Low-resistance electrode design

A solution for designing a semiconductor device, in which two or more attributes of a pair of electrodes are determined to, for example, minimize resistance between the electrodes, is provided. Each electrode can include a current feeding contact from which multiple fingers extend, which are interdigitated with the fingers of the other electrode in an alternating pattern. The attributes can include a target depth of each finger, a target effective width of each pair of adjacent fingers, and one or more target attributes of the current feeding contacts. Subsequently, the device and/or a circuit including the device can be fabricated.

TECHNICAL FIELD

The disclosure relates generally to semiconductor devices, and more particularly, to an improved electrode design for semiconductor devices.

BACKGROUND ART

On state resistance is an important characteristic of a semiconductor device, such as a semiconductor device used in any of various switching applications. A low on state resistance in a planar device, such as a field effect transistor (FET) or a diode, is generally achieved by the use of multi-finger structures for pairs of electrodes. The multi-finger structures increase the total periphery of each electrode.

The development and implementation of devices with a semiconductor structure with extremely low sheet resistance, e.g., 200-300 Ohms/square, presents new considerations in the design of devices with low on state resistance. For example, when a low on state resistance device is manufactured using semiconductor layer(s) with very low sheet resistance, the resistance of the semiconductor layer becomes comparable to that of the metal electrodes supplying the current to the device. In this case, the current density along the device finger becomes non-uniform. As a result, the device resistance does not decrease inversely proportionally to the finger width (e.g., the size of the dimension of the finger that is perpendicular to the direction of the current flow between the electrodes). Instead, as the finger width is increased, the device resistance per unit finger width first decreases, flattens over a range of widths, and then increases as the width is further increased.

SUMMARY OF THE INVENTION

Aspects of the invention provide a solution for designing a semiconductor device, in which two or more attributes of a pair of electrodes are determined to, for example, minimize resistance between the electrodes. Each electrode can include a current feeding contact from which multiple fingers extend, the fingers are interdigitated with the fingers of the other electrode in an alternating pattern. The attributes can include a target depth of each finger, a target effective width of each pair of adjacent fingers, and one or more target attributes of the current feeding contacts. Subsequently, the device and/or a circuit including the device can be fabricated. In this manner, a low total device impedance can be achieved.

A first aspect of the invention provides a method of designing a semiconductor device, the method comprising: configuring a plurality of attributes of an interface between a first electrode to a semiconductor structure of the semiconductor device and a second electrode to the semiconductor structure to substantially minimize a total resistance for the semiconductor device, wherein each electrode includes a current feeding contact and a plurality of fingers extending therefrom, and wherein the plurality of fingers of the first and second electrodes are adjacent to each other in an alternating pattern, the configuring including: determining a target depth for each of the plurality of fingers based on a characteristic contact transfer length for a junction between the fingers and the semiconductor device; determining a target effective width of each pair of adjacent fingers of the first electrode and the second electrode based on the target depth, an impedance of at least one finger per unit width and an impedance of a portion of the semiconductor structure between the pair of adjacent fingers per unit width; and determining at least one target attribute of the current feeding contact of each of the first and second electrodes based on the target depth, the target effective width, an impedance of the current feeding contact per unit width and an impedance of a pair of adjacent fingers with the semiconductor structure there between per unit width.

A second aspect of the invention provides a method of fabricating a semiconductor device, the method comprising: designing the semiconductor device, wherein the designing includes configuring a plurality of attributes of an interface between a first electrode to a semiconductor structure of the semiconductor device and a second electrode to the semiconductor structure to substantially minimize a total resistance for the semiconductor device, wherein each electrode includes a current feeding contact and a plurality of fingers extending therefrom, and wherein the plurality of fingers of the first and second electrodes are adjacent to each other in an alternating pattern, the configuring including: determining a target depth for each of the plurality of fingers based on a characteristic contact transfer length for a junction between the fingers and the semiconductor device; determining a target effective width of each pair of adjacent fingers of the first electrode and the second electrode based on the target depth, an impedance of at least one finger per unit width and an impedance of a portion of the semiconductor structure between the pair of adjacent fingers per unit width; and determining at least one target attribute of the current feeding contact of each of the first and second electrodes based on the target depth, the target effective width, an impedance of the current feeding contact per unit width and an impedance of a pair of adjacent fingers with the semiconductor structure there between per unit width; and forming each of the first and second electrodes on the semiconductor structure according to the design.

A third aspect of the invention provides a computer system comprising: a design system configured to generate a device design for a semiconductor device by performing a method comprising: configuring a plurality of attributes of an interface between a first electrode to a semiconductor structure of the semiconductor device and a second electrode to the semiconductor structure to substantially minimize a total resistance for the semiconductor device, wherein each electrode includes a current feeding contact and a plurality of fingers extending therefrom, and wherein the plurality of fingers of the first and second electrodes are adjacent to each other in an alternating pattern, the configuring including: determining a target depth for each of the plurality of fingers based on a characteristic contact transfer length for a junction between the fingers and the semiconductor device; determining a target effective width of each pair of adjacent fingers of the first electrode and the second electrode based on the target depth, an impedance of at least one finger per unit width and an impedance of a portion of the semiconductor structure between the pair of adjacent fingers per unit width; and determining at least one target attribute of the current feeding contact of each of the first and second electrodes based on the target depth, the target effective width, an impedance of the current feeding contact per unit width and an impedance of a pair of adjacent fingers with the semiconductor structure there between per unit width.

Other aspects of the invention provide methods, systems, program products, and methods of using and generating each, which include and/or implement some or all of the actions described herein. The illustrative aspects of the invention are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a solution for designing a semiconductor device, in which two or more attributes of a pair of electrodes are determined to, for example, minimize resistance between the electrodes. Each electrode can include a current feeding contact from which multiple fingers extend, the fingers are interdigitated with the fingers of the other electrode in an alternating pattern. The attributes can include a target depth of each finger, a target effective width of each pair of adjacent fingers, and one or more target attributes of the current feeding contacts. Subsequently, the device and/or a circuit including the device can be fabricated. In this manner, a low total device impedance can be achieved. As used herein, unless otherwise noted, the term “set” means one or more (i.e., at least one) and the phrase “any solution” means any now known or later developed solution.

Turning to the drawings,FIGS. 1A and 1Bshow top and side views, respectively, of an illustrative device10according to an embodiment. In general, device10comprises a planar semiconductor device with a lateral layout. To this extent, device10can comprise a highly resistive substrate12with a semiconductor structure14comprising one or more semiconducting layers formed thereon. Substrate12can comprise any type of substrate, such as sapphire, silicon, silicon carbide, or any other semiconductor or dielectric materials. Semiconductor structure14can comprise, for example: a group-III nitride heterostructure, which includes two or more layers of materials selected from the group-III nitride material system (e.g., AlXInYGa1-X-YN, where 0≦X, Y≦1, and X+Y≦1 and/or alloys thereof); a group-III arsenide heterostructure, which includes two or more layers of materials selected from the group-III arsenide material system (e.g., AlXGa1-XAs, where 0≦X≦1 and/or alloys thereof); and/or the like.

Device10is shown including a pair of electrodes20,22. In order to design the semiconductor device10, a type of contact to the semiconductor structure14can be selected for each electrode20,22using any solution. Each electrode20,22can comprise, for example, a metal, and form any type of contact to the semiconductor structure14. For example, each electrode20,22can form an ohmic contact, a capacitively-coupled contact, a composite conducting capacitive coupled contact, and/or the like to the semiconductor structure14. Electrodes20,22can form the same type of contact or different types of contacts to the semiconductor structure14for a particular device10, depending on the desired application and/or operating characteristics for the device10.

Also as part of designing the semiconductor device10, a shape configuration of the electrodes20,22and the corresponding locations of the electrodes20,22can be designed. The shape configuration can be selected/designed to provide one or more benefits to the operation of the semiconductor device10using any solution. For example, as illustrated, each electrode20,22can comprise a multi-finger electrode having a comb configuration, which includes multiple fingers, such as fingers20A-20C,22A-22C, respectively, electrically connected in parallel by a current feeding contact24A,24B from which each finger20A-20C,22A-22C, respectively extends. The respective fingers20A-20C,22A-22C of each electrode20,22can be physically arranged in an alternating manner (e.g., interdigitated). In operation, current can flow from one electrode20,22to the other electrode20,22. For example, electrode20can comprise a source contact, while electrode22comprises a drain contact for the device10. In this case, under forward bias operating conditions of device10, current can flow from the fingers20A-20C of electrode20to the fingers22A-22C of electrode22via the semiconductor structure14. To this extent, the current can enter fingers20A-20C via current feeding contact24A and leave fingers22A-22C via current feeding contact24B.

FIG. 1Cshows a top view of an illustrative pair of adjacent fingers20A,22A according to an embodiment. As illustrated, each finger20A,22A comprises a corresponding depth, dFING, and each pair of fingers20A,22A on electrodes20,22comprises a corresponding effective width, W. The effective width W of each pair of fingers20A,22A is a size of the dimension of the pair of fingers20A,22A that is perpendicular to the current flow direction between the electrodes20,22. Since the current flows between adjacent fingers20A,22A primarily in a horizontal direction inFIGS. 1A-1C, the effective width W can correspond to the portions of the vertical lengths of the fingers20A,22A that are horizontally adjacent to one another. Moreover, the semiconductor structure14(FIG. 1A) separating each pair of fingers20A,22A has a corresponding length, LSC, over which the current travels. Additional measurements of electrodes20,22include a depth of the current feeding contact24A,24B, dCMB, of each electrode and a total length of the comb interface between the two electrodes20,22, LCMB, as shown inFIG. 1A.

An embodiment of the invention provides a solution for designing and/or fabricating electrodes20,22of a semiconductor device10to achieve a lower total resistance for device10, which includes determining one or more target attributes of the electrodes20,22. For example, a target effective width W of the respective electrodes20,22can be determined according to an embodiment. Subsequently, the design of the semiconductor device10can include configuring the electrodes20,22according to the target effective width W. For example, one or more attributes of the electrodes20,22can be selected according to the target effective width W.

When the electrodes20,22comprise multi-finger electrodes as shown inFIGS. 1A,1B, the effective width W of each electrode20,22corresponds to the effective width W of the respective pairs of fingers20A,22A and20B,22B. The target effective width W can be selected to reduce the impedance of the device10, e.g., by reducing non-uniformity of the current along the electrodes20,22in a direction that is perpendicular to the current flow between the electrodes20,22. Such non-uniformity can occur due to a finite resistance of the electrodes20,22deposited over a highly-conducting semiconductor structure14. In an illustrative embodiment, the target effective width W of electrodes20,22corresponds to the effective width at which the impedance of the device10is close to the impedance obtained with ideally conducting metal electrodes20,22.

FIG. 2shows an illustrative diagram of the dependence of the impedance of a device, such as device10(FIG. 1), on the effective width W of the electrodes20A,22A according to an embodiment. As illustrated in the inset of the diagram, the input current can enter the top electrode20A from the left and leave the bottom electrode22A from the right. If a resistance of the metal of each electrode20A,22A is zero, the top electrode20A and bottom electrode22A would be equipotential, and the current density through the semiconductor structure14(FIG. 1) between the electrodes20A,22A would be uniform from left to right in the inset. However, due to a finite resistance of the metal of the electrodes20A,22A, the current creates a voltage drop along the electrodes20A,22A causing a current density to decrease in the central part of the electrodes20A,22A.

The non-uniformity of the current along the effective width W of the electrodes20A,22A can be derived from the signal propagation along a distributed line formed by a series impedance of the electrodes20A,22A and a shunting admittance of the semiconductor structure14connected between the electrodes20A,22A. From the general transmission line theory, the propagation constant in such a Z-Y line is γFING=√{square root over (Z1Y1)}, where Z1and Y1are the line series impedance and shunting admittance per unit length, respectively. For the transmission line described herein, Z1=2×ZFINGand Y1=YSC=1/ZSC, where ZFINGis an impedance of a metal electrode20A,22A per unit width and ZSCis an impedance of the semiconducting structure14between the electrodes20A,22A per unit width. ZFINGcan be calculated, for example, using the formula ZFING=RSHFING/dFING, where RSHFINGis the sheet resistance of a finger and dFINGis the depth of the finger. A factor of two can be used in calculating Z1to account for the resistances of the two electrodes20A,22A connected in series in each unit cell of the Z-Y transmission line. Similarly, the resistances of the two electrodes20A,22A can be summed when calculating Z1.

From these equations, the propagation constant for the distributed transmission line along the fingers can be calculated by the equation, γFING=√{square root over (2ZFING/ZSC)}. As the effective width W of the pair of electrodes20A,22A increases, a contribution of the metal electrodes20A,22A to the total device impedance increases as well. As a result, the device impedance, Z0, decreases slower with the effective width W as compared to a structure with zero-resistivity electrodes20A,22A. When the product, γFING*W, exceeds unity, the device impedance Z0can increase with the effective width W.

An embodiment determines a target effective width W of electrodes20A,22A as an effective width where γFING*W≦2. In this case, the total impedance per pair of electrodes20A,22A is close to the impedance of a structure with ideally conducting electrodes20A,22A (e.g., zero metal resistance). Similarly, an embodiment provides fabrication of multi-finger electrodes20,22(FIG. 1) in which a target effective width W of each finger pair, such as a pair of fingers20A,22A and a pair of fingers20B,22B, is determined such that γFING*W≦2 to achieve a low total impedance Z0of the corresponding device10.

Returning toFIGS. 1A-1C, one or more other attributes of the electrodes20,22can be determined based on a total device resistance. For example, an embodiment provides a solution for designing and/or fabricating electrodes20,22that includes determining one or more of: a total number of fingers20A-20B,22A-22B for each electrode20,22comprising a comb configuration; a target depth, dFING, of each finger20A-20B,22A-22B; a target length of each finger20A-20B,22A-22B; a target depth, dCMB, of the current feeding contact24A,24B of each electrode20,22, respectively; a total length, LCMB, of the interdigitated comb configuration of the two electrodes20,22; and/or the like.

For example, a target depth, dFING, of each finger20A-20B,22A-22B can be determined based on a characteristic contact transfer length, LTR, of a junction between electrodes20,22and semiconductor structure14. The characteristic contact transfer length LTRcan be determined, for example, using the transmission line measurement (TLM) technique. The target depths of the fingers can be selected to provide a minimal resistance to the current flow through the fingers. In an embodiment, a target depth dFINGfor the first finger20C and last finger22C of the interface of electrodes20,22can comprise a depth dFING≧3*LTR(e.g., since each includes only a single adjacent finger in the structure), while the remaining fingers of each electrode20,22can comprise a depth dFING≧6*LTR(e.g., since each includes two adjacent fingers in the structure). Additionally, the design can include determining a target effective width W of each pair of fingers. For example, when the target depths dFINGof the fingers are as described above, the target effective width W of each pair of fingers can be selected such that γFING*W≦2. As illustrated inFIG. 2, such an effective width will provide a minimal impedance for the device. A total width of each finger can comprise the effective width W plus approximately the length of the semiconductor between adjacent fingers, LSC. Similarly, a total width between the current feeding contacts24A,24B can comprise the effective width W plus approximately 2*LSC.

An embodiment of the design further includes determining a target total length LCMBof the interdigitated comb configuration of the two electrodes20,22. To this extent, the design can include determining an impedance of the current feeding contacts24A,24B, ZCMB, to determine the target total length LCMB. ZCMBcan be calculated, for example, using the formula ZCMB=RSHCMB/dCMB, where RSHCMBis the sheet resistance of the current feeding contact and dCMBis the depth of the current feeding contact. ZCMBcan be used to determine a propagation constant of the distributed transmission line along the current feeding contacts24A,24B, γCMB. For example, γCMBcan be calculated by the equation, γCMB=√{square root over (2ZCMB/ZFINGSC)}, where ZFINGSCis the unit-length resistance of the pair of fingers with the semiconductor material in between. ZFINGSCcan be calculated, for example, using the formula ZFINGSC=RSHSC×LSC/W*(2dFING+LSC), where RSHSCis the sheet resistance of the semiconductor structure14. Subsequently, LCMBcan be selected to provide a minimal resistance to the current flow. For example, similar to the effective width W as illustrated inFIG. 2, LCMBcan be selected such that LCMB*γCMB≦2. In particular, a similar relationship between the total length of the comb configuration can apply as described above with respect to the relationship of the effective width W of each finger pair. Since γCMBdepends on dCMB, an embodiment of the design can determine a target depth dCMBof the current feeding contact24A,24B of each electrode20,22that satisfies LCMB*γCMB≦2. For example, when a target total device impedance Z0is given, the design can derive a required number of finger pairs and LCMBfrom the total impedance. In any event, from LCMB, a total length of the current feeding contact of each electrode20,22can be derived, e.g., by subtracting a depth of an end finger20C,22C and the length of the semiconductor between adjacent fingers, LSC.

It is understood that the various target attributes can be determined for alternative electrode configurations. For example,FIGS. 3A and 3Bshow illustrative circular electrode configurations according to embodiments. As illustrated inFIG. 3A, the electrode configuration includes a pair of electrodes30,32, each of which comprises a current feeding contact34A,34B, respectively, and a plurality of electrode fingers30A-30B,32A-32B, respectively. Each electrode finger30A-30B,32A-32B comprises a partial elliptical (e.g., circular) shape. In this case, a solution for designing and/or fabricating electrodes30,32can include determining one or more of: a total number of fingers30A-30B,32A-32B for each electrode30,32; a target depth of each finger30A-30B,32A-32B; a target length of each finger30A-30B,32A-32B; a target depth of the current feeding contact34A,34B of each electrode30,32, respectively; a total length of the interdigitated comb configuration of the two electrodes30,32; and/or the like. Electrodes30,32can be configured to include any number of pairs of fingers30A-30B,32A-32B.

Additionally, as illustrated inFIG. 3B, each finger30A-30B,32A-32B can comprise a plurality of extensions, such as extensions36A,36B, one or more target attributes of which also can be determined as described herein. The target attributes of the extensions can be determined similar to the target attributes of the fingers shown inFIG. 1Cand described herein, where each extension corresponds to a finger, and each finger corresponds to a current feeding contact. For example, a target depth of each extension, dEXT, can be determined similar to the target depth of each finger dFINGdescribed herein, and a target effective width of a pair of adjacent extensions, WEXT, can be determined similar to the target effective width of a pair of adjacent fingers W described herein. To this extent, the target depth of each extension dEXTcan be selected to such that dEXT≧3*LTR(for the end extensions) or dEXT≧6*LTR(for all interior extensions). Similarly, the target effective width of each pair of adjacent extensions WEXTcan be selected such that γEXT*WEXT≦2, where γEXT=√{square root over (2ZEXT/ZSC)}. While extensions36A,36B are only shown and described with reference to the partially elliptical fingers, it is understood that the fingers ofFIG. 1Acould be similarly configured with extensions between the pairs of fingers.

To this extent, target value(s) for one or more of the various attributes can be determined using similar formulas as described herein. However, one or more of the formulas can be modified to account for the circular configuration of electrodes30,32. For example, the target effective width W for a pair of fingers, such as fingers30B,32B, can be approximately correspond to an inner circumference of the larger finger (e.g., finger32B). To this extent, a diameter, d, of the larger finger can be selected such that πdγ≦2, where γ is calculated based on the impedance of the electrodes30,32and a corresponding semiconductor structure using the same formula described above in calculating γFING.

Aspects of the invention described herein can be applied to the design and fabrication of various types of devices for which operation of the device is improved when resistance between electrodes20,22is reduced. For example, device10or a device with the electrode configuration ofFIG. 3can comprise a diode, such as a large periphery diode, a high current Schottky diode, a p-n junction diode, and/or the like. To this extent, one or more electrodes20,22or30,32can form a non-linear contact with the semiconductor structure14. For example, electrodes20,22can comprise different diode contacts, e.g., ohmic and Schottky or contacts to p- and n-regions of the semiconductor structure14, and/or the like.

FIG. 4shows a portion of another illustrative electrode configuration for a device40according to an embodiment. Device40can include multi-finger electrodes20,22formed on a semiconductor structure14. The fingers of electrodes20,22can be designed to have the target effective width W as described herein. Additionally, device40includes a series of gate electrodes42A-42D formed between the fingers of the electrodes20,22. Gate electrodes42A-42D can be operated to selectively allow the flow of current between the fingers of electrodes20,22using any solution.

In any event, after the electrodes have been configured, a device, such as device10(FIG. 1) or device40, can be fabricated using any solution. To this extent, the fabrication of the device10,40can include forming each of the corresponding electrodes of approximately the target effective width W as determined herein. Devices10,40can each be implemented as a component in any of various types of circuits. For example, device40can be configured to operate as a field effect transistor (FET) within a circuit. To this extent, device40can comprise a FET that is used as a solid-state switch, an amplifier in a switching mode (e.g., class E, F), and/or the like.

While shown and described herein as a method of designing and/or fabricating a semiconductor device, it is understood that aspects of the invention further provide various alternative embodiments. For example, in one embodiment, the invention provides a method of designing and/or fabricating a circuit that includes one or more of the semiconductor devices designed and fabricated as described herein.

To this extent,FIG. 5shows an illustrative flow diagram for fabricating a circuit126according to an embodiment. Initially, a user can utilize a device design system110to generate a device design112using a method described herein. The device design112can be used by a device fabrication system114to generate a set of physical devices116according to the features defined by the device design112. Similarly, the device design112can be provided to a circuit design system120(e.g., as an available component for use in circuits), which a user can utilize to generate a circuit design122. The circuit design122can include a device designed using a method described herein. In any event, the circuit design122and/or one or more physical devices116can be provided to a circuit fabrication system124, which can generate a physical circuit126according to the circuit design122. The physical circuit126can include one or more devices116designed using a method described herein.

In another embodiment, the invention provides a device design system110for designing and/or a device fabrication system114for fabricating a semiconductor device116by applying the method described herein. In this case, the system110,114can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the semiconductor device116as described herein. Similarly, an embodiment of the invention provides a circuit design system120for designing and/or a circuit fabrication system124for fabricating a circuit126that includes at least one device116designed and/or fabricated using a method described herein. In this case, the system120,124can comprise a general purpose computing device, which is programmed to implement a method of designing and/or fabricating the circuit126including at least one semiconductor device116as described herein.

In still another embodiment, the invention provides a computer program fixed in at least one computer-readable medium, which when executed, enables a computer system to implement a method of designing and/or fabricating a semiconductor device as described herein. For example, the computer program can enable the device design system110to generate the device design112as described herein. To this extent, the computer-readable medium includes program code, which implements some or all of a process described herein when executed by the computer system. It is understood that the term “computer-readable medium” comprises one or more of any type of tangible medium of expression, now known or later developed, from which a copy of the program code can be perceived, reproduced, or otherwise communicated by a computing device. For example, the computer-readable medium can comprise: one or more portable storage articles of manufacture; one or more memory/storage components of a computing device; paper; and/or the like.

In another embodiment, the invention provides a method of providing a copy of program code, which implements some or all of a process described herein when executed by a computer system. In this case, a computer system can process a copy of the program code to generate and transmit, for reception at a second, distinct location, a set of data signals that has one or more of its characteristics set and/or changed in such a manner as to encode a copy of the program code in the set of data signals. Similarly, an embodiment of the invention provides a method of acquiring a copy of program code that implements some or all of a process described herein, which includes a computer system receiving the set of data signals described herein, and translating the set of data signals into a copy of the computer program fixed in at least one computer-readable medium. In either case, the set of data signals can be transmitted/received using any type of communications link.

In still another embodiment, the invention provides a method of generating a device design system110for designing and/or a device fabrication system114for fabricating a semiconductor device as described herein. In this case, a computer system can be obtained (e.g., created, maintained, made available, etc.) and one or more components for performing a process described herein can be obtained (e.g., created, purchased, used, modified, etc.) and deployed to the computer system. To this extent, the deployment can comprise one or more of: (1) installing program code on a computing device; (2) adding one or more computing and/or I/O devices to the computer system; (3) incorporating and/or modifying the computer system to enable it to perform a process described herein; and/or the like.