| Cover - USD ($) | 12 Months Ended | |
| Dec. 31, 2020 | Feb. 28, 2021 | |
| Cover [Abstract] | ||
| Document Type | 10-K/A | |
| Document Annual Report | true | |
| Document Period End Date | Dec. 31, 2020 | |
| Current Fiscal Year End Date | --12-31 | |
| Document Transition Report | false | |
| Entity File Number | 000-55832 | |
| Entity Registrant Name | Transphorm, Inc. | |
| Entity Incorporation, State or Country Code | DE | |
| Entity Tax Identification Number | 82-1858829 | |
| Entity Address, Address Line One | 75 Castilian Drive | |
| Entity Address, City or Town | Goleta, | |
| Entity Address, State or Province | CA | |
| Entity Address, Postal Zip Code | 93117 | |
| City Area Code | 805 | |
| Local Phone Number | 456-1300 | |
| Entity Well-known Seasoned Issuer | No | |
| Entity Voluntary Filers | No | |
| Entity Current Reporting Status | Yes | |
| Entity Interactive Data Current | Yes | |
| Entity Filer Category | Non-accelerated Filer | |
| Entity Small Business | true | |
| Entity Emerging Growth Company | true | |
| Entity Ex Transition Period | false | |
| ICFR Auditor Attestation Flag | false | |
| Entity Shell Company | false | |
| Entity Public Float | $ 0 | |
| Entity Common Stock, Shares Outstanding | 40,278,796 | |
| Documents Incorporated by Reference | None. | |
| Amendment Description | Transphorm, Inc. (“we,” “us,” “our,” the “Company,” or “Transphorm”) is filing this Amendment No. 1 on Form 10-K/A (“Amendment No. 1”) to amend our Annual Report on Form 10-K for the fiscal year ended December 31, 2020 (“Original Filing”), filed with the U.S. Securities and Exchange Commission (“SEC”) on March 12, 2021 (“Original Filing Date”), to include the information required by Items 10 through 14 of Part III of Form 10-K. This information was previously omitted from the Original Filing in reliance on General Instruction G(3) to Form 10-K. Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended, this Amendment No. 1 also contains new certifications by our principal executive officer and principal financial officer as required by Section 302 of the Sarbanes-Oxley Act of 2002. Accordingly, Item 15 of Part IV is amended to include the currently dated certifications as exhibits.Except as expressly noted in this Amendment No. 1, this Amendment No. 1 does not reflect events that may have occurred subsequent to the Original Filing Date or modify or otherwise update any other disclosures contained in the Original Filing, including, without limitation, the financial statements. Accordingly, this Amendment No. 1 should be read in conjunction with the Original Filing. | |
| Entity Central Index Key | 0001715768 | |
| Document Fiscal Year Focus | 2020 | |
| Document Fiscal Period Focus | FY | |
| Amendment Flag | true |
