# Patent

## Title
Bipolar semiconductor device with sub-cathode enhancement regions

## Abstract

There are disclosed herein various implementations of a bipolar semiconductor device with sub-cathode enhancement regions. Such a bipolar semiconductor device includes a drift region having a first conductivity type situated over an anode layer having a second conductivity type opposite the first conductivity type. The bipolar semiconductor device also includes first and second depletion trenches, each having a depletion electrode. In addition, the bipolar semiconductor device includes a first control trench situated between the first and second depletion trenches, the first control trench extending into the drift region and being adjacent to cathode diffusions. An enhancement region having the first conductivity type is localized in the drift region between the first control trench and one or both of the first and second depletion trenches. In one implementation, the bipolar semiconductor device may be an insulated-gate bipolar transistor (IGBT).

## Background
BACKGROUND
Bipolar semiconductor devices suitable for use as power switches, such as insulated-gate bipolar transistors (IGBTs), for example, may be implemented in a variety of applications. For instance, IGBTs may be used as power switches in motor drive inverters, as well as in direct-current (DC) to DC power converters. In these and other power applications, on-state voltage drop (VON), turn-off losses (EOFF), and turn-off delay time (Td,OFF) are important operating parameters.
However, conventional techniques for producing desirable on-state characteristics, such as low VON, can undesirably result in increased EOFF and longer Td,OFF. As switching speed increases, switching losses, including EOFF, typically represent a significant portion of total power loss by a bipolar power switch. Consequently, IGBTs and other bipolar switching devices having desirable on-state characteristics and reduced EOFF and Td,OFF during fast switching are highly sought after in the art.
SUMMARY
The present disclosure is directed to a bipolar semiconductor device with sub-cathode enhancement regions, substantially as shown in and/or described in connection with at least one of the figures, and as set forth in the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 presents a cross-sectional view showing a portion of an exemplary bipolar semiconductor device with sub-cathode enhancement regions, according to one implementation.
FIG. 2 presents a cross-sectional view showing a portion of an exemplary bipolar semiconductor device with sub-cathode enhancement regions, according to another implementation.
FIG. 3 presents a cross-sectional view showing a portion of an exemplary bipolar semiconductor device with sub-cathode enhancement regions, according to yet another implementation.
FIG. 4 presents a cross-sectional view showing a portion of an exemplary bipolar semiconductor device with sub-cathode enhancement regions, according to a further implementation.



DETAILED DESCRIPTION
The following description contains specific information pertaining to implementations in the present disclosure. One skilled in the art will recognize that the present disclosure may be implemented in a manner different from that specifically discussed herein. The drawings in the present application and their accompanying detailed description are directed to merely exemplary implementations. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present application are generally not to scale, and are not intended to correspond to actual relative dimensions.
FIG. 1 presents a cross-sectional view showing unit cell 150 of exemplary bipolar semiconductor device 100 with sub-cathode enhancement regions, according to one implementation. As shown in FIG. 1, bipolar semiconductor device 100 is implemented as a vertical power device including P type anode layer 110 at bottom surface 104 of semiconductor substrate 102, and N type drift region 114 situated over P type anode layer 110. In addition, P type inversion region 116 is situated over N type drift region 114. As further shown in FIG. 1, bipolar semiconductor device 100 includes N type buffer or field stop layer 112 (hereinafter “buffer layer 112”), as well as N type cathode diffusions 132 and P type contacts 134 formed in P type inversion region 116. It is noted that although single unit cell 150 is shown in FIG. 1 for conceptual clarity, bipolar semiconductor device 100 includes a plurality of unit cells, each of which may include the features attributed to unit cell 150 and described in the present application.
According to the exemplary implementation shown in FIG. 1, unit cell 150 of bipolar semiconductor device 100 includes control trench 120 and first and second depletion trenches 122a and 122b, each extending from top surface 106 of semiconductor substrate 102, through P type inversion region 116, and further extending into N type drift region 114. As shown in FIG. 1, control trench 120 is adjacent to N type cathode diffusions 132, and includes trench insulator 124 and control trench electrode 126. As further shown in FIG. 1, each of first depletion trench 122a and second depletion trench 122b includes trench insulator 124 and depletion electrode 128, but neither first depletion trench 122a nor second depletion trench 122b is bordered by cathode diffusions 132. It is noted that control trench 120 is situated between first depletion trench 122a and second depletion trench 122b. 
Unit cell 150 of bipolar semiconductor device 100 also includes N type enhancement region 140. N type enhancement region 140 is localized in N type drift region 114 between control trench 120 and one or both of first depletion trench 122a and second depletion trench 122b. As shown in FIG. 1, N type enhancement region 140 extends under cathode diffusions 132 and may therefore be characterized as a sub-cathode enhancement region.
It is emphasized that although the exemplary implementation shown in FIG. 1 depicts N type enhancement region 140 as localized in N type drift region 114 between control trench 120 and each of first and second depletion trenches 122a and 122b, in other implementations, N type enhancement region 140 may be localized in N type drift region 114 between control trench 120 and one, but not both of, first and second depletion trenches 122a and 122b. It is further emphasized that N type enhancement region 140 is absent from regions of N type drift region 114 that do not lie between control trench 120 and first depletion trench 122a, and from regions of N type drift region 114 that do not lie between control trench 120 and second depletion trench 122b. 
In operation, bipolar semiconductor device 100 is configured to produce conduction channels through P type inversion region 116 in regions beneath N type cathode diffusions 132 and immediately adjacent control trench 120. Thus, when bipolar semiconductor device 100 is turned on, conduction channels (not shown as such in FIG. 1) are produced as N type conduction channels through P type inversion region 116 so as to enable transfer of charge carriers between N type cathode diffusions 132 and P type anode layer 110.
Although the implementation shown in FIG. 1 depicts bipolar semiconductor device 100 as having P type anode layer 110, N type buffer layer 112, N type drift region 114, N type enhancement region 140, P type inversion region 116, N type cathode diffusions 132, and as being configured to produce N type conduction channels, that representation is merely exemplary. In other implementations, the described polarities can be reversed. That is to say, bipolar semiconductor device 100 may have an N type layer corresponding to P type anode layer 110, a P type buffer layer, a P type drift region, a P type enhancement region, an N type inversion region, P type diffusions corresponding to N type cathode diffusions 132, and may be configured to produce P type conduction channels adjacent control trench 120.
According to one exemplary implementation, bipolar semiconductor device 100 may take the form of an insulated-gate bipolar transistor (IGBT). In that implementation, P type anode layer 110 corresponds to a P type collector layer, P type inversion region 116 corresponds to a P type base, and N type cathode diffusions 132 correspond to N type emitter diffusions of the IGBT. Moreover, when bipolar semiconductor device 100 is implemented as an IGBT, control trench 120 corresponds to a gate trench of the IGBT, including a gate insulator and a gate electrode corresponding respectively to trench insulator 124 and control trench electrode 126.
Semiconductor substrate 102 may be a silicon (Si) substrate or a silicon carbide (SiC) substrate, for example. In some implementations, semiconductor substrate 102 may include N type drift region 114 and P type inversion region 116 formed in an epitaxial silicon layer of semiconductor substrate 102. Formation of such an epitaxial silicon layer may be performed by any suitable method, as known in the art, such as chemical vapor deposition (CVD) or molecular beam epitaxy (MBE), for example. More generally, however, N type drift region 114 and P type inversion region 116 may be formed in any suitable elemental or compound semiconductor layer included in semiconductor substrate 102.
Thus, in other implementations, N type drift region 114 and P type inversion region 116 need not be formed through epitaxial growth, and/or need not be formed of silicon. For example, in one alternative implementation, N type drift region 114 and P type inversion region 116 can be formed in a float zone silicon layer of semiconductor substrate 102. In other implementations, N type drift region 114 and P type inversion region 116 can be formed in either a strained or unstrained germanium layer formed as part of semiconductor substrate 102. Moreover, in some implementations, semiconductor substrate 102 may include additional layers, such as N type buffer layer 112 situated between P type anode layer 110 and N type drift region 114, as shown in FIG. 1.
P type inversion region 116 may be formed by implantation and thermal diffusion. For example, boron (B) dopants may be implanted into semiconductor substrate 102 and diffused to form P type inversion region 116. Moreover, P type contacts 134 of P type inversion region 116 may be more highly doped regions of P type inversion region 116 utilizing the same dopant species used to form P type inversion region 116.
Trench insulator 124 may be formed using any material and any technique typically employed in the art. For example, trench insulator 124 may be formed of silicon oxide, and may be deposited or thermally grown to line control trench 120 and first and second depletion trenches 122a and 122b. Control trench electrode 126 may also be formed using any material typically utilized in the art. For example, control trench electrode 126 may be formed of doped polysilicon or metal.
Like control trench electrode 126, depletion electrodes 128 may be formed using any material typically utilized in the art, such as doped polysilicon or metal. Moreover, although identified by different reference numbers in FIG. 1, depletion electrodes 128 may be formed of the same material as control trench electrode 126, and may be fabricated in the same processing step utilized to produce control trench electrode 126. However, unlike control trench electrode 126, depletion electrodes 128 may be electrically floating, or may be electrically tied to N type cathode diffusions 132. It is noted that although not explicitly shown in FIG. 1, depletion electrodes 128 can be electrically connected to one another and/or to N type cathode diffusions 132 in the third dimension relative to the cross-sectional perspective shown by FIG. 1.
N type cathode diffusions 132 may be selectively formed in P type inversion region 116 using any conventional techniques known in the art. For example, phosphorus (P) or arsenic (As) dopants may be implanted into P type inversion region 116 and diffused to form N type cathode diffusions 132. As may be the case for depletion electrodes 128, and although also not explicitly shown in FIG. 1, N type cathode diffusions 132 can be electrically connected to one another in the third dimension relative to the cross-sectional perspective shown by FIG. 1.
N type enhancement region 140 may have a doping concentration greater than that of N type drift region 114 and less than that of N type cathode diffusions 132. In one implementation, N type enhancement region 140 may have a doping concentration substantially equal to that of N type buffer layer 112. For example, N type enhancement region 140 may have a doping concentration of from approximately 1×1015/cm3 to approximately 1×1016/cm3, while the doping concentration of N type drift region 114 is typically from approximately 1×1013/cm3 to approximately 2×1014/cm3.
According to the exemplary implementation shown in FIG. 1, N type enhancement region 140 adjoins P type inversion region 116 and terminates in N type drift region 114, above N type buffer layer 112. In other words, N type enhancement region 140 adjoins P type inversion region 116 while being spaced apart from N type buffer layer 112 by N type drift region 114.
The reduced charge in the upper portion of N type drift region 114 due to localization of N type enhancement region 140 between control trench 120 and one or both of first and second depletion trenches 122a and 122b allows the depletion region that begins to form when bipolar semiconductor device 100 is turned off to expand more rapidly. In addition, the Miller capacitance of bipolar semiconductor device 100 is reduced due to the overall reduction in charge between top surface 106 and P type anode layer 110. Consequently, turn-off losses (EOFF) and turn-off delay time (Td,OFF) are substantially improved, i.e., reduced, in the implementation shown in FIG. 1, when compared to conventional bipolar semiconductor devices, such as conventional IGBTs. Moreover, these advantages may be achieved while maintaining the on-state voltage drop (VON) of bipolar semiconductor device 100 at a desirable level.
Continuing to FIG. 2, FIG. 2 presents a cross-sectional view showing unit cell 250 of exemplary bipolar semiconductor device 200 with sub-cathode enhancement regions, according to another implementation. As shown in FIG. 2, bipolar semiconductor device 200 is implemented as a vertical power device including P type anode layer 210 at bottom surface 204 of semiconductor substrate 202, and N type drift region 214 situated over P type anode layer 210. In addition, P type inversion region 216 is situated over N type drift region 214. As further shown in FIG. 2, bipolar semiconductor device 200 includes N type buffer layer 212, as well as N type cathode diffusions 232 and P type contacts 234 formed in P type inversion region 216. It is noted that although single unit cell 250 is shown in FIG. 2 for conceptual clarity, bipolar semiconductor device 200 includes a plurality of unit cells, each of which may include the features attributed to unit cell 250 and described in the present application.
According to the exemplary implementation shown in FIG. 2, unit cell 250 of bipolar semiconductor device 200 includes first control trench 220a and second control trench 220b, each extending from top surface 206 of semiconductor substrate 202, through P type inversion region 216, and further extending into N type drift region 214. As shown in FIG. 2, each of first control trench 220a and second control trench 220b is adjacent to N type cathode diffusions 232, and includes trench insulator 224 and control trench electrode 226.
In addition, unit cell 250 of bipolar semiconductor device 200 includes first and second depletion trenches 222a and 222b. As further shown in FIG. 2, each of first depletion trench 222a and second depletion trench 222b includes trench insulator 224 and depletion electrode 228, but neither first depletion trench 222a nor second depletion trench 222b is bordered by cathode diffusions 232. It is noted that both of first control trench 220a and second control trench 220b are situated between first and second depletion trenches 222a and 222b. 
Unit cell 250 of bipolar semiconductor device 200 also includes N type enhancement region 240. N type enhancement region 240 is localized in N type drift region 214 between first depletion trench 222a and first control trench 220a, and between second control trench 220b and second depletion trench 222b. As shown in FIG. 2, N type enhancement region 240 extends under cathode diffusions 232 and may therefore be characterized as a sub-cathode enhancement region. Moreover, and as further shown in FIG. 2, in some implementations, N type enhancement region 240 adjoins each of first control trench 220a, first depletion trench 222a, second control trench 220b, and second depletion trench 222b. It is emphasized, however, that N type enhancement region 240 is localized so as to be confined between a control trench and its adjacent depletion trench. As a result, no N type enhancement region 240 is present between first control trench 220a and second control trench 220b. 
Bipolar semiconductor device 200 corresponds in general to bipolar semiconductor device 100, in FIG. 1. That is to say, semiconductor substrate 202, P type anode layer 210, N type buffer layer 212, and N type drift region 214, in FIG. 2, correspond respectively in general to semiconductor substrate 102, P type anode layer 110, N type buffer layer 112, and N type drift region 114, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above.
In addition, P type inversion region 216, P type contacts 234, and N type cathode diffusions 232, in FIG. 2, correspond respectively in general to P type inversion region 116, P type contacts 134, and N type cathode diffusions 132, in FIG. 1 and may share any of the characteristics attributed to those corresponding features, above. Each of first control trench 220a and second control trench 220b including trench insulator 224 and control trench electrode 226, in FIG. 2, correspond in general to control trench 120 including trench insulator 124 and control trench electrode 126, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above.
Moreover, first and second depletion trenches 222a and 222b, each including trench insulator 224 and depletion electrode 228, correspond respectively in general to first and second depletion trenches 122a and 122b, each including trench insulator 124 and depletion electrode 128, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above. Furthermore, N type enhancement region 240, in FIG. 2, corresponds in general to N type enhancement region 140, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above. In other words, N type enhancement region 240 may have a doping concentration of from approximately 1×1015/cm3 to approximately 1×1016/cm3.
It is noted that, like bipolar semiconductor device 100, in FIG. 1, bipolar semiconductor device 200, in FIG. 2, may take the form of an IGBT. In that implementation, P type anode layer 210 corresponds to a P type collector layer, P type inversion region 216 corresponds to a P type base, and N type cathode diffusions 232 correspond to N type emitter diffusions of the IGBT. Moreover, when bipolar semiconductor device 200 is implemented as an IGBT, first and second control trenches 220a and 220b correspond respectively to first and second gate trenches of the IGBT, each including a gate insulator and a gate electrode corresponding respectively to trench insulator 224 and control trench electrode 226.
The absence of N type enhancement regions 240 from the area between first control trench 220a and second control trench 220b allows the depletion region that begins to form when bipolar semiconductor device 200 is turned off to expand more rapidly. In addition, the Miller capacitance of bipolar semiconductor device 200 is reduced due to the reduced charge between top surface 206 and P type anode layer 210 in the region between first control trench 220a and second control trench 220b. Consequently, EOFF and Td,OFF are substantially improved, i.e., reduced, in the implementation shown in FIG. 2, when compared to conventional bipolar semiconductor devices, such as conventional IGBTs. Moreover, these advantages may be achieved while maintaining the VON of bipolar semiconductor device 200 at a desirable level.
Moving to FIG. 3, FIG. 3 presents a cross-sectional view showing unit cell 350 of exemplary bipolar semiconductor device 300 with sub-cathode enhancement regions, according to yet another implementation. As shown in FIG. 3, bipolar semiconductor device 300 is implemented as a vertical power device including P type anode layer 310 at bottom surface 304 of semiconductor substrate 302, and N type drift region 314 situated over P type anode layer 310. In addition, P type inversion region 316 is situated over N type drift region 314. As further shown in FIG. 3, bipolar semiconductor device 300 includes N type buffer layer 312, as well as N type cathode diffusions 332 and P type contacts 334 formed in P type inversion region 316. It is noted that although single unit cell 350 is shown in FIG. 3 for conceptual clarity, bipolar semiconductor device 300 includes a plurality of unit cells, each of which may include the features attributed to unit cell 350 and described in the present application.
According to the exemplary implementation shown in FIG. 3, unit cell 350 of bipolar semiconductor device 300 includes control trench 320 and first, second, and third depletion trenches 322a, 322b, and 322c, each extending from top surface 306 of semiconductor substrate 302, through P type inversion region 316, and further extending into N type drift region 314. As shown in FIG. 3, control trench 320 is adjacent to N type cathode diffusions 332, and includes trench insulator 324 and control trench electrode 326. As further shown in FIG. 3, each of first depletion trench 322a, second depletion trench 322b, and third depletion trench 322c includes trench insulator 324 and depletion electrode 328, but none of first depletion trench 322a, second depletion trench 322b, or third depletion trench 322c is bordered by cathode diffusions 332.
It is noted that control trench 320 is situated between first depletion trench 322a and second depletion trench 322b. It is further noted that although third depletion trench 322c is shown to be situated adjacent first depletion trench 322a, in FIG. 1, in other implementations, third depletion trench 322c may be situated adjacent second depletion trench 322b. In other words, in some implementations, first depletion trench 322a is situated between control trench and third depletion trench 322c, while in other implementations, second depletion trench 322b is situated between control trench 320 and third depletion trench 322c. 
Unit cell 350 of bipolar semiconductor device 300 also includes N type enhancement region 340. N type enhancement region 340 is localized in N type drift region 314 between control trench 320 and one or both of first depletion trench 322a and second depletion trench 322b. As shown in FIG. 3, N type enhancement region 340 extends under cathode diffusions 332 and may therefore be characterized as a sub-cathode enhancement region.
It is emphasized that although the exemplary implementation shown in FIG. 3 depicts N type enhancement region 340 as localized in N type drift region 314 between control trench 320 and each of first and second depletion trenches 322a and 322b, in other implementations, N type enhancement region 340 may be localized in N type drift region 314 between control trench 320 and one, but not both of, first and second depletion trenches 322a and 322b. It is further emphasized that N type enhancement region 340 is absent from regions of N type drift region 314 that do not lie between control trench 320 and first depletion trench 322a, and from regions of N type drift region 314 that do not lie between control trench 320 and second depletion trench 322b. That is to say, N type enhancement region 340 is absent between third depletion trench 322c and first depletion trench 322a. Furthermore, in implementations in which third depletion trench 322c is situated adjacent second depletion trench 322b, N type enhancement region 340 is absent between third depletion trench 322c and second depletion trench 322b. 
Bipolar semiconductor device 300 corresponds in general to bipolar semiconductor device 100, in FIG. 1. That is to say, semiconductor substrate 302, P type anode layer 310, N type buffer layer 312, and N type drift region 314, in FIG. 3, correspond respectively in general to semiconductor substrate 102, P type anode layer 110, N type buffer layer 112, and N type drift region 114, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above.
In addition, P type inversion region 316, P type contacts 334, and N type cathode diffusions 332, in FIG. 3, correspond respectively in general to P type inversion region 116, P type contacts 134, and N type cathode diffusions 132, in FIG. 1 and may share any of the characteristics attributed to those corresponding features, above. Control trench 320 including trench insulator 324 and control trench electrode 326, in FIG. 3, corresponds in general to control trench 120 including trench insulator 124 and control trench electrode 126, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above.
Moreover, first and second depletion trenches 322a and 322b, each including trench insulator 324 and depletion electrode 328, correspond respectively in general to first and second depletion trenches 122a and 122b, each including trench insulator 124 and depletion electrode 128, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above. In addition, third depletion trench 322c including trench insulator 324 and depletion electrode 328, corresponds in general to either of first or second depletion trenches 122a and 122b including trench insulator 124 and depletion electrode 128, in FIG. 1.
N type enhancement region 340, in FIG. 3, corresponds in general to N type enhancement region 140, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above. In other words, N type enhancement region 340 may have a doping concentration of from approximately 1×1015/cm3 to approximately 1×1016/cm3.
It is noted that, like bipolar semiconductor device 100, in FIG. 1, bipolar semiconductor device 300, in FIG. 3, may take the form of an IGBT. In that implementation, P type anode layer 310 corresponds to a P type collector layer, P type inversion region 316 corresponds to a P type base, and N type cathode diffusions 332 correspond to N type emitter diffusions of the IGBT. Moreover, when bipolar semiconductor device 300 is implemented as an IGBT, control trench 320 corresponds to a gate trench of the IGBT, including a gate insulator and a gate electrode corresponding respectively to trench insulator 324 and control trench electrode 326.
The absence of N type enhancement region 340 from the area between third depletion trench 322c and first depletion trench 322a allows the depletion region that begins to form when bipolar semiconductor device 300 is turned off to expand more rapidly. In addition, the Miller capacitance of bipolar semiconductor device 300 is reduced due to the overall reduction in charge between top surface 306 and P type anode layer 310. Consequently, EOFF and Td,OFF are substantially improved, i.e., reduced, in the implementation shown in FIG. 3, when compared to conventional bipolar semiconductor devices, such as conventional IGBTs. Moreover, these advantages may be achieved while maintaining the VON of bipolar semiconductor device 300 at a desirable level.
Referring now to FIG. 4, FIG. 4 presents a cross-sectional view showing unit cell 450 of exemplary bipolar semiconductor device 400 with sub-cathode enhancement regions, according to a further implementation. As shown in FIG. 4, bipolar semiconductor device 400 is implemented as a vertical power device including P type anode layer 410 at bottom surface 404 of semiconductor substrate 402, and N type drift region 414 situated over P type anode layer 410. In addition, P type inversion region 416 is situated over N type drift region 414. As further shown in FIG. 4, bipolar semiconductor device 400 includes N type buffer layer 412, as well as N type cathode diffusions 432 and P type contacts 434 formed in P type inversion region 416. It is noted that although single unit cell 450 is shown in FIG. 4 for conceptual clarity, bipolar semiconductor device 400 includes a plurality of unit cells, each of which may include the features attributed to unit cell 450 and described in the present application.
According to the exemplary implementation shown in FIG. 4, unit cell 450 of bipolar semiconductor device 400 includes first control trench 420a and second control trench 420b, each extending from top surface 406 of semiconductor substrate 402, through P type inversion region 416, and further extending into N type drift region 414. As shown in FIG. 4, each of first control trench 420a and second control trench 420b is adjacent to N type cathode diffusions 432, and includes trench insulator 424 and control trench electrode 426.
In addition, unit cell 450 of bipolar semiconductor device 400 includes first, second, and third depletion trenches 422a, 422b, and 422c. As further shown in FIG. 4, each of first depletion trench 422a, second depletion trench 422b, and third depletion trench 422c includes trench insulator 424 and depletion electrode 428, but none of first depletion trench 422a, second depletion trench 422b, or third depletion trench 422c is bordered by cathode diffusions 432. It is noted that both of first control trench 420a and second control trench 420b are situated between first and second depletion trenches 422a and 422b, while third depletion trench 422c is situated adjacent one of first depletion trench 422a or second depletion trench 422b. That is to say, in some implementations, first depletion trench 422a is situated between first control trench 420a and third depletion trench 422c, while in other implementations, second depletion trench 422b is situated between second control trench 420b and third depletion trench 422c. 
Unit cell 450 of bipolar semiconductor device 400 also includes N type enhancement region 440. N type enhancement region 440 is localized in N type drift region 414 between first depletion trench 422a and first control trench 420a, and between second control trench 420b and second depletion trench 422b. As shown in FIG. 4, N type enhancement region 440 extends under cathode diffusions 432 and may therefore be characterized as a sub-cathode enhancement region. Moreover, and as further shown in FIG. 4, in some implementations, N type enhancement region 440 adjoins each of first control trench 420a, first depletion trench 422a, second control trench 420b, and second depletion trench 422b. 
It is emphasized, however, that N type enhancement region 440 is localized so as to be confined between a control trench and its closest adjacent depletion trench. As a result, no N type enhancement region 440 is present between first control trench 420a and second control trench 420b. It is further emphasized that N type enhancement region 440 is absent between third depletion trench 422c and first depletion trench 422a. Furthermore, in implementations in which third depletion trench 422c is situated adjacent second depletion trench 422b, N type enhancement region 440 is absent between second depletion trench 422b and third depletion trench 422c. 
Bipolar semiconductor device 400 corresponds in general to bipolar semiconductor device 100, in FIG. 1. That is to say, semiconductor substrate 402, P type anode layer 410, N type buffer layer 412, and N type drift region 414, in FIG. 4, correspond respectively in general to semiconductor substrate 102, P type anode layer 110, N type buffer layer 112, and N type drift region 114, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above.
In addition, P type inversion region 416, P type contacts 434, and N type cathode diffusions 432, in FIG. 4, correspond respectively in general to P type inversion region 116, P type contacts 134, and N type cathode diffusions 132, in FIG. 1 and may share any of the characteristics attributed to those corresponding features, above. Each of first control trench 420a and second control trench 420b including trench insulator 424 and control trench electrode 426, in FIG. 4, correspond in general to control trench 120 including trench insulator 124 and control trench electrode 126, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above.
Moreover, first and second depletion trenches 422a and 422b, each including trench insulator 424 and depletion electrode 428, correspond respectively in general to first and second depletion trenches 122a and 122b, each including trench insulator 124 and depletion electrode 128, in FIG. 1, and may share any of the characteristics attributed to those corresponding features, above. In addition, third depletion trench 422c including trench insulator 424 and depletion electrode 428, corresponds in general to either of first or second depletion trenches 122a and 122b including trench insulator 124 and depletion electrode 128, in FIG. 1.
Furthermore, N type enhancement region 440, in FIG. 4, corresponds in general to N type enhancement region 140, in FIG. 1, and may share any of the characteristics attributed to that corresponding feature, above. In other words, N type enhancement region 440 may have a doping concentration of from approximately 1×1015/cm3 to approximately 1×1016/cm3.
It is noted that, like bipolar semiconductor device 100, in FIG. 1, bipolar semiconductor device 400, in FIG. 4, may take the form of an IGBT. In that implementation, P type anode layer 410 corresponds to a P type collector layer, P type inversion region 416 corresponds to a P type base, and N type cathode diffusions 432 correspond to N type emitter diffusions of the IGBT. Moreover, when bipolar semiconductor device 400 is implemented as an IGBT, first and second control trenches 420a and 420b correspond respectively to first and second gate trenches of the IGBT, each including a gate insulator and a gate electrode corresponding respectively to trench insulator 424 and control trench electrode 426.
The absence of N type enhancement region 440 from the area between first control trench 420a and second control trench 420b, and between third depletion trench 422c and either of first and second depletion trenches 422a and 422b allows the depletion region that begins to form when bipolar semiconductor device 400 is turned off to expand more rapidly. In addition, the Miller capacitance of bipolar semiconductor device 400 is reduced. Consequently, EOFF and Td,OFF are substantially improved, i.e., reduced, in the implementation shown in FIG. 4, when compared to conventional bipolar semiconductor devices.
Thus, the present application discloses implementations of a bipolar semiconductor device with sub-cathode enhancement regions. As disclosed in the present application, by localizing or confining enhancement regions between a control trench and its closest adjacent depletion trench, the present solution enables a bipolar semiconductor device to have lower EOFF and shorter Td,OFF when compared to conventional devices, such as conventional IGBTs. Moreover, these advantages may be achieved while maintaining the VON of the bipolar semiconductor device at a desirable level.
From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described herein, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.

## Claims

1. A bipolar semiconductor device comprising a plurality of unit cells, each of said plurality of unit cells comprising:
a drift region having a first conductivity type situated over an anode layer having a second conductivity type opposite said first conductivity type;
first and second depletion trenches, each having a depletion electrode;
a first control trench situated between said first and second depletion trenches, said first control trench extending into said drift region and being adjacent to cathode diffusions;
an enhancement region having said first conductivity type localized in said drift region between said first control trench and at least one of said first and second depletion trenches,
a third depletion trench adjacent to one of said first and second depletion trenches, wherein said enhancement region does not extend between said third depletion trench and either of said first and second depletion trenches,
wherein said first control trench extends deeper into said drift region than said enhancement region.

2. The bipolar semiconductor device of claim 1, wherein said enhancement region extends between said first control trench and each of said first and second depletion trenches.

3. The bipolar semiconductor device of claim 1, wherein a doping concentration of said enhancement region is greater than a doping concentration of said drift region.

4. The bipolar semiconductor device of claim 1, wherein a doping concentration of said enhancement region is less than a doping concentration of said cathode diffusions.

5. The bipolar semiconductor device of claim 1, wherein said depletion electrodes are electrically coupled to said cathode diffusions.

6. The bipolar semiconductor device of claim 1, wherein each of said plurality of unit cells further comprises a buffer layer having said first conductivity type situated between said anode layer and said drift region.

7. The bipolar semiconductor device of claim 1, wherein said first conductivity is N type and said second conductivity is P type.

8. The bipolar semiconductor device of claim 1, wherein each of said plurality of unit cells further comprises a second control trench situated between said first and second depletion trenches, said enhancement region extending between said second control trench and one of said first and second depletion trenches.

9. The bipolar semiconductor device of claim 8, wherein each of said plurality of unit cells further comprises a third depletion trench adjacent to one of said first and second depletion trenches, wherein said enhancement region does not extend between said third depletion trench and either of said first and second depletion trenches.

10. An insulated-gate bipolar transistor (IGBT) comprising a plurality of IGBT unit cells, each of said plurality of IGBT unit cells comprising:
a drift region having a first conductivity type situated over a collector having a second conductivity type opposite said first conductivity type;
first and second depletion trenches, each having a depletion electrode;
a first gate trench situated between said first and second depletion trenches, said first gate trench extending into said drift region and being adjacent to emitter diffusions;
an enhancement region having said first conductivity type localized in said drift region between said first gate trench and at least one of said first and second depletion trenches,
a third depletion trench adjacent to one of said first and second depletion trenches, wherein said enhancement region does not extend between said third depletion trench and either of said first and second depletion trenches,
wherein said first gate trench extends deeper into said drift region than said enhancement region.

11. The IGBT of claim 10, wherein said enhancement region extends between said first gate trench and each of said first and second depletion trenches.

12. The IGBT of claim 10, wherein a doping concentration of said enhancement region is greater than a doping concentration of said drift region.

13. The IGBT of claim 10, wherein a doping concentration of said enhancement region is less than a doping concentration of said emitter diffusions.

14. The IGBT of claim 10, wherein said depletion electrodes are electrically coupled to said emitter diffusions.

15. The IGBT of claim 10, wherein each of said plurality of IGBT unit cells further comprises a buffer layer having said first conductivity type situated between said collector and said drift region.

16. The IGBT of claim 10, wherein said first conductivity is N type and said second conductivity is P type.

17. The IGBT of claim 10, wherein each of said plurality of IGBT unit cells further comprises a second gate trench situated between said first and second depletion trenches, said enhancement region extending between said second gate trench and one of said first and second depletion trenches.

18. The IGBT of claim 17, wherein each of said plurality of IGBT unit cells further comprises a third depletion trench adjacent to one of said first and second depletion trenches, wherein said enhancement region does not extend between said third depletion trench and either of said first and second depletion trenches.