10. STOCK-BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Stock-based compensation expense by line item
Stock-based compensation expense	$ 2,266,000	$ 2,070,000	$ 1,877,000
Cost of revenues
Stock-based compensation expense by line item
Stock-based compensation expense	234,000	259,000	282,000
Research and development
Stock-based compensation expense by line item
Stock-based compensation expense	1,310,000	1,141,000	980,000
Selling, general and administrative
Stock-based compensation expense by line item
Stock-based compensation expense	722,000	670,000	615,000
2007 Employee Stock Purchase Plan
Stock-based compensation expense by line item
Stock-based compensation expense	$ 211,000	$ 207,000	$ 150,000