Business and Basis of Presentation (Details)			1 Months Ended							3 Months Ended						9 Months Ended
	Jul. 16, 2021 USD ($)	Feb. 12, 2020 $ / shares shares	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 USD ($) $ / shares shares	Aug. 31, 2021 USD ($)	Jul. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2021 USD ($) segment shares	Dec. 31, 2020 USD ($) shares	Oct. 04, 2021 USD ($) $ / shares 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	53,066,922					40,531,996		53,066,922			40,531,996			53,066,922				50,325,662
Common stock, forfeited and cancelled (in shares) | shares		682,699
Shares issued under company plans (in shares) | shares		2,461,923	2,957,904	2,506,531				2,543,125	2,320,318	2,957,904	2,506,531		2,543,125	2,320,318	2,327,423	2,957,904	2,320,318		2,458,091
Number of Shares | shares		15,461	2,742,065							2,742,065						2,742,065				186,535
Conversion price per share on convertible note payable (in dollars per share) | $ / shares		$ 5.12
Number of common stock shares converted per preferred share (in shares) | shares																		3,120,000
Net cash used in operating activities																$ (13,910,000)	$ (7,964,000)
Proceeds from issuance of common stock, net					$ 50,900,000											49,773,000	13,646,000
Restricted cash			$ 575,000							$ 575,000						575,000
Restricted cash in current asset			500,000					$ 0		500,000			$ 0			500,000
Cash and cash equivalents			40,467,000					9,500,000		40,467,000			9,500,000			40,467,000
Provision for doubtful accounts										0						0	110,000
Recovery from doubtful accounts														$ (36,000)
Goodwill impairment charges										0				0		0	0
Impairment of intangible assets										0				0		0	0
Revenue from product and license fees										4,604,000				2,013,000		19,123,000	10,271,000
Term of cooperation and development agreement.									3 years
Cumulative effect of government rate change													(83,000)		$ 505,000	423,000
Grant reimbursement										$ 80,000				$ 47,000		$ 151,000	$ 353,000
Number of anti-dilutive shares (in shares) | shares										6,947,976				3,285,058		6,947,976	3,285,058
Number of reportable segments | segment																1
Yaskawa
Cash and Cash Equivalents [Line Items]
Conversion price per share on convertible note payable (in dollars per share) | $ / shares		$ 5.12		$ 5.00							$ 5.00							$ 5.00
Yaskawa Note
Cash and Cash Equivalents [Line Items]
Accrued interest on promissory note			0					523,000		$ 0			523,000			$ 0
Yaskawa Note | Yaskawa
Cash and Cash Equivalents [Line Items]
Principal and unpaid interest of convertible note																		$ 15,600,000
Principal amount of long term loan facility			15,000,000							15,000,000						15,000,000
Accrued interest on promissory note			600,000							$ 600,000						$ 600,000
Number of common stock shares converted per preferred share (in shares) | shares																		3,120,000
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
Share conversion in connection with the Merger (in shares) | shares										3,120,000						3,120,000
Yaskawa
Cash and Cash Equivalents [Line Items]
Letter of intent for development activities									$ 4,000,000
Proceeds from collaborators						$ 750,000			1,000,000
Revenue recognized for collaboration development activities			375,000	$ 375,000			$ 333,000	333,000	$ 333,000	$ 375,000	$ 375,000	$ 333,000	333,000	$ 333,000
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					75,000		75,000			75,000			$ 75,000
License and Service
Cash and Cash Equivalents [Line Items]
Revenue from product and license fees	$ 8,000,000									$ 0				$ 0		$ 8,000,000	$ 5,000,000
Stock options
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares										2,957,904				2,320,318		2,957,904	2,320,318
Warrant
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares										2,742,065				159,415		2,742,065	159,415
Restricted Stock Awards (RSA)
Cash and Cash Equivalents [Line Items]
Number of anti-dilutive shares (in shares) | shares										1,248,007				805,325		1,248,007	805,325
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			$ 65,000					$ 444,000		$ 65,000			$ 444,000			$ 65,000
Percentage of cash and cash equivalents in foreign subsidiary			0.20%					4.70%		0.20%			4.70%			0.20%
Revenue from product and license fees										$ 9,000				$ 392,000		$ 506,000	$ 702,000
UNITED STATES
Cash and Cash Equivalents [Line Items]
Revenue from product and license fees										$ 4,600,000				$ 1,600,000		$ 18,600,000	$ 9,600,000