| Cover Page | Mar. 16, 2021 |
| Cover [Abstract] | |
| Entity Central Index Key | 0001715768 |
| Amendment Flag | false |
| Document Type | S-3 |
| 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 Filer Category | Non-accelerated Filer |
| Entity Small Business | true |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
