BALANCE SHEET COMPONENTS - Schedule of property and equipment, net (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Accumulated depreciation and amortization	$ (2,353)	$ (2,283)
Property and equipment, net	245	294
Depreciable Property, Plant and Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,586	2,572
Property and equipment, net	233	289
Research and development equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,569	1,558
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	803	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	119	119
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 12	$ 5