GaN on Si Inductor Cavity for Heat Dissipation and Impedance
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Solution Overview
Problem
Gallium nitride (GaN) RF power transistors on silicon substrates face challenges with heat dissipation and integration, leading to lower quality factor (Q) passive components and limited current-carrying capability due to thinner substrates, which restrict the realization of high impedance transmission lines and inductors, resulting in higher system losses.
Innovation Solution
The device incorporates a semiconductor substrate with a top and bottom conductive structure, including a cavity filled with a dielectric medium of lower dielectric constant than the substrate, which increases the quality factor and characteristic impedance of inductors and transmission lines, and allows for higher impedance transmission lines by electrically isolating conductive features and using through-wafer vias for interconnects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If thinner substrates are used to dissipate heat in GaN transistors, then heat dissipation is improved, but the quality factor of passive components and characteristic impedance of transmission lines deteriorate
Solution Approach 1:
The substrate is segmented by forming cavities that remove portions of the substrate beneath conductive features. This segmentation isolates the passive components from the lossy substrate material, allowing the substrate to remain thin for heat dissipation while the cavities preserve the quality factor by eliminating substrate losses in the passive component regions.
Solution Approach 2:
The substrate structure is made non-uniform by forming cavities only beneath specific conductive features (inductors, transmission lines) while leaving other areas intact. This local modification allows different regions of the substrate to serve different functions: thin regions for heat dissipation and cavity regions for maintaining high quality factor in passive components.
2Temperature
If thinner substrates are used to dissipate heat, then heat dissipation is improved, but the maximum realizable impedance of transmission lines is limited
Solution Approach 1:
Cavities are formed beneath transmission line conductors to segment the substrate, removing lossy material that would otherwise limit the achievable impedance. This allows transmission lines to achieve higher impedances than would be possible with a continuous thin substrate, while still maintaining the thin substrate for heat dissipation in active device regions.
3Productivity
If higher levels of integration are implemented to lower assembly cost, then integration is improved, but the quality factor of passive components deteriorates due to thinner substrates
Solution Approach 1:
The substrate is designed with locally differentiated properties: regions beneath passive components contain cavities to maintain high quality factor, while other regions remain as continuous substrate to support integrated active devices and provide heat dissipation. This local quality approach enables high-level integration while preserving passive component performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances the quality factor and self-resonance frequency of inductors and transmission lines, increasing their impedance and reducing losses, thereby improving the performance of GaN on Si devices.
Implementation Method 1
a cavity (160) filled with a dielectric medium (166) of lower dielectric constant than the substrate
Data Source
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AI summary
An embodiment of a device includes a semiconductor substrate, a transistor formed at the first substrate surface, a first conductive feature formed over the first substrate surface and electrically coupled to the transistor, and a second conductive feature covering only a portion of the second substrate surface to define a first conductor-less region. A cavity vertically aligned with the first conductive feature within the first conductor-less region extends into the semiconductor substrate. A dielectric medium may be disposed within the cavity and have a dielectric constant less than a dielectric constant of the semiconductor substrate. A method for forming the device may include forming a semiconductor substrate, forming a transistor on the semiconductor substrate, forming the first conductive feature, forming the second conductive feature, forming the conductor-less region, forming the cavity, and filling the cavity with the dielectric medium.