary 86, the dimension x between the lines 88 and 90 is the greatest distance between any point in a nonjunction area and the nearest point in a junction area, is less than 100 micrometers, should be less than two diffusion lengths of the prevelant minority carriers in the semiconductor material of the device and preferably is on the order of about one such diffusion length. With respect to the detector 78, consisting of eight strips 80, the shortest closed-path boundary enclosing the p-n junction area of this device is designated by the numeral 92. Again, any line within this boundary 92 and perpendicular to the surface is less than 100 micrometers from a line perpendicular to a p-n junction underlying one of the strips 80.

With reference to Figure 6b, there is shown a plan view of an array of semiconductor devices having a network or grid pattern constructed in a manner such as illustrated in Figures 2a, 2b or 2c. Six detectors are shown in the array and one of these detectors 94 is shown having the shortest closed-path boundary 96 that could be drawn on the semiconductor surface to be exposed to incident radiation to enclose all of the p-n junction area underlying the grid 98 of n-type conductivity material on or in the p-type conductivity substrate 100.

The point 102 designates any line within the boundary 96 and perpendicular to the surface, whereas the point 104 designates the nearest imaginary line perpendicular to the p-n junction area underlying the network 98. The dimension x between these lines again is less than 100 micrometers. In an array of semiconductor devices, such as those illustrated in Figure 6b, two or more of the devices may be connected in parallel.

Thus, conductive means 106 is shown interconnecting the network or grid structure 98 of the detector 94 with the network or grid structure 108 of the second device 110.

With reference to Figure 6c, there is shown a plan view of an array of semiconductor devices having the structural configuration illustrated in Figures 3a, 3b or 3c. Circular regions 112 of n-type conductivity material or Schottky barrier metal in or on a p-type conductivity substrate are arranged in a hexagonal pattern with the discrete regions 112 being ohmically connected together by conductive means 116. In Figure 6c, eight semiconductor devices are illustrated. The device 118 has a broken line 120 drawn around it to designate the shortest closedpath boundary enclosing the p-n junction area of the device. The point 122 designates any line within the boundary 120 and perpendicular to the device surface and the point 124 indicates the nearest line perpendicular to a p-n junction area underlying one of the circular p-n junctions.

Again, the dimension x between the lines 122 and 124 is less than 100 micrometers and preferably less than two diffusion lengths and on the order of about one diffusion length.

In the preferred form of the invention, the semiconductor device is a photovoltaic diode made in the Schottky barrier metal form generally illustrated in Figure 3c. The semiconductor material is epitaxially grown to a thickness of a few micrometers on an insulating substrate of barium fluoride or strontium fluoride. The barrier metal preferably is lead or indium and the semiconductor material is selected from the group of IV-VI semiconductor materials consisting of: PbS PbSe PbTe Pb,~xSnxTe (0 < x < O.4) Pb1~xSnxSe (0 < x < 0.2) PbSe, xTex (O < x < l) PbS1~xSex (O < x < l) Pb,~,Ge,Te (0 < x < 0. 1) Pbl~xCdxTe (O < x < 0. 1) At present, it is preferred that the strips of n-type conductivity material or the diameter of the circular regions of n-type conductivity material illustrated in the drawings have a dimension of about 0.05 L where L is the diffusion length of the prevelant photogenerated minority carriers.

For the group IV-VI semiconductor materials listed above, L is about fifteen micrometers, and therefore, the dimension x in the drawings would be less than about 30 micrometers (2L) and the barrier 22 would be located less than fifteen micrometers from the p-n junction area.

With the semiconductor device configurations illustrated in Figures la, lb and lc, calculations have revealed that an increase in detectivity D* of a factor of 3.5, as compared to conventional devices, can be obtained and the capacitance can be reduced to 0.05 of that for a conventional device. Similar values are obtained for semiconductor devices having the configurations shown in Figures 2a, 2b and 2c. As to the configuration of circular areas shown in Figures 3a, 3b and 3c, calculations have shown that the detectivity D* can be increased by a factor of 9.8 over the detectivity of conventional devices. Also, the capacitance of the device can be reduced by a factor of 8x10-4 of that of the conventional device.

As an example of a group IV-VI photovoltaic semiconductor device in accordance with the invention, a Schottky barrier diode may be made from Pb on ptype epitaxial PbTe with an insulating BaF2 substrate. At the typical operating temperature of 77"K, the minority carrier diffusion length L is estimated to be of the order of fifteen micrometers. The optical absorption coefficient for three to five micrometer radiation is on the order of one micrometer so that efficient absorption is obtained with layer of the order of one micrometer thick. Junction dimensions (width, diameter, etc.) of about one micrometer (approximately 0.1 of a diffusion length L) are attainable with standard photolithographic techniques. The strip width and p-n junction circular area diameter value previously mentioned was chosen to fit this dimension. Thus, depending upon the geometry of the device constructed in accordance with the invention, the Johnson-noise-limited detectivity D* of PbTe photovoltaic detectors may be increased by a factor of approximately three to ten and their capacitance may be reduced by a factor of approximately 20 to 1000.

In terms of the application of the invention to photovoltaic detectors of infrared radiation employed in thermal imaging systems, the best projected performance for a conventional diode array with 180 field of view and at 1700K is about one-third of the background-limited D*.

The semiconductor device of the invention is estimated to give fully backgroundlimited performance at about 1900K for three to five micrometer systems. The cooling power required for operation of the detectors of the invention is estimated to be one-fifth of the cooling power required for the conventional diode array.

In addition to the group IV-VI semiconductor materials described above, the following group Ill-V semiconductor compounds are considered suitable for the device: GaAs, InAs, GaSb and InSb.

Moreover, the following group Ill-V pseudobinary alloys are believed suitable as the semiconductor material used in the device: Gal~xInxAs (0 < x < 1), GaAsl~xSbx (0 < xsl) and InAsl XSbX (0 < x < 1). Also, Hg1 Cd Te (0 < x < 0.4) is believed to be a suitable semiconductor material for use in the device of the invention if the Cd content is large enough to give a positive energy gap.

As an example of a group Ill-V semiconductor device, consideration may be given to InSb as the semiconductor material. The 77"K energy gap is 0.228 eV, which corresponds to a cutoff wavelength of approximately 5.4 micrometers. Thus, InSb is suitable for detection of radiation in the three to five micrometer atmospheric transmission band. At the typical operating temperature of 77"K, electron life times in p-type material are about 4x 10-10 sec. (R.

A. Laff and H. Y. Fan, Phys. Rev., Volume 121, page 53, 1961).

Electron mobilities at 770K range from about 105cm V-l seen' for material with impurity concentrations of 1016 to 1017 cm-3 to about lO6cm2V-1sec-1 for the purest crystals (S. G. Parker, J. Electrochem. soc., Volume 112, page 80, 1965). This gives diffusion lengths for minority electrons in the range from five to sixteen micrometers at 77"K. For such a diffusion length, of the order of ten micrometers, the improvements of the invention may be obtained by diffusing n-type dopants into ptype material to give an array of circular junction areas that are approximately one micrometer in diameter and approximately ten micrometers apart. These circular junction areas, of course, would be ohmically connected in parallel and the confining barrier required for the device of the invention may be achieved by increasing the acceptor concentration, for example, from lO17cm-3 to 1018cm-3, for the InSb that is more than approximately one micrometer below the surface of the semiconductor device.

With respect to Hg,,Cd,Te for use as a material in the device of the invention, the energy gap for this material is given by D.

Long and J. L. Schmit, "Semiconductors and Semimetals", Volume 5, page 175, edited by R. K. Willardson and A. C. Beer, Academic Press, New York, 1970, as: -0.25+ 1 1.59x+0.327x3+5.233 x 104T (12.08x). This permits selection of the cutoff wavelength for the spectral region of interest with a particular operating temperature. Two energy gaps of special interest are approximately 0.24 eV and 0.09 eV, which correspond to cut-off wavelengths of five micrometers and fourteen micrometers, respectively, for detectors that would operate in the three to five micrometer and eight to fourteen micrometer atmospheric windows. The design considerations for use of Hg1,Cd,Te do not change greatly over the approximate composition range 0.l < x < 0.4. For illustration, the energy gap of 0.09 eV and an operating temperature of 77 K is chosen.

In this case, x is approximately equal to 0.20.

The lifetime of Hg,~,Cd,Te has been shown to be limited by radiative recombination and to depend upon the composition and the carrier concentration.

For p-type material at 770K with x=0.2 and p=lO17cm-3, the minority carrier lifetime is approximately equal to 2x lO-8sec, which, together with an electron mobility of approximately 3x 105cm2V-1sec-1, permits the minority carrier diffusion length L to be calculated. using the above numbers, the diffusion length L is approximately 60 micrometers.

Confinement of the photogenerated minority carriers to a region with thickness much less than the diffusion length may be achieved by creating a potential barrier a few micrometers below the surface. For effective confinement, this barrier should correspond to a potential energy of at least kT and an energy of at least 3kT is preferred. This barrier may be achieved by providing a region of Hg1,Cd1Te wherein x is equal to 0.2 and the acceptor concentration of p-type material is 1017cm-3.

A second region of the semiconductor material Hg,~,Cd,Te and a barrier is achieved by increasing the carrier concentration of the material to between 1018 and 1019cm-3. Also, a barrier may be obtained by grading the composition of the semiconductor material toward a larger Cd content so that it has a larger energy gap Eg.

For a 3kT barrier at 770K, the potential energy difference for electrons needs to be about 0.02eV. Thus, the barrier may be achieved by making the surface p-type with x=0.20 (Eg=0.09) and the underlying material similarly p-type, but with x approximately equal to 0.21 corresponding to E8 approximately equal to 0.11. Such a structure may be achieved using the vapor transport method described by G. Cohen Solal, Y. Marfaing, F. Bailly and M. Rodot, Compt. Rend., Volume 261, page 931, 1965.

The improvements described above may then be made by diffusing into the material a dopant to give an array of ohmicallyconnected six micrometer diameter n-type regions that are approximately 100 micrometers apart in the p-type Hg11Cd1Te.

The various geometric configurations illustrated in the drawings are illustrative of the principles involved in the application of the present invention. Various other configurations may be utilized.

WHAT WE CLAIM IS: 1. A photovoltaic semiconductor device for detecting electromagnetic radiation having a photon energy greater than or equal to the energy bandgap value of the semiconductor material in which absorption of said radiation occurs, said device comprising a semiconductor material for absorption of said radiation, said semiconductor material having a surface area to be exposed to incident radiation to be detected, said semiconductor material having within it a p-n junction area substantially parallel to said semiconductor surface area, said p-n junction area being a network, either in the sense of being formed by strips of interconnected p-n junctions or in the sense of consisting of a plurality of discrete p-n junctions electrically connected in parallel with one another, the shortest imaginary closed-path boundary that can be drawn on said semiconductor surface area to enclose all of said p-n junction area enclosing a total area on said semiconductor device that is greater than twice said p-n junction area substantially parallel to said semiconductor surface area, any imaginary line perpendicular to said semiconductor surface area and within said closed-path boundary but outside the p-n junction area being less than 100 micrometers from at least one imaginary line perpendicular to and passing through said p-n junction area, and said device having a barrier for confining photogenerated minority carriers to a region within said semiconductor material on the side of said barrier nearer said p-n junction area, said barrier being located less than 50 micrometers from said p-n junction area measured in a direction perpendicular to said p-n junction area.

2. A device as in Claim 1, said device further comprising a substrate made from an insulating material, said semiconductor material being located on said insulating material, the boundary thus formed between said insulating material and said semiconductor material forming said barrier.

3. A device according to claim 2, wherein the substrate is composed of BaF2 or SrF2.

4. A device as in Claim 1 wherein said semiconductor material has first and second regions of the same conductivity type, said second region having a larger majority carrier concentration than said first region, the boundary between said first and second regions defining said barrier, the prevelant photogenerated minority carriers in said semiconductor material being confined to said first region.

5. A device as in Claim 1, said device further comprising a second semiconductor material on which said first-mentioned semiconductor material is positioned, said second semiconductor material having a larger bandgap energy, between its valence and conduction bands, than the corresponding bandgap energy of said firstmentioned semiconductor material, said barrier being the heterojunction formed between said first-mentioned semiconductor material and said second semiconductor material.

6. A device as in Claim 1, wherein said barrier is a potential energy barrier having a magnitude greater than or equal to kT where k is Boltzman's constant and T is the absolute temperature of said semiconductor material.

7. A device as in any one of Claims 1 to 6, wherein said p-n junction area is defined by a mesa structure formed on said semiconductor material by selective removal of said semiconductor material.

8. A device as in any one of Claims 1 to 6, wherein said semiconductor material has a planar surface, said planar surface having discrete volumes of majority carrier concentration opposite in conductivity type to the majority carrier concentration in the remainder of said semiconductor material.

9. A device as in any one of Claims 1 to 6, which further comprises a Schottky barrier metal placed on a surface of said semiconductor material, said p-n junction area resulting from the presence of said Schottky barrier metal.

10. A device according to any one of Claims 1 to 9 wherein said semiconductor material is a group IV-VI semiconductor material selected from: PbS, PbSe, PbTe, Pb1~xSNxTe (wherein 0 < x < 0.4) Pbl~xSNxSe (wherein O < x < 0.2) PbSe,~xTex (wherein 0 < x < 1), PbS, xSex (wherein 0 < x < l), Pbt~xGexTe (wherein 0 < x < 0.1), and Pbt~xCdxTe (wherein Osx < 0.1).

11. A semiconductor according to any one of Claims 1 to 10 wherein the spacing beneath said imaginary lines is less than 30 micrometers and said barrier is less than 15 micrometers from said p-n junction area measured in a direction perpendicular to said p-n junction area.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. regions that are approximately 100 micrometers apart in the p-type Hg11Cd1Te. The various geometric configurations illustrated in the drawings are illustrative of the principles involved in the application of the present invention. Various other configurations may be utilized. WHAT WE CLAIM IS:

1. A photovoltaic semiconductor device for detecting electromagnetic radiation having a photon energy greater than or equal to the energy bandgap value of the semiconductor material in which absorption of said radiation occurs, said device comprising a semiconductor material for absorption of said radiation, said semiconductor material having a surface area to be exposed to incident radiation to be detected, said semiconductor material having within it a p-n junction area substantially parallel to said semiconductor surface area, said p-n junction area being a network, either in the sense of being formed by strips of interconnected p-n junctions or in the sense of consisting of a plurality of discrete p-n junctions electrically connected in parallel with one another, the shortest imaginary closed-path boundary that can be drawn on said semiconductor surface area to enclose all of said p-n junction area enclosing a total area on said semiconductor device that is greater than twice said p-n junction area substantially parallel to said semiconductor surface area, any imaginary line perpendicular to said semiconductor surface area and within said closed-path boundary but outside the p-n junction area being less than 100 micrometers from at least one imaginary line perpendicular to and passing through said p-n junction area, and said device having a barrier for confining photogenerated minority carriers to a region within said semiconductor material on the side of said barrier nearer said p-n junction area, said barrier being located less than 50 micrometers from said p-n junction area measured in a direction perpendicular to said p-n junction area.

12. A photovoltaic semiconductor device

substantially as described with reference to any one of the accompanying drawings.