OVER-VOLTAGE PROTECTION

Related Applications

[0001] This application claims the benefit of U.S. patent application number 13/957,698, filed August 2, 2013, which claims the benefit of U.S. provisional patent application number 61 /692,768, filed August 24, 2012, the disclosure of which is incorporated herein by reference in its entirety. Field of the Disclosure

[0002] The present disclosure relates to electronic devices that include overvoltage and current surge protection.

Background

[0003] Gallium nitride (GaN) is commonly cited as a superior material for high- voltage power devices due to its wide bandgap and associated high electric field required for avalanche breakdown. Ideal bulk GaN crystals have critical fields in excess of 3,000,000 V per centimeter. However, in practice, a high electric field needed for avalanche breakdown is lowered by non-idealities that are present within the structure of a GaN device. During high voltage operation of a GaN device, electrical breakdown will typically occur at defects and/or at locations with a concentrated electric field. An example of such a breakdown location is a corner of a Schottky gate. An ideal structure comprising a bulk crystal such as silicon carbide (SiC) or GaN will avalanche uniformly in a high electric field region. As a result, avalanche energy is distributed uniformly, which greatly enhances the survivability of a device made up of an ideal bulk crystal. For example, vertical p-n junctions fabricated in SiC homoepitaxial layers

demonstrate avalanche breakdown ruggedness. However, breakdown in defective GaN layers will typically occur at defects within defective GaN layers. A resulting high energy density typically causes irreversible damage to a device that includes defective GaN layers. [0004] Another factor impacting breakdown ruggedness is the nature of the metal semiconductor contacts that carry a breakdown current. Previous work with SiC Schottky diodes has demonstrated that Schottky contacts can be degraded by avalanche current. In response to this problem, junction barrier Schottky diodes have been developed to urge avalanche breakdown to occur across a bulk p-n junction with ohmic contacts rather than Schottky contacts. Thus, the breakdown ruggedness of GaN high electron mobility transistors (HEMTs) may be limited by breakdown events in highly localized areas within a semiconductor due to crystal defects and/or electric field concentration.

Moreover, the breakdown ruggedness of GaN HEMTs may be limited by an electrical breakdown of adjacent dielectric layers and/or high current flow through the Schottky gate electrode during breakdown events. Thus, there is a need to provide overvoltage protection for a GaN device to ensure that the GaN device handles a typically destructive breakdown voltage without being damaged.

Summary

[0005] A gallium nitride (GaN) device with leakage current-based over-voltage protection is disclosed. The GaN device includes a drain and a source disposed on a semiconductor substrate. The GaN device also includes a first channel region within the semiconductor substrate and between the drain and the source. The GaN device further includes a second channel region within the

semiconductor substrate and between the drain in the source. The second channel region has an enhanced drain induced barrier lowering (DIBL) that is greater than the DIBL of the first channel region. As a result, a drain voltage will be safely clamped below a destructive breakdown voltage once a substantial drain current begins to flow through the second channel region.

[0006] Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings. Brief Description of the Drawings

[0007] The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.

[0008] Figure 1 is a graph of a prior art transfer curve of drain current versus gate voltage illustrating the effect of drain induced barrier lowering (DIBL).

[0009] Figure 2 is a circuit schematic that represents a model of a transistor having a first channel region and a second channel region in which the second channel region has an enhanced DIBL in accordance with the present disclosure.

[0010] Figure 3 is a plan view of an exemplary layout for the transistor of Figure 2.

[0011] Figure 4 is a graph of an exemplary transfer curve of drain current versus gate voltage comparing a first DIBL effect for a typical first channel region to a second DIBL effect for a second channel region that has an enhanced DIBL in accordance with the present disclosure.

[0012] Figure 5 depicts a cross-sectional view of a first segmented gate embodiment having a Schottky gate with an insulator region between gate segments.

[0013] Figure 6 depicts a cross-sectional view of a second segmented gate embodiment having an insulated gate with an insulator between gate segments and between the gate segments and a semiconductor substrate.

[0014] Figure 7 depicts a cross-sectional view of a third segmented gate embodiment having a Schottky gate with an insulator between gate segments connected by a gate strap.

[0015] Figure 8 depicts a cross-sectional view of a fourth segmented gate embodiment having an insulated gate with the insulator between gate segments and a semiconductor region, and between a gate strap and the semiconductor substrate. Detailed Description

[0016] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0017] It will be understood that when an element such as a layer, region, or substrate is referred to as being "over," "on," "in," or extending "onto" another element, it can be directly over, directly on, directly in, or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly over," "directly on," "directly in," or extending "directly onto" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

[0018] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.

[0019] Figure 1 is a graph of a prior art transfer curve of drain current versus gate voltage illustrating the effect of drain induced barrier lowering (DIBL). In Figure 1 , the drain current is plotted on a logarithmic scale. At a high drain bias voltage, the transfer curve shifts to the left reflecting the lower threshold voltage. DIBL is defined as a ratio of AV to AV _D , wherein AV is how much the transfer curve shifts and AV _D is a change in drain bias voltage. Physically, DIBL is associated with drain bias voltage V _D , which lowers a barrier that prevents electrons from flowing from a source and a drain of a transistor, when the transistor is in a non-conducting off state.

[0020] Figure 2 is a schematic of a circuit model that represents a transistor 10 having a first channel region 12 and a second channel region 14 in which the second channel region 14 has an enhanced DIBL in accordance with the present disclosure. The transistor 10 is modeled using a symbolic transistor M1 in parallel with a symbolic transistor M2. However, it is to be understood that the first channel region 12 and the second channel region 14 can comprise a same channel periphery of the transistor 10. Moreover, the second channel region 14 has an enhanced DIBL that is greater than the DIBL of the first channel region 12. The enhanced DIBL is symbolically represented by the symbol for the transistor M2 being smaller than the symbol for the transistor M1 . In this way, the first channel region 12 is designated as a main channel and the second channel region 14 is designated as a protective channel that leaks current once a protective channel drain voltage V _PC is reached. The protective channel drain voltage V _PC is below a voltage that is potentially destructive for the transistor 10.

[0021] Figure 3 is a plan view of an exemplary layout for the transistor 10. The first channel region 12 and the second channel region 14 span between a drain D1 to a source S1 . In Figure 3, the first channel region 12 and the second channel region 14 are shown as being separated and as being approximately the same size. In this case, the second channel region 14 makes up around about 50% of the total channel periphery that includes both the first channel region 12 and the second channel region 14. However, it is to be understood that the first channel region 12 and the second channel region 14 can overlap and be one in the same. In that case, the second channel region 14 would make up 100% of a total channel periphery. In other embodiments, the second channel region 14 accounts for a range of around about 0.1 % to around about 10% of the total channel periphery made up of the first channel region 12 and the second channel region 14. In yet other embodiments, the second channel region 14 accounts for around about 10% to around about 50% of the total channel periphery made up of the first channel region 12 and the second channel region 14. In still further embodiments, the second channel region 14 accounts from around about 50% to around about 100% of the total channel periphery made up of the first channel region 12 and the second channel region 14. In some of these embodiments, the first channel region 12 and the second channel region 14 are separate, while in other of these embodiments the first channel region 12 and the second channel region 14 overlap.

[0022] Figure 4 is a graph of an exemplary transfer curve of drain current versus gate voltage comparing a first DIBL effect for a typical first channel region to a second DIBL effect for a second channel region that has an enhanced DIBL in accordance with the present disclosure. In particular, Figure 4 represents a case in which a gate periphery of the second channel region 14 has a relatively high DIBL with respect to a relatively low DIBL of a gate periphery of the first channel region 12. For example, the DIBL of the gate periphery of the first channel region 12 has a negligible DIBL of less than around about 0.001 V/V, which is typical for a DIBL of a traditional GaN HEMT. In contrast, the DIBL of the gate periphery for the second channel region 14 ranges from around about 0.003 V/V to around about 0.03 V/V. As a result, at a high V _D , the second channel region 14 will conduct more current than the first channel region 12 as V _D effectively turns on the second channel region 14. The DIBL of the second channel region 14 is enhanced such that it begins to turn on at a V _D that is less than a destructive breakdown voltage of the first channel region 12. Thus, V _D will be safely clamped below the destructive breakdown voltage once a substantial drain current begins to flow through the second channel region 14. In the disclosed embodiments, enhanced DIBL is defined as AV/AV _PC , where AV is a threshold voltage shift and V _PC is a drain voltage at which the second channel region 14 begins to conduct current. A AV for an enhanced DIBL GaN HEMT of the present disclosure ranges from around about 5 V to around about 15 V. In this case, AV is approximately a difference between the unshifted threshold voltage at low V _D and a voltage of a negative voltage rail. A V _PC for the second channel region 14 of an enhanced DIBL GaN HEMT of the present disclosure ranges from around about 600 V to around about 1400 V.

[0023] In exemplary embodiments, the DIBL of the second channel region 14 is enhanced by segmenting a gate G1 near the second channel region 14, and/or decreasing the gate length, and/or changing the dimensions of a field plate near the second channel region 14, and/or changing the spacing between the gate G1 and the drain D1 near the second channel region 14, and/or altering an epitaxial layer stack including doping near the second channel region 14. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth therein. Rather, these exemplary embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. As such, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments.

[0024] Referring back to Figure 3, segmenting the gate G1 at a location along a gate axis 1 6 near the second channel region 14 will enhance the DIBL for the second channel region 14. Figures 5 through 8 depict four exemplary

segmented gate embodiments for enhancing the DIBL of the second channel region 14. Each of Figures 5 through 8 represents a cross-sectional view cut along the gate axis 1 6. The drain D1 and the source S1 shown in Figure 3 are not shown in Figures 5 through 8 because the drain D1 and the source S1 are located on either side of the gate G1 , and showing them in Figures 5 through 8 would obscure the details of the segmented gate embodiments.

[0025] Figure 5 depicts a cross-sectional view of a first segmented gate embodiment with an insulator 18 between a first gate segment G1 A and a second gate segment G1 B comprising the gate G1 . Moreover, the first gate segment G1 A and the second gate segment G1 B are coupled electrically by a conductor that is not visible in the cross-sectional view of Figure 5. In this particular embodiment, the gate G1 is in Schottky contact with a semiconductor substrate 20, and an amount of DIBL enhancement for the second channel region 14 is controlled by an interval L between the first gate segment G1 A and the second gate segment G1 B.

[0026] Figure 6 depicts a cross-sectional view of a second segmented gate embodiment wherein the gate G1 has the insulator 18 extending between the first gate segment G1 A and the second gate segment G1 B. Further still, the insulator 18 separates the first gate segment G1 A and the second gate segment G1 B from the semiconductor substrate 20. Thus, the gate G1 in this segmented embodiment is an insulated gate. Similar to the first segmented gate

embodiment, the amount of DIBL enhancement for the second channel region 14 is controlled by the interval L between the first gate segment G1 A and the second gate segment G1 B. However, in this second embodiment, the DIBL of the second channel region 14 can also be controlled locally by varying a thickness Ti of the insulator 18 separating the first gate segment G1 A and the second gate segment G1 B from the semiconductor substrate 20.

[0027] Figure 7 depicts a cross-sectional view of a third segmented gate embodiment having a gate strap 22 that electrically couples the first gate segment G1 A to the second gate segment G1 B that are in direct contact with the semiconductor substrate 20. Thus, the gate G1 is a Schottky gate in this third segmented gate embodiment. Similar to the first and second segmented gate embodiments, the amount of DIBL enhancement for the second channel region 14 is controlled by the interval L between the first gate segment G1 A and the second gate segment G1 B. However, in this third segmented gate embodiment, the DIBL of the second channel region 14 can also be controlled locally by varying a distance D _G s between the gate strap 22 and semiconductor substrate 20.

[0028] Figure 8 depicts a cross-sectional view of a fourth segmented gate embodiment wherein the insulator 18 extends between the semiconductor substrate 20 and the first gate segment G1 A and the second gate segment G1 B, thereby making the gate G1 an insulated gate. The insulator 18 also extends between the gate strap 22 and the semiconductor substrate 20. Similar to the first, second, and third segmented gate embodiments, the amount of DIBL enhancement for the second channel region 14 is controlled by the interval L between the first gate segment G1 A and the second gate segment G1 B.

However, in this fourth segmented gate embodiment, the DIBL of the second channel region 14 can also be controlled locally by varying the distance D _G s of the gate strap 22 and by varying the thickness Ti of the insulator 18 separating the first gate segment G1 A and the second gate segment G1 B from the semiconductor substrate 20.

[0029] In each of the four exemplary segmented gate embodiments, the first channel region 12 extends into the semiconductor substrate 20 adjacent to both the first gate segment G1 A and the second gate segment G1 B. The second channel region 14 extends into the semiconductor substrate 20 and being sandwiched between the first channel region 12. Each of the four exemplary segmented gate embodiments depicted in Figures 5 through 8 have unique electrical characteristics due to each particular structure made up of the gate G1 , the insulator 18, and the semiconductor substrate 20. The electrical

characteristics are also tunable by varying the dopant levels of the

semiconductor substrate adjacent to the second channel region 14. The unique electrical characteristics affect the DIBL of the second channel region 14 and can be altered to suit a particular application. Also, while the exemplary segmented gate embodiments of Figures 5 through 8 show only the first gate segment G1 A and the second segment G1 B, it is to be understood that additional gate segments can be included without deviating from the scope of the present disclosure. Moreover, while the exemplary embodiments of the present disclosure are directed to GaN transistors, it is to be understood that other GaN devices such as GaN diodes and GaN thyristors can include a main channel region and a protective channel region as described in the present disclosure.

[0030] Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.