Cover - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 28, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
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	Portions of the registrant’s Definitive Proxy Statement for its 2021 Annual Meeting of Stockholders, which will be filed within 120 days of the fiscal year ended December 31, 2020, are incorporated by reference into Part III of this report (or information will be provided by amendment to this Form 10-K), as noted herein.
Entity Central Index Key	0001715768
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false