Modulated super junction power MOSFET devices

Abstract

A semiconductor device—e.g., a super junction power MOSFET—includes a number of columns of one type of dopant formed in a region of another type of dopant. Generally speaking, the columns are modulated in some manner. For example, the widths (e.g., diameters) of some columns are greater than the widths of other columns.

Description

BACKGROUND

To conserve power, it is important to reduce power losses in transistors. In a metal oxide semiconductor field effect transistor (MOSFET) device, and in particular in the class of MOSFETs known as power MOSFETs, power losses can be reduced by reducing the device's on-resistance (Rdson).

Breakdown voltage provides an indication of a device's ability to withstand breakdown under reverse voltage conditions. Breakdown voltage is inversely related to Rdson, and so is adversely affected when Rdson is reduced. To address this issue, super junction (SJ) power MOSFETs, which include alternating p-type and n-type regions at the active regions of the device, were introduced. When the charges in the alternating p-type and n-type regions in a SJ power MOSFET are balanced (the charges in the p-type regions, Q_p, are equal to the charges in the n-type regions, Q_n), then breakdown voltage is at its peak value, thereby enabling the device to better withstand breakdown.

As Q_n is increased relative to Q_p, Rdson advantageously decreases. However, an n-channel SJ power MOSFET device operated with Q_n greater than Q_p will suffer from lower unclamped inductive switching (UIS) ruggedness, because the field peak at breakdown will occur closer to the base of the inherent parasitic bipolar transistor. Therefore, the device is generally operated with Q_p greater than Q_n. However, as Q_p is increased relative to Q_n, the breakdown voltage decreases and, consequently, the breakdown voltage will be less than its peak value for an n-channel SJ power MOSFET device operated in this manner.

SUMMARY

In an embodiment according to the invention, an SJ power MOSFET device includes a number of columns of one type of dopant formed in a region of another type of dopant. For example, in an n-channel device, p-type columns are formed in an n-type region. Generally speaking, in embodiments according to the invention, the columns are modulated in some manner.

In one embodiment, the modulated columns have different widths. For example, the widths of some columns are greater than the widths of other columns. In another embodiment, the modulated columns have different cross-sectional shapes. For example, some columns may have a circular cross-section, while other columns may have a squarish cross-section or a hexagonal cross-section. In contrast, conventional SJ power MOSFET devices have columns that are the same size (width) and shape.

The modulated columns can be arranged in different ways. For example, larger-width columns can be interleaved with smaller-width columns in alternating fashion. That is, a row of larger-width columns can be next to a row of smaller-width columns, which in turn is next to another row of larger-width columns followed by another row of smaller-width columns, and so on. As another example, each narrow-width column can be surrounded by wider-width columns.

Modulation of the columns results in a combination of higher and lower amounts of charge in the columns. For example, consider an n-channel device according to the present invention. In such a device, modulating the columns results in higher values for Q_p in the larger (wider) columns and lower values for Q_p in the smaller (narrower) columns. However, the lowest values of Q_p are greater than the charge balance value; that is, the lowest values of Q_p are greater than Q_n. Consequently, the breakdown voltage will be higher than the breakdown voltage corresponding to the highest value of Q_p, but lower than the breakdown voltage corresponding to the lowest value of Q_p. This will lead to a lower slew rate of the breakdown voltage at the higher Q_p values that provide better UIS ruggedness. Breakdown voltage and UIS ruggedness are not compromised as the field peak at breakdown will occur away from the base region of the inherent parasitic bipolar transistor because, as noted above, the lower Q_p values are greater than the charge balance value. Also, process sensitivity of an SJ power MOSFET device with modulated columns is improved relative to conventional forms of such devices.

These and other objects and advantages of the present invention will be recognized by one skilled in the art after having read the following detailed description, which are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Like numbers denote like elements throughout the drawings and specification.

FIG. 1 is a cross-sectional view of a portion of a semiconductor device (e.g., a super junction power MOSFET device) in an embodiment according to the present invention.

FIG. 2 is a cross-sectional view of an array of columns in a semiconductor device (e.g., a super junction power MOSFET device) in an embodiment according to the present invention.

FIG. 3 is a cross-sectional view of an array of columns in a semiconductor device (e.g., a super junction power MOSFET device) in another embodiment according to the present invention.

FIG. 4 is a cross-sectional view of an array of columns in a semiconductor device (e.g., a super junction power MOSFET device) in yet another embodiment according to the present invention.

FIG. 5 is a flowchart of a method for fabricating a device (e.g., a super junction power MOSFET device) in an embodiment according to the present invention.

DETAILED DESCRIPTION

In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.

The figures are not drawn to scale, and only portions of the structures, as well as the various layers that form those structures, may be shown in the figures.

As used herein, the letter “n” refers to an n-type dopant and the letter “p” refers to a p-type dopant. A plus sign “+” or a minus sign “−” is used to represent, respectively, a relatively high or relatively low concentration of the dopant.

The term “channel” is used herein in the accepted manner. That is, current moves within a FET in a channel, from the source connection to the drain connection. A channel can be made of either n-type or p-type semiconductor material; accordingly, a FET is specified as either an n-channel or p-channel device. The disclosure is presented in the context of an n-channel device, specifically an n-channel super junction (SJ) power MOSFET; however, embodiments according to the present invention are not so limited. That is, the features described herein can be utilized in a p-channel device. The disclosure can be readily mapped to a p-channel device by substituting, in the discussion, n-type dopant and materials for corresponding p-type dopant and materials, and vice versa.

FIG. 1 is a cross-sectional view of a portion of a semiconductor device 100 (e.g., an SJ power MOSFET device) in an embodiment according to the present invention. The device 100 may include elements and structures instead of or in addition to those shown and described.

Generally speaking, the device 100 has a channel of a first type dopant, and a number of columnar regions formed using a second type dopant that is different from the first type dopant, where the columnar regions/columns are in a region of the first type dopant. In the example of FIG. 1, the first type dopant is n-type and the second type dopant is p-type, and the device 100 is an n-channel device.

More specifically, the device 100 includes a drain electrode 102 on the bottom surface of an n⁺ drain layer or substrate 104. In the FIG. 1 embodiment, the device 100 also includes p⁻ drift regions or p-type columns 106 formed in n⁻ drift regions or n-type regions 108, above the substrate 104. The p-type (p⁻) columns 106 and n-type (n⁻) regions 108 form what is known as a super junction. The columns 106 and the regions 108 are located within the active region of the device 100. A termination region or termination regions (not shown) are disposed along the edges of the device 100, around the active region.

In one embodiment, the columns 106 of p-type dopant are separated from the adjacent regions 108 of n-type dopant by isolation layers or columns (e.g., a layer/column of dielectric or oxide; not shown). The isolation layers keep the p-type columns 106 and the n-type regions 108 from diffusing into one another when the structure is heated during fabrication, to prevent breakdown voltage from being adversely affected by the fabrication process.

In the FIG. 1 embodiment, there is a p-type base region 110 at the top of each of the columns 106 (in the orientation of FIG. 1). There may also be a p-type (p⁺) contact region 112 and an n-type (n⁺) source region 114 at each of the columns 106, as shown. The layer of the device 100 above the substrate 104 and that includes the elements 106, 108, 110, 112, and 114 may be referred to as the epitaxial layer 116. The epitaxial layer 116 may include elements and structures instead of or in addition to those shown and described.

In the FIG. 1 embodiment, a layer of source metal 118 is coupled to a source electrode 120, and a gate structure 122 is coupled to a gate electrode 124. The gate structure 122 is separated from its neighboring elements and structures by an isolation layer 126.

FIG. 2 is a cross-sectional view along the cut line A-A (FIG. 1) showing an array of columns 216 and 226 in an embodiment according to the present invention. The columns 216 and 226 are examples of the columns 106 of FIG. 1. In other words, the set of columns 106 represented in FIG. 1 includes the columns 216 and 226. As described in more detail below, according to embodiments of the invention, the columns 216 and 226 are modulated. The modulated columns 216 and 226 are located in the active region of the device 100 but not in the termination region(s).

In the embodiment of FIG. 2, the columns 216 and 226 are modulated in terms of their dimensions (e.g., their widths or diameters) and thus in terms of their charge (Q_p) values. More specifically, the columns 216 have substantially the same first width (diameter) and the columns 226 have substantially the same second width, where the first width is different from the second width. The term “substantially the same” is used herein to mean that the there might be some differences in the widths among the columns 216 from one column to the next, but the widths of the columns 216 are within specified tolerances such as design and manufacturing tolerances. Similarly, there might be some differences in the widths of the columns 226 from one column to the next, but the widths of the columns 226 are within specified tolerances such as design and manufacturing tolerances. In other words, in the embodiment of FIG. 2, each of the columns 216 has the same width (within specified tolerances) along its entire length (below the base region 110 of FIG. 1), and each of the columns 226 has the same width (within specified tolerances) along its entire length (below the base region 110).

In the example of FIG. 2, the first width is greater than the second width. That is, the columns 216 are wider than the columns 226. Specifically, in one embodiment, the same first width is specified for each of the columns 216, and the same second width (different from the first width) is specified for each of the columns 226, and the average value of the widths of the columns 216 is greater than the average value of the widths of the columns 226. In one embodiment, a width (diameter) of 2.0 micrometers is specified for the wider columns, and a width (diameter) of 1.96 micrometers is specified for the narrower columns.

Significantly, specific locations are assigned to the columns 216 and 226 according to their respective widths. In other words, certain locations within the active region of the device 100 are identified as being the locations where wider columns (e.g., the columns 216) are to be formed. Similarly, certain locations within the active region of the device 100 are identified as being the locations where narrower columns (e.g., the columns 226) are to be formed.

In the example of FIG. 2, rows of the wider columns 216 are interleaved with rows of the narrower columns 226. More specifically, a row of the wider columns 216 is next to a row of the narrower columns 226, which is next to another row of the wider columns 216, which is next to another row of the narrower columns 226, and so on. In such an arrangement, the breakdown voltage would be around the arithmetic mean of the breakdown voltage if all the columns were wider columns and the breakdown voltage if all of the columns were narrower columns.

In general, modulation of the columns results in a combination of higher and lower amounts of charge in the columns. For example, consider an n-channel device according to the present invention. In such a device, modulating the columns results in higher values for Q_p in the larger (wider) columns and lower values for Q_p in the smaller (narrower) columns. However, the lowest values of Q_p are greater than the charge balance value; that is, the lowest values of Q_p are greater than Q_n. Consequently, the breakdown voltage will be higher than the breakdown voltage corresponding to the highest value of Q_p, but lower than the breakdown voltage corresponding to the lowest value of Q_p. In other words, by mixing larger columns with smaller columns, it is possible to elevate the breakdown voltage relative to the breakdown voltage that would be realized if only larger columns were used.

In Table 1, the breakdown voltages (in volts, V) and on-resistances (in ohms) are presented for examples of n-channel devices with conventional columns and for examples of n-channel devices with modulated (different width) columns. In these examples, modulated p-type columns are arranged as shown in FIG. 2, the wider columns have a diameter of 2.0 micrometers, and the narrower columns have a diameter of 1.96 micrometers. In each example, the p-type columns have a dose of 5.6E+13 ions per square centimeter. In one example, the JFet (n-type) dose is 5.0E+11 ions per square centimeter; in the other example, the JFet dose is 1.0E+12 ions per square centimeter.

TABLE 1
Example Breakdown Voltages and On-Resistances
	p-Column	JFet	Rdson	Breakdown
Type	Dose	Dose	(ohms)	Voltage (V)
Modulated	5.6E+13	5.0E+11	82.1	670
Conventional	5.6E+13	5.0E+11	82.4	620
Modulated	5.6E+13	1.0E+12	80.6	655
Conventional	5.6E+13	1.0E+12	80.8	626

As can be seen from Table 1, by modulating columns, the breakdown voltage has increased by 50 V in the first example and by 29 V in the second example, relative to conventional designs.

The use of modulated columns also leads to a lower slew rate of the breakdown voltage at the higher Q_p values that provide better UIS ruggedness. Breakdown voltage and UIS ruggedness are not compromised as the field peak at breakdown will occur away from the base region of the inherent parasitic bipolar transistor because, as noted above, the lower Q_p values are greater than the charge balance value.

Also, process sensitivity of an SJ power MOSFET device with modulated columns is improved relative to conventional forms of such devices. Arranging the different-sized columns as in FIG. 2, for example, can have the effect of smoothing of the process variations of the widths (diameters) of the columns that arise due to photolithographic limitations.

FIG. 3 is a cross-sectional view along the cut line A-A (FIG. 1) showing an array of columns 316 and 326 in another embodiment according to the present invention. The columns 316 and 326 are examples of the columns 106 of FIG. 1. In other words, the set of columns 106 represented in FIG. 1 includes the columns 316 and 326. As described above, according to embodiments of the invention, the columns 316 and 326 are modulated. The modulated columns 316 and 326 are located in the active region of the device 100 but not in the termination region(s).

In the embodiment of FIG. 3, the columns 316 and 326 are modulated in terms of their dimensions (e.g., their widths or diameters) and thus in terms of their charge (Q_p) values. More specifically, the columns 316 have substantially the same first width (diameter) and the columns 326 have substantially the same second width, where the first width is different from the second width. In the example of FIG. 3, the first width is greater than the second width. In one embodiment, a width (diameter) of 2.0 micrometers is specified for the wider columns, and a width (diameter) of 1.96 micrometers is specified for the narrower columns.

As presented above, specific locations are assigned to the columns 316 and 326 according to their respective widths. In the example of FIG. 3, each of the narrower columns 326 is surrounded by wider columns 316. For example, the narrower column labeled A is surrounded by six wider columns labeled B, C, D, E, F, and G. In essence, the columns B-G form a ring around the column A; in the example of FIG. 3, the ring is hexagonal in shape.

The arrangement of columns in FIG. 3 can also be described as follows. A first row includes only wider columns 316. Next to the first row is a second row that includes narrower columns 326 interleaved with wider columns 316. In the example of FIG. 3, every other column in the second row is a wider column—there is a narrower column between each wider column, and vice versa. Next to the second row is a third row that is similar to the first row; next to the third row is a fourth row that is similar to the second row; and so on.

Modulated columns can be arranged differently from the examples of FIGS. 2 and 3. Furthermore, the modulated columns do not need to have cross-sections that are circular in shape, as in the examples of FIGS. 2 and 3. The modulated columns may have cross-sections that are, for example, squarish or hexagonal (where the cross-sections are orthogonal to the longitudinal axes of the columns), as shown in the example of FIG. 4. Also, the modulated columns do not all have to have the same shape. For example, some columns may be circular, and other columns may be hexagonal, also shown in the example of FIG. 4.

FIG. 4 is a cross-sectional view along the cut line A-A (FIG. 1) showing an array of circular columns 416 and hexagonal columns 426 in an embodiment according to the present invention. The columns 416 and 426 are examples of the columns 106 of FIG. 1. In other words, the set of columns 106 represented in FIG. 1 includes the columns 416 and 426. As described above, according to embodiments of the invention, the columns 416 and 426 are modulated. The modulated columns 416 and 426 are located in the active region of the device 100 but not in the termination region(s).

In the embodiment of FIG. 4, the columns 416 and 426 are modulated in terms of their shape and thus in terms of their charge (Q_p) values. More specifically, the columns 416 are circular in shape, and the columns 426 are hexagonal in shape. In the example of FIG. 4, the arrangement of the different-shaped columns 416 and 426 is analogous to the arrangement of the different-dimensioned columns in FIG. 2. The columns 416 and 426 can be arranged in a different manner, such as one analogous to the arrangement of FIG. 3.

In general, modulated columns can be arranged according to their dimensions and/or according to their shapes. In the examples of FIGS. 2 and 3, the columns are arranged according to their dimensions (widths, diameters). If the columns 116 (FIG. 1) have different shapes, then the columns could be arranged in a manner similar to that shown in FIGS. 2 and 3, but according to their shapes, as in the example of FIG. 4. If the columns 116 (FIG. 1) have different shapes, and there are different dimensions among columns having the same shape, then the columns can be arranged according to both their shapes and their dimensions.

An SJ power MOSFET device with modulated columns can be fabricated in much the same way as a conventional SJ power MOSFET device without modulated columns, except that the processes used to control the dimensions and shapes of the columns can be modified to permit columns with different widths and/or different shapes to be formed. Thus, in one embodiment, different-sized columns and/or different-shaped columns can be formed in the same process step(s), and other process step(s) can be designed to account for the differences in the sizes and/or shapes of the columns.

FIG. 5 is a flowchart 500 of a method for fabricating a device (e.g., the device 100 of FIG. 1) in an embodiment according to the present invention. The operations described below can be performed in a different order than that described, and operations described as separate blocks may be combined and performed in the same process step (that is, in the same time interval, after the preceding process step and before the next process step). Furthermore, fabrication processes and steps may be performed along with the processes and steps discussed herein; that is, there may be a number of process steps before, in between and/or after the steps shown and described herein. Importantly, embodiments according to the present invention can be implemented in conjunction with these other (perhaps conventional) processes and steps without significantly perturbing them. Generally speaking, embodiments according to the present invention can replace portions of a conventional process without significantly affecting peripheral processes and steps.

In block 502 of FIG. 5, a first set of specific locations at which a first set (type) of columns is to be formed is identified in an active region of the device. A first width and shape is specified for each column in the first set of columns.

In block 504, a second set of specific locations at which a second set (type) of columns is to be formed is identified in the active region of the device. A second width and shape is specified for each column in the second set of columns. The first width is different from the second width.

In block 506, the first set of columns is formed at the first set of locations. Each column in the first set of columns is formed with the same first width, within tolerances.

In block 508, the second set of columns is formed at the second set of locations. Each column in the second set of columns is formed with the same second width, within tolerances.

The device may be designed to include other sets (types) of columns that have different widths and/or different shapes than those in the first set and those in the second set. If so, then operations such as those in blocks 502 and 506 can be repeated for each such set.

In summary, embodiments of SJ power MOSFET devices are described. The features described herein can be used in low voltage devices as well as high voltage devices as an alternative to split-gate, dual-trench, and other conventional high voltage super junction devices.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. A super junction power metal oxide semiconductor field effect transistor (MOSFET) device having a channel of first type dopant, said device comprising: a plurality of columns, wherein said plurality of columns comprises: a first plurality of columns comprising second type dopant that is different from said first type dopant, said first plurality of columns formed in a region comprising said first type dopant, each column in said first plurality of columns having substantially a same first width; anda second plurality of columns comprising said second type dopant, said second plurality of columns formed in said region comprising said first type dopant, each column in said second plurality of columns having substantially a same second width;wherein said first width is different from said second width, and wherein said second plurality of columns comprises columns that are surrounded by columns of said first plurality of columns, and wherein each column in said first plurality of columns is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with source metal, and wherein each column in said second plurality of columns is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with said source metal.

2. The super junction power MOSFET device of claim 1 wherein an average value of said first width across said first plurality of columns is greater than an average value of said second width across said second plurality of columns.

3. The super junction power MOSFET device of claim 1 wherein said first plurality of columns comprises a first column having a first cross-section that has a first shape and wherein said second plurality of columns comprises a second column having a second cross-section that has a second shape that is different from said first shape, said first cross-section orthogonal to an axis of said first column and said second cross-section orthogonal to an axis of said second column.

4. The super junction power MOSFET device of claim 3 wherein said first shape and said second shape are selected from the group consisting of: a circular shape; a squarish shape; and a hexagonal shape.

5. The super junction power MOSFET device of claim 1 wherein said first plurality of columns and said second plurality of columns are at an active region of said device but are outside termination regions of said device.

6. The super junction power MOSFET device of claim 1 wherein each column in said first plurality of columns has substantially a same first width along its entire length below its said respective base region, and wherein each column in said second plurality of columns has substantially a same second width along its entire length below its said respective base region.

7. A semiconductor device comprising: a substrate comprising a first type dopant;a super junction structure coupled to said substrate and comprising a plurality of columnar regions, said plurality of columnar regions comprising a first plurality of columnar first regions formed in a third region and a second plurality of columnar second regions formed in said third region, said third region comprising said first type dopant, said columnar first regions and said columnar second regions each comprising second type dopant that is different from said first type dopant, wherein each of said columnar first regions has a first width measured orthogonal to the longitudinal axis of said first region, and wherein each of said columnar second regions has a second width measured orthogonal to the longitudinal axis of said second region, said first width different from said second width;wherein said second plurality of columnar second regions comprises columnar regions that are surrounded by columnar regions of said first plurality of columnar first regions, and wherein each columnar region of said first plurality of columnar first regions is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with source metal, and wherein each columnar region of said second plurality of columnar second regions is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with said source metal.

8. The semiconductor device of claim 7 wherein an average value of said first width across said first plurality of columnar first regions is greater than an average value of said second width across said second plurality of columnar second regions.

9. The semiconductor device of claim 7 wherein each columnar region in said first plurality of columnar first regions has a first cross-section that has a first shape and wherein each columnar region in said second plurality of columnar second regions has a second cross-section that has a second shape that is different from said first shape, said first cross-section orthogonal to axes of said columnar first regions and said second cross-section orthogonal to axes of said columnar second regions.

10. The semiconductor device of claim 9 wherein said first shape and said second shape are selected from the group consisting of: a circular shape; a squarish shape; and a hexagonal shape.

11. The semiconductor device of claim 7 wherein said first plurality of columnar first regions and said second plurality of columnar second regions are at an active region of said device but are outside termination regions of said device.

12. The semiconductor device of claim 7 wherein each of said columnar first regions has substantially a same first width along its entire length below its said respective base region, the first width measured orthogonal to the longitudinal axis of said columnar first regions, wherein each of said columnar second regions has substantially a same second width along its entire length below its said respective base region, the second width measured orthogonal to the longitudinal axis of said columnar second regions, and wherein said first width is different from said second width.

13. A semiconductor device comprising a gate, source, and drain, said semiconductor device comprising: a substrate of said first type dopant; andan epitaxial layer coupled to said substrate, said epitaxial layer doped with said first type dopant, said epitaxial layer having formed therein a plurality of columns, wherein said plurality of columns comprises: a first plurality of columns comprising second type dopant that is different from said first type dopant, each column in said first plurality of columns having substantially a same first width; anda second plurality of columns comprising said second type dopant, each column in said second plurality of columns having substantially a same second width;wherein said first width is different from said second width, and wherein said second plurality of columns comprises columns that are surrounded by columns of said first plurality of columns, and wherein each column in said first plurality of columns is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with source metal, and wherein each column in said second plurality of columns is electrically and physically in contact with a respective base region of said second type dopant that is electrically and physically in contact with a respective contact region of said second type dopant that is physically and electrically in contact with said source metal.

14. The semiconductor device of claim 13 wherein an average value of said first width across said first plurality of columns is greater than an average value of said second width across said second plurality of columns.

15. The semiconductor device of claim 13 wherein said first plurality of columns comprises a first column having a first cross-section that has a first shape and wherein said second plurality of columns comprises a second column having a second cross-section that has a second shape that is different from said first shape, said first cross-section orthogonal to an axis of said first column and said second cross-section orthogonal to an axis of said second column.

16. The semiconductor device of claim 13 wherein said first plurality of columns and said second plurality of columns are at an active region of said device but are outside termination regions of said device.

17. The semiconductor device of claim 13 wherein each column in said first plurality of columns has substantially a same first width along its entire length below its said respective base region, and wherein each column in said second plurality of columns has substantially a same second width along its entire length below its said respective base region.