Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Loss before tax expense	$ 17,909	$ 15,283
Tax expense	0	0
Valuation allowance	52,200	49,400
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Loss before tax expense	10,300	10,600
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	245,300
Operating loss carryforwards subject to expiration	207,500
Domestic Tax Authority | Tax Year 2018
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	37,800
Domestic Tax Authority | Tax Year 2020
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	37,800
Domestic Tax Authority | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforward	4,500
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Loss before tax expense	(5,200)
Foreign Tax Authority | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	7,600	4,700
Foreign Tax Authority | Transphorm Japan, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	800	1,000
Foreign Tax Authority | Transphorm Ailzu, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	6,800	3,700
Foreign Tax Authority | Transphorm Epi, Inc. | National Tax Agency, Japan
Operating Loss Carryforwards [Line Items]
Loss before tax expense	(25)	(1)
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	152,400
Tax expense	1	$ 1
State and Local Jurisdiction | Tax Year 2019
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	37,800
State and Local Jurisdiction | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research tax credit carryforward	$ 3,800