10. STOCK-BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Stock-based compensation expense by line item
Stock-based compensation expense	$ 661	$ 716	$ 1,299	$ 1,539
Cost of Sales [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	49	64	109	134
Research and Development Expense [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	390	418	741	888
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	$ 222	$ 234	$ 449	$ 517