Balance Sheets Details - Accrued liabilities (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of accrued liabilities
Accrued compensation and related charges	$ 4,417,000	$ 3,307,000
Preferred stock dividends payable	2,901,000	2,901,000
Payable in connection with construction	1,457,000	1,447,000
Payable in connection with repurchase of subsidiaries shares	1,439,000	151,000
Accrued income taxes	760,000	171,000
Payable in connection with land restoration of Nanjing JinMei factory	750,000	703,000
Accrued professional services	675,000	630,000
Accrued product warranty	609,000	387,000	$ 236,000
Current portion of operating lease liabilities	445,000	319,000
Advance from customers	374,000	396,000
Other tax payable	295,000	50,000
Other personnel-related costs	101,000	180,000
Accrual for sales returns	81,000	26,000
Other accrued liabilities	1,691,000	1,013,000
Accrued liabilities, Total	$ 15,995,000	$ 11,681,000