Cover - shares	9 Months Ended
	Sep. 30, 2020	Nov. 05, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
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 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
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		35,266,496
Entity Central Index Key	0001715768
Document Fiscal Year Focus	2020
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q3
Amendment Flag	false