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