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Business and Basis of Presentation (Details) - USD ($)
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 12, 2020
Mar. 31, 2021
Dec. 31, 2020
Feb. 29, 2020
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Feb. 11, 2020
Jan. 01, 2020
Dec. 31, 2018
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Merger conversion rate (in shares) 0.08289152527                    
Common stock, shares issued (in shares) 4,171,571 40,531,996 40,531,996   40,531,996   40,531,996 4,220,998 50,325,662    
Common stock, forfeited and cancelled (in shares) 682,699                    
Shares issued under company plans (in shares) 2,461,923 2,543,125 2,320,318   2,543,125   2,320,318 2,473,198   2,473,198 2,377,180
Number of shares (in shares) 15,461 159,415     159,415       186,535    
Conversion price per share on convertible note payable (in dollars per share) $ 5.12                    
Maximum number of shares of common stock issuable (in shares) 3,076,171                    
Proceeds from issuance of common stock, net of offering cost   $ 1,000,000.0 $ 13,600,000 $ 19,700,000 $ 950,000 $ 19,741,000 $ 33,387,000 $ 0      
Yaskawa Note                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Maximum number of shares of common stock issuable (in shares) 3,076,171                    
Principal amount of long term loan facility     15,000,000.0       15,000,000.0        
Accrued interest on promissory note   $ 523,000 $ 486,000   $ 523,000   $ 486,000 $ 336,000      
Transphorm Technology 2007 Stock Plan and 2015 Equity Incentive Plan                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Shares issued under company plans (in shares) 29,703,285                    
Series 1 Preferred Stock                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Convertible preferred stock, shares issued (in shares) 51,680,254                    
Convertible preferred stock, shares outstanding (in shares) 51,680,254                    
Share conversion in connection with the Merger (in shares) 12,433,953                    
Series 2 Preferred Stock                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Convertible preferred stock, shares issued (in shares) 38,760,190                    
Convertible preferred stock, shares outstanding (in shares) 38,760,190                    
Conversion of stock (in shares) 7,499,996                    
Series 3 Preferred Stock                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Convertible preferred stock, shares issued (in shares) 31,850,304                    
Convertible preferred stock, shares outstanding (in shares) 31,850,304                    
Conversion of stock (in shares) 4,000,000                    
Common Stock                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Price per share (in dollars per share) $ 4.00                    
Share conversion in connection with the Merger (in shares)           23,933,949 23,933,949        
Transphorm Technology                      
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]                      
Share conversion in connection with the Merger (in shares) 28,105,520