Business and Basis of Presentation (Details)		1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 12, 2020 shares $ / shares	Feb. 29, 2020 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2020 USD ($) segment shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2020 shares	Feb. 11, 2020 shares	Dec. 31, 2019 USD ($) shares	Jun. 30, 2019 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		35,266,496		35,266,496			50,325,662	4,220,998
Common stock, forfeited and cancelled (in shares)	682,699
Shares issued under company plans (in shares)	2,461,923		2,327,423	2,453,812	2,327,423	2,453,812	2,454,687		2,473,130	2,342,440	2,377,180
Number of shares (in shares)	15,461		15,461		15,461			186,535
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
Cash and cash equivalents | $			$ 4,369,000		4,369,000				$ 2,875,000
Significant losses on accounts receivable | $			0		0				0
Goodwill impairment charges | $			0	$ 0	0	0
Impairment of intangible assets | $			0	0	0	0
Revenue from product and license fees | $			1,929,000	994,000	9,358,000	2,011,000
Cumulative effect of government rate change | $			505,000
Grant reimbursement | $			$ 81,000	0	$ 380,000	$ 0
Number of anti-dilutive shares (in shares)			3,155,064		3,155,064	2,469,273
Number of reportable segments | segment					1
Common Stockholder
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue from related parties | $				178,000		$ 293,000
EPI Gen 4 wafer growth sales | Common Stockholder
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue from related parties | $			$ 280,000	0	$ 280,000	0
License and Service
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue from product and license fees | $			$ 0	$ 0	$ 5,000,000.0	$ 0
Stock options
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of anti-dilutive shares (in shares)			2,327,423	2,453,812	2,327,423	2,453,812
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	15,461	15,461	15,461
Restricted Stock Awards (RSA)
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of anti-dilutive shares (in shares)			812,180
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
Award vesting period					1 year
Maximum
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property and equipment, estimated useful life					7 years
Award vesting period					4 years
JAPAN
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Cash and cash equivalents | $			$ 41,000		$ 41,000				$ 55,000
Percentage of cash and cash equivalents in foreign subsidiary			0.90%		0.90%				1.90%
Revenue from product and license fees | $			$ 289,000	$ 6,000	$ 321,000	$ 25,000
UNITED STATES
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Revenue from product and license fees | $			1,600,000	$ 988,000	9,000,000.0	$ 2,000,000.0
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				$ 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
Principal amount of long term loan facility | $			15,000,000.0		15,000,000.0
Accrued interest on promissory note | $			$ 448,000		$ 448,000				$ 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) | $ / 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