BALANCE SHEET COMPONENTS - Schedule of property and equipment, net (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization	$ (2,419)	$ (2,283)
Total property, plant and equipment, net	388	294
Depreciable Property, Plant And Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,807	2,572
Total property, plant and equipment, net	388	289
Research and development equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,763	1,558
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	808	800
Autos
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	54	54
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	41	41
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	141	119
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 0	$ 5