11. STOCK-BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($)	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Stock-based compensation expense by line item
Stock-based compensation expense	$ 2,566,000	$ 2,266,000	$ 2,070,000
Cost of revenues
Stock-based compensation expense by line item
Stock-based compensation expense	257,000	234,000	259,000
Research and development
Stock-based compensation expense by line item
Stock-based compensation expense	1,487,000	1,310,000	1,141,000
Selling, general and administrative
Stock-based compensation expense by line item
Stock-based compensation expense	822,000	722,000	670,000
2007 Employee Stock Purchase Plan
Stock-based compensation expense by line item
Stock-based compensation expense	$ 220,000	$ 211,000	$ 207,000