| Cover Page | Apr. 20, 2021 | Aug. 11, 2020 |
| Cover [Abstract] | ||
| Entity Central Index Key | 0001715768 | |
| Amendment Flag | false | |
| Document Type | 8-K | |
| Document Period End Date | May 6, 2021 | |
| Entity Registrant Name | Transphorm, Inc. | |
| Entity Incorporation, State or Country Code | DE | |
| Entity File Number | 000-55832 | |
| 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 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | true | |
| Entity Ex Transition Period | false |
