METHOD AND STRUCTURE FOR MULTI-CELL DEVICES WITHOUT PHYSICAL ISOLATION

Abstract

The present invention relates to multi-cell devices fabricated on a common substrate that are more desirable than single cell devices, particularly in photovoltaic applications. Multi-cell devices operate with lower currents, higher output voltages, and lower internal power losses. Prior art multi-cell devices use physical isolation to achieve electrical isolation between cells. In order to fabricate a multicell device on a common substrate, the individual cells must be electrically isolated from one another. In the prior art, isolation generally required creating a physical dielectric barrier between the cells, which adds complexity and cost to the fabrication process. The disclosed invention achieves electrical isolation without physical isolation by proper orientation of interdigitated junctions such that the diffusion fields present in the interdigitated region essentially prevent the formation of a significant parasitic current which would be in opposition to the output of the device.

Claims

1-19. (canceled)

20. A multi-cell device providing electrical isolation of cells, comprising: a monolithic substrate; ohmic contacts on the substrate for external connection so that a multi-cell current flows between the ohmic contacts; a plurality of cells fabricated adjacently on the substrate, each cell of the plurality of cells includes a P-N junction creating a diffusion field between P-type material and N-type material, wherein the P-type material and the N-type material are interdigitated; and a bus structure on the substrate to carry the multi-cell current flow, the bus structure extending from a first side of one of the plurality of cells to an adjacent side of another of the plurality of cells, wherein the P-N junction of each cell is oriented such that the diffusion field within each P-N junction is in a perpendicular orientation to the multi-cell current flow resulting in electrical isolation between the cells.

21. The device of claim 20 wherein the P-type material and the N-type material are rectangular and a spacing between a P+ region and a N+ region of the P-N junction of each of the plurality of cells is less than a length of each of the plurality of cells and formed so that the diffusion field is strong enough to impact majority carriers.

22. The device of claim 20 wherein the P-N junctions of the plurality of cells are connected in parallel.

23. The device of claim 20 wherein a spacing between a P+ region and a N+ region of the P-N junction of each of the plurality of cells is less than a width of each of the plurality of cells.

24. The device of claim 20 wherein a length of each P-N junction of the cells is greater than 10 times a spacing between a P+ region and a N+ region of the respective cell.

25. The device of claim 20 where the substrate is selected from the group consisting of a homogeneous bulk semiconductor material, a substrate with an epitaxial layer, and a semi-insulating material with a thin epitaxial layer.

26. The device of claim 20 wherein the multi-cell device is a photovoltaic device.

27. A multi-cell device providing electrical isolation of cells, comprising: ohmic contacts for two external connections on the multi-cell device having an electrical potential, an electric field in a substrate, and a current flow between the two external connections; and a plurality of cells fabricated on the substrate, the cells being spaced apart from one another and interconnected by a respective bus extending between sides of adjacent cells, each cell of the plurality of cells having a diffusion field resulting from a presence of photogenerated or bias generated carriers, and each cell itself is configured and oriented to provide electrical isolation from the other cells wherein the diffusion field is perpendicular to a multi-cell current flow, a parasitic current flow and the electric field in the substrate between the two external connections, wherein: a spacing between a P+ region and a N+ region of each of the plurality of cells is less than a length of the respective cell and formed so that the diffusion field is strong enough to prevent majority carrier generation; a region of the substrate directly between each of two adjacent cells and beneath the bus consists of material that is a same composition as material of substrate regions having the plurality of cells disposed thereon; and the region of the substrate directly between each of two adjacent cells and beneath the bus does not have physical isolation.

28. The device of claim 27 wherein an orientation of each of the plurality of cells relative to the direction of the electric field in the substrate provides electrical isolation for each of the plurality of cells.

29. The device of claim 27 wherein a spacing between the P+ region and the N+ region of each of the plurality of cells is less than a width of the respective cell.

30. The device of claim 27 wherein a length of each cell of the plurality of cells is greater than 10 times a spacing between the P+ region and the N+ region of the cell.

31. A semiconductor device comprising: a substrate; a first bus bar on the substrate, the first bus bar extending along a first axis; a first cell on the substrate directly connected to a first side of the first bus bar; and a second cell on the substrate directly connected to a second side of the first bus bar so that the first bus bar extends from the first cell to the second cell, wherein each cell has a plurality of elongated continuous P-type regions interdigitated with a plurality of elongated continuous N-type regions along a second axis that is perpendicular to the first axis, the P-type and N-type regions having a small separation distance to form a plurality of P-N junctions that generate a high diffusion field along the second axis to provide electrical isolation of the first and second cells and prevent majority carrier generation, and wherein the region of the substrate directly between the first and second cells and beneath the first bus bar does not have a physical isolation structure.

32. The semiconductor device of claim 31, further comprising: a second bus bar connected to the first cell and a third bus bar connected to the second cell and wherein a region of the substrate directly between the cells and beneath the bus barz consists of material that is a same composition as material of substrate regions having the first and second cells disposed thereon.

33. The semiconductor device of claim 32, further comprising a third cell on the substrate connected to the third bus bar, wherein the third cell has a plurality of P-type regions interdigitated with a plurality of N-type regions to form a plurality of P-N junctions oriented and configured to provide electrical isolation of the third cell.

33. The semiconductor device of claim 32, further comprising a first ohmic contact connected to the second bus bar and a second ohmic contact connected to the third bus bar for providing external connection with a current flow between the first and second ohmic contacts.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein:

[0011] FIG. 1A depicts a cross section view of a prior art multi-cell device employing trenches for providing physical isolation between cells;

[0012] FIG. 1B depicts an equivalent schematic representation of the multi-cell device;

[0013] FIG. 2A depicts a layout of a multicell device where an orientation of an interdigitated pattern does not provide effective cell isolation by a transverse electric field;

[0014] FIG. 2B depicts the equivalent circuit of the multi-cell device for the string shown in FIG. 2A.

[0015] FIG. 3A depicts a layout of a multicell device where an orientation of an interdigitated pattern does provide effective cell isolation by a transverse diffusion field;

[0016] FIG. 3B depicts an equivalent schematic representation of the multi-cell device and

[0017] FIG. 4 depicts experimental data taken from a multi-cell device without physical isolation and having an orientation configuration of an interdigitated pattern that provides effective cell isolation by a transverse diffusion field.

DETAILED DESCRIPTION

[0018] Turning to FIG. 1A, FIG. 1A depicts an epitaxial layer on a substrate of semi-insulating material. As shown in FIG. 1A, prior art multicell devices on a common substrate require the device to be electrically isolated by forming an insulator or a physical barrier between cells. The extremely high resistivity of the substrate material insulates the cell vertically. As depicted, the horizontal insulation is typically achieved by cutting trenches through an epitaxial layer into the substrate material.

[0019] FIG. 1A depicts a cross section view of a prior art multi-cell device 100 employing trenches for providing physical isolation between cells. FIG. 1B depicts an equivalent schematic representation of the multi-cell device 100. FIG. 1A depicts N+ ohmic contact regions 124, 134, 144 and P+ ohmic contact regions 122, 132, 142 formed in P-type absorption regions 110, 112, 114. These regions were originally a single epitaxial layer grown on a semi-insulating substrate starting wafer 116. The use of this type of substrate, which is a material of high resistivity, provides vertical isolation for the cells. As shown, the single epitaxial layer has been divided into cells 126, 136, 146 by the trench configuration 128, 138. The trenches are lined with silicon dioxide 118 to provide horizontal isolation and top surface isolation. Metallization layers 120, 130, 140, 150 provide external and internal connections for the device 100. Only the serial string connections are shown. There is well-known multi-cell methodology in the prior art that utilizes both trenches 128, 138 and multiple epitaxial layers that is even more complex than the configuration shown in FIG. 1A. This methodology employs what is commonly known as a lateral collection layer. In contrast to the cross section of the prior art, the invention may use a single epitaxial layer, but does not have the complexities involved in trench or multi-layer epitaxial fabrication.

[0020] Turning to FIG. 2A, FIG. 2A depicts a junction layout of a multi-cell device 200 where an interdigitated pattern does not provide effective electrical or physical isolation. The path of current in the diffusion fields 290 of the cells 225, 235, 245, 255 and total current flow 280 is parallel and generally in the same direction as the parasitic current path and the output current 280 flowing through the device 200. The cells are connected edge to edge internally by metallization 230, 240, 250 and externally by metallization 220, 260. The equivalent circuit for the string shown in FIG. 2A is shown in FIG. 2B. The current in the parasitic feedback resistor 260 is not opposed, which degrades the output of the device. This configuration 200 does not provide isolating fields and is an example of an ineffective configuration. This is a very ineffective isolation method and gives poor results. For simplicity, only one pair of junctions is shown for each cell. However, cells may comprise multiple interdigitated junctions connected by buss bars.

[0021] Turning to FIG. 3A, FIG. 3A depicts a top view of a multi-cell device 300 that provides effective electrical isolation according to the disclosed invention by selective placement and orientation of the P-N interdigitated junctions. FIG. 3A depicts four cells 322, 332, 342, 352 connected in a series configuration. The cells 322, 332, 342, 352 each have interdigitated junctions connected in parallel. These cells or sub-cells are connected by bus bars 330, 340, 350 to build up output voltage and minimize internal losses. The cells have external connections 320, 360. FIG. 3B depicts an equivalent schematic representation of the multi-cell device 300. As depicted in FIG. 3A, the diffusion field current flow 390 is perpendicular to the total current flow 380, to an electric field present in the substrate, and the direction of the parasitic path. This perpendicular orientation of the diffusion field and diffusion current 390 is in opposition to the electric field in the substrate and results in electrical isolation approximately equal to that provided by physical isolation without the use of physical isolation, such as trenches.

[0022] Turning to FIG. 4, FIG. 4 shows the forward and reverse characteristics of a string of five diodes fabricated on a common bulk substrate with no physical isolation according to the disclosed invention. The three curves are generated from three separate diode strings. The voltage achieved is consistent with the expected forward voltage of germanium material. A single germanium diode would have a voltage of approximately 200 mV in forward bias, so the experimentally demonstrated accumulated voltage of approximately one volt from the string of diodes, as shown in FIG. 4, is what would be expected from a string of five electrically isolated diodes biased in the forward direction. If electrical isolation had not been achieved, the tested string (on a common substrate) output would look like a single forward biased diode with an internal resistor in series, resulting in a lower voltage output.

[0023] The illustration of the invention, as provided herein, is based primarily on the operation of a photovoltaic device, however, the invention is not restricted to photovoltaic devices and applies equally well to a forward biased string of diodes used for any purpose.

[0024] The effectiveness of the configuration employed, as in FIG. 3 according to the disclosed invention, and the ineffectiveness of the configuration employed in FIG. 2, have been verified by experimentation. Based on an experimental sample, there is no significant difference in performance between physical isolation by the prior art and the field enabled electrical isolation provided by the disclosed invention. In general, the electrical isolation, according to the invention, is improved by providing an interdigitated diode configuration in which the separation distance between the P+ region of the cell and the N+ region of the cell is small relative to the length of the junctions. A small separation distance between the P+ region of the cell and the N+ region of the cell is preferred, so that a high diffusion field is obtained. In a preferred embodiment, the separation distance is nominally 5 to 50 micrometers, and the length of the cell is at least ten times the separation distance. In general terms, these dimensions and this ratio of dimensions provide a high blocking field and a weak parasitic field.

[0025] In the course of trying to improve the output current of a multi-cell device on a bulk substrate without physical isolation, a number of cell layouts were experimentally tested. Superior results were obtained for the layout as depicted in FIGS. 3A and 3B where the device geometry provides multi-cell functionality without physical isolation between cells. In a preferred embodiment of the invention, the individual cells are formed by an interdigitated construction of junctions with like junctions connected by bus bars. The bus bar connection between cells alternates P to N in a fashion to form a diode string which has a beginning-to-end dimension. The individual junctions within a cell have a long dimension that is parallel to the direction of the beginning-to-end dimension of the diode string. The junctions may be formed by diffusion, ion implant, Schottky barrier, or other known processes. The preferred embodiment of the invention is alternatively described as a multiple cell device on a common substrate wherein the junctions and ohmic contact regions of each cell are in the form of long interdigitated fingers, and the long direction of these fingers is parallel to the long direction of the parasitic path formed by ohmic contacts of some of the fingers to the common substrate, and the long direction of these finger is parallel to the beginning-to-end path of the diode string.