Note 4 - Stock-based Compensation - Stock-based Compensation Expense by Financial Statement Line Item (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020
Total stock-based compensation expense	$ 1,120	$ 839	$ 3,513	$ 2,597
Cost of Sales [Member]
Total stock-based compensation expense	36	(23)	148	35
Research and Development Expense [Member]
Total stock-based compensation expense	190	179	629	470
Selling, General and Administrative Expenses [Member]
Total stock-based compensation expense	$ 894	$ 683	$ 2,736	$ 2,092