11. STOCK-BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($)	12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2020
Stock-based compensation expense by line item
Stock-based compensation expense	$ 2,993,000	$ 2,854,000	$ 2,566,000
Cost of Sales [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	248,000	346,000	257,000
Research and Development Expense [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	1,676,000	1,509,000	1,487,000
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	1,069,000	999,000	822,000
Employee Stock Purchase Plan Year 2007
Stock-based compensation expense by line item
Stock-based compensation expense	$ 260,000	$ 276,000	$ 220,000