Business and Basis of Presentation (Details)		1 Months Ended			3 Months Ended				9 Months Ended
	Feb. 12, 2020 USD ($) shares $ / shares	Mar. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($)	Feb. 29, 2020 USD ($)	Jun. 30, 2021 USD ($) segment shares	Mar. 31, 2021 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) shares	Mar. 31, 2021 USD ($) shares	Mar. 31, 2020 shares	Feb. 11, 2020 shares
Cash and Cash Equivalents [Line Items]
Merger conversion rate (in shares) | shares	0.08289152527
Common stock, shares issued (in shares) | shares	4,171,571	40,531,996			40,662,020	40,531,996			40,531,996		50,325,662
Common stock, forfeited and cancelled (in shares) | shares	682,699
Shares issued under company plans (in shares) | shares	2,461,923	2,543,125			2,462,414	2,543,125		2,454,687	2,543,125	2,458,091
Number of Shares | shares	15,461				159,415						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) | shares	3,076,171
Proceeds from Issuance of Common Stock | $		$ 1,000,000.0	$ 13,600,000	$ 19,700,000
Cash and cash equivalents | $		9,500,000			$ 2,462,000	$ 9,500,000			$ 9,500,000
Provision for doubtful accounts | $					0			$ 60,000
Goodwill impairment charges | $					0			0
Impairment of intangible assets | $					0			0
Revenue from product and license fees | $					3,216,000			6,329,000
Term of cooperation and development agreement.			3 years
Revenue recognized for collaboration development activities | $					334,000			0
Deferred revenue | $		505,000			1,016,000	505,000			505,000
Cumulative effect of government rate change | $						(83,000)	$ 505,000		423,000
Grant reimbursement | $					$ 26,000			$ 226,000
Number of anti-dilutive shares (in shares) | shares					3,543,866			2,470,148
Number of reportable segments | segment					1
Billed Contracts Receivable | $					$ 750,000			$ 0
Yaskawa Note
Cash and Cash Equivalents [Line Items]
Maximum number of shares of common stock issuable (in shares) | shares	3,076,171
Principal amount of long term loan facility | $					15,000,000.0
Accrued interest on promissory note | $		523,000			561,000	523,000			523,000
Transphorm Technology 2007 Stock Plan and 2015 Equity Incentive Plan
Cash and Cash Equivalents [Line Items]
Shares issued under company plans (in shares) | shares	29,703,285
Series 1 Preferred Stock
Cash and Cash Equivalents [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares	51,680,254
Convertible preferred stock, shares issued (in shares) | shares	51,680,254
Share conversion in connection with the Merger (in shares) | shares	12,433,953
Series 2 Preferred Stock
Cash and Cash Equivalents [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares	38,760,190
Convertible preferred stock, shares issued (in shares) | shares	38,760,190
Conversion of stock (in shares) | shares	7,499,996
Series 3 Preferred Stock
Cash and Cash Equivalents [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares	31,850,304
Convertible preferred stock, shares issued (in shares) | shares	31,850,304
Conversion of stock (in shares) | shares	4,000,000
Common Stock
Cash and Cash Equivalents [Line Items]
Price per share (in dollars per share) | $ / shares	$ 4.00
Yaskawa
Cash and Cash Equivalents [Line Items]
Letter of intent for development activities | $	$ 4,000,000.0		$ 4,000,000.0
Proceeds from collaborators | $			$ 1,000,000.0
Revenue recognized for collaboration development activities | $					334,000	333,000
Deferred revenue | $					750,000			0
Transphorm Technology
Cash and Cash Equivalents [Line Items]
Share conversion in connection with the Merger (in shares) | shares	28,105,520
Other Assets
Cash and Cash Equivalents [Line Items]
Restricted cash | $		75,000			225,000	75,000			75,000
License and Service
Cash and Cash Equivalents [Line Items]
Revenue from product and license fees | $					$ 0			$ 5,000,000.0
Stock options
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares					2,462,414			2,454,687
Warrant
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares					159,415			15,461
Restricted Stock Awards (RSA)
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares					922,037
Minimum
Cash and Cash Equivalents [Line Items]
Property and equipment, estimated useful life					3 years
Intangible assets, estimated useful life					3 years
Award vesting period					1 year
Maximum
Cash and Cash Equivalents [Line Items]
Property and equipment, estimated useful life					7 years
Intangible assets, estimated useful life					10 years
Award vesting period					4 years
JAPAN
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents | $		$ 444,000			$ 134,000	$ 444,000			$ 444,000
Percentage of cash and cash equivalents in foreign subsidiary		4.70%			5.40%	4.70%			4.70%
Revenue from product and license fees | $					$ 471,000			$ 21,000
UNITED STATES
Cash and Cash Equivalents [Line Items]
Revenue from product and license fees | $					$ 2,700,000			$ 6,300,000