Greenthread Ltd v. Monolithic Power Systems Inc

This appeal continues a multi-front patent dispute between Greenthread Ltd., a patent assertion entity, and Monolithic Power Systems Inc. (MPS), a major publicly traded fabless semiconductor company specializing in power management solutions. Greenthread, which acquired a portfolio of patents from inventor and company director G.R. Mohan Rao, has engaged in a widespread litigation campaign, asserting its patents against numerous significant players in the semiconductor industry, including Intel, Samsung, Dell, and Texas Instruments. The defendant, MPS, is a high-growth company with a market capitalization approaching $80 billion and a history of successfully defending itself in patent litigation.

The dispute centers on semiconductor fabrication technology. Greenthread alleges that MPS's power semiconductor devices infringe U.S. Patent No. 11,121,222, titled "Semiconductor device with graded dopant regions." This patent, and others in its family, claim methods and structures for creating a gradual change in the concentration of chemical impurities (dopants) within a semiconductor device, a technique used to control electrical properties and improve device performance. The infringement allegations target the fundamental architecture of MPS's integrated circuits.

The case arrives at the U.S. Court of Appeals for the Federal Circuit in a posture shaped by proceedings in another forum. Greenthread initially sued MPS in the U.S. District Court for the District of Delaware in May 2023 (Case No. 1:23-cv-00579). However, MPS responded aggressively by filing inter partes review (IPR) petitions with the Patent Trial and Appeal Board (PTAB), challenging the validity of Greenthread's patents. In April 2024, the district court granted a stay, pausing the infringement case pending the outcome of the PTAB's validity review. The current appeal to the Federal Circuit, filed in April 2026, is likely a challenge to a final written decision from one of those IPRs, making this appeal a critical test of the patent's validity. The case is notable for MPS's successful use of the PTAB to halt costly district court litigation and for its place within Greenthread's broad, multi-year assertion campaign against the semiconductor industry.

Key Legal Developments and Case Outcome

This litigation followed a path increasingly common in high-tech patent disputes, where validity challenges at the Patent Trial and Appeal Board (PTAB) take precedence over district court infringement proceedings. The dispute moved from the District of Delaware to the PTAB and is now at the Federal Circuit, all without a trial on the merits of infringement.

Chronological Developments:

• 2023-05-26: Complaint Filed in District Court
  Greenthread Ltd. initiated the litigation by filing a patent infringement complaint against Monolithic Power Systems Inc. (MPS) in the U.S. District Court for the District of Delaware (Case No. 1:23-cv-00579). The complaint asserted that MPS's power semiconductor devices, such as the MP86905 monolithic half-bridge, infringed six of Greenthread's patents, including U.S. Patent No. 11,121,222. This suit was part of a broader campaign by Greenthread against numerous semiconductor companies.

• 2023–2024: Parallel PTAB Proceedings Initiated
  In response to the lawsuit, MPS and other defendants in Greenthread's litigation campaign filed multiple inter partes review (IPR) petitions with the PTAB, challenging the validity of the asserted patents. Semiconductor Components Industries, LLC (d/b/a onsemi) and Texas Instruments Inc. were among the petitioners challenging the '222 patent in cases IPR2023-01242 and IPR2023-01244. These IPRs became the central battleground for the dispute.

• 2024-04-23: District Court Case Stayed
  The Delaware District Court granted a stay of the infringement case pending the resolution of the IPRs by the PTAB. This decision effectively paused all district court activities, including discovery and claim construction, allowing the PTAB to first rule on whether the patent claims asserted by Greenthread were valid. The court then administratively closed the case for statistical purposes, pending the outcome of the PTAB proceedings.

• 2025-02 (approx.): PTAB Issues Final Written Decision
  The PTAB issued a Final Written Decision in IPR2023-01242, finding all challenged claims of the '222 patent unpatentable. The decision came after the Board instituted the trial, received briefing from both sides, and held an oral hearing on November 13, 2024. This outcome was a significant victory for the petitioners, including MPS, as it invalidated the patent claims Greenthread was asserting in district court.

• 2025-04-24: USPTO Director Vacates and Remands PTAB Decision
  In a significant procedural turn, the Acting USPTO Director vacated the PTAB's Final Written Decisions in the IPRs against the '222 patent (and another Greenthread patent) and remanded the cases. The decision was based on Greenthread's argument that the petitions were time-barred under 35 U.S.C. § 315(b) because the petitioner, onsemi, was allegedly in privity with Intel, which had been sued by Greenthread more than a year before the IPRs were filed. The Director found that the PTAB had improperly denied Greenthread's requests for discovery into the relationship between onsemi and Intel and had failed to properly consider Greenthread's arguments on the matter.

• 2026-04-22: Appeal Filed at the Federal Circuit
  Following a subsequent final decision from the PTAB after the remand, Greenthread filed an appeal with the U.S. Court of Appeals for the Federal Circuit (Case No. 26-1723). This appeal directly challenges the PTAB's final validity determination for U.S. Patent No. 11,121,222, making the Federal Circuit the new venue for resolving the patent's viability.

Present Posture:

As of May 1, 2026, the district court case remains stayed and administratively closed. The dispositive action has moved to the Federal Circuit. The appeal (Case No. 26-1723) is in its early stages. The outcome of this appeal will determine whether the claims of the '222 patent are valid. If the Federal Circuit affirms the PTAB's finding of unpatentability, Greenthread's case against MPS concerning this patent will be effectively terminated. If the court reverses the PTAB, the stay in the District of Delaware would likely be lifted, and the infringement litigation could resume.

Based on counsel's consistent representation of Greenthread Ltd. in the related district court and Patent Trial and Appeal Board (PTAB) proceedings, the following attorneys are identified as counsel for the plaintiff-appellant.

As the appeal was filed on April 22, 2026, formal notices of appearance at the Federal Circuit may still be pending. The list below reflects the counsel of record from the underlying and prerequisite PTAB and district court cases.

Lead Counsel

Firm: Heim, Payne & Chorush, LLP (Houston, TX)

This Houston-based intellectual property litigation boutique has served as Greenthread's lead counsel throughout its litigation campaign, including in the District of Delaware case (No. 1:23-cv-00579) and the associated inter partes review (IPR) proceedings before the PTAB (IPR2023-01242).

• Michael A. Heim (Lead Counsel)
  • Role: Partner
  • Note: Heim is a founding partner of the firm and a veteran trial lawyer known for securing significant verdicts and settlements in high-stakes patent infringement cases for patent holders.
• Blaine F. FASE (Of Counsel)
  • Role: Partner
  • Note: Fase has extensive experience in patent litigation and appeals, representing clients in district courts, the PTAB, and the Federal Circuit across various technologies.
• William B. "Trey" Condon III (Of Counsel)
  • Role: Partner
  • Note: Condon's practice focuses on complex patent litigation, including representing clients in cases involving semiconductor technology and software.

Additional Counsel

Firm: Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. (Boston, MA)

Mintz Levin served as counsel alongside Heim, Payne & Chorush in the critical PTAB proceedings that led to the current appeal.

• James W. Mears (Of Counsel)
  • Role: Member (Partner)
  • Note: Mears appeared for Greenthread in the PTAB proceedings (IPR2023-01242) and specializes in IP litigation, with a focus on representing patent owners before the PTAB.

District Court Local Counsel (Inactive for Appeal)

Firm: Stamoulis & Weinblatt LLC (Wilmington, DE)

This firm served as required local counsel for the now-stayed case in the U.S. District Court for the District of Delaware. Their role is specific to the district court and they are not expected to be active counsel in the Federal Circuit appeal.

• Stamatios Stamoulis (Local Counsel)
  • Role: Partner
  • Note: A well-known Delaware patent litigator who frequently serves as local counsel for plaintiffs in the District of Delaware.
• Richard C. Weinblatt (Local Counsel)
  • Role: Partner
  • Note: Co-founder of the firm, specializing in intellectual property and commercial litigation in Delaware's federal and state courts.

Defendant-Appellee Counsel of Record

As the appeal (Case No. 26-1723) was filed recently, on April 22, 2026, formal notices of appearance at the Federal Circuit may still be pending. The attorneys listed below represented Monolithic Power Systems, Inc. (MPS) in the prerequisite Patent Trial and Appeal Board (PTAB) and underlying district court proceedings and are anticipated to continue their representation.

Lead Counsel

Firm: Perkins Coie LLP (San Diego, CA)

This national law firm has a prominent intellectual property practice and has represented MPS in other patent litigation matters, including a successful defense against Bel Power Solutions. Perkins Coie served as counsel for MPS in its inter partes review (IPR) petitions that led to the current appeal.

• John D. Esterhay (Lead Counsel)
  • Role: Partner
  • Note: Esterhay's name appears on PTAB filings for MPS in related IPRs and he has extensive experience representing clients in patent litigation and PTAB trials involving semiconductor technology.
• Anita Chou (Of Counsel)
  • Role: Paralegal
  • Note: Chou's name appears on PTAB filings on behalf of MPS. While not an attorney, she is part of the core legal team managing the case.

Based on PTAB filings in related matters, Foley & Lardner LLP also represented MPS. It is not yet clear which firm will take the lead in the Federal Circuit appeal.

Firm: Foley & Lardner LLP (San Diego, CA)

This firm is also listed as representing MPS in its PTAB challenges against Greenthread's patents.

• Miguel Bombach (Lead Counsel)
  • Role: Partner
  • Note: Bombach is listed as lead counsel on PTAB petitions filed by MPS against other Greenthread patents from the same family as the '222 patent.
• John P. Schnurer (Of Counsel)
  • Role: Partner
  • Note: Schnurer is listed as back-up counsel on MPS's PTAB petitions and has a practice focused on patent litigation, particularly within the electronics and software industries.

District Court Local Counsel (Inactive for Appeal)

Firm: Potter Anderson & Corroon LLP (Wilmington, DE)

This firm served as local counsel for MPS in the U.S. District Court for the District of Delaware (Case No. 1:23-cv-00579). Their role is specific to the now-stayed district court case and they are not expected to be active counsel in the Federal Circuit appeal. The specific attorneys who appeared have not been identified in the available search results.

In-House Counsel

• Saria Tseng (In-House)
  • Role: EVP, Strategic Corporate Development, General Counsel and Corporate Secretary
  • Firm: Monolithic Power Systems, Inc.
  • Note: As General Counsel since 2004, Tseng oversees all legal matters for the company, including management of its intellectual property litigation strategy.