Business and Basis of Presentation (Details)			1 Months Ended	3 Months Ended
	Feb. 12, 2020 shares $ / shares	Feb. 11, 2020 shares	Feb. 29, 2020 USD ($)	Mar. 31, 2020 USD ($) segment shares	Mar. 31, 2019 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares
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	50,325,662		35,135,520		4,220,998
Common stock, forfeited and cancelled (in shares)		682,699
Shares issued under company plans (in shares)	2,461,923			2,458,091	2,371,545	2,473,130	2,377,180
Number of shares (in shares)	15,461	186,535		15,461
Conversion price per share on convertible note payable (in dollars per share) | $ / shares	$ 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 | $			$ 19,700,000	$ 19,741,000	$ 0
Goodwill impairment charges | $				0	0
Impairment of intangible assets | $				0	0
Grant reimbursement | $				$ 72,000	$ 0
Award vesting period				4 years
Number of anti-dilutive shares (in shares)				2,473,552	2,397,702
Number of reportable segments | segment				1
Revenue from product and license fees | $				$ 1,100,000	$ 529,000
Cash and cash equivalents | $				$ 14,648,000		$ 2,875,000
Stock options
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of anti-dilutive shares (in shares)				2,458,091	2,371,545
Warrant
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of anti-dilutive shares (in shares)				15,461	26,157
Minimum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property and equipment, estimated useful life				3 years
Intangible assets, estimated useful life				3 years	10 years
Maximum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property and equipment, estimated useful life				7 years
JAPAN
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Percentage of cash and cash equivalents in foreign subsidiary				0.20%		1.90%
Revenue from product and license fees | $				$ 11,000	$ 7,000
Cash and cash equivalents | $				31,000		$ 55,000
UNITED STATES
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue from product and license fees | $				1,100,000	$ 522,000
Other Assets
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Cash included in other assets | $				75,000		75,000
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
Long-term loan facility | $				15,000,000.0
Accrued interest on promissory note | $				$ 373,000		$ 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
The 2020 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)				0
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) | $ / shares	$ 4.00
Share conversion in connection with the Merger (in shares)				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