Note 3 - Stock-based Compensation - Stock-based Compensation Expense By Financial Statement Line Item (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total stock-based compensation expense	$ 849	$ 397	$ 1,758	$ 646
Cost of Sales [Member]
Total stock-based compensation expense	30	21	59	24
Research and Development Expense [Member]
Total stock-based compensation expense	161	92	291	103
Selling, General and Administrative Expenses [Member]
Total stock-based compensation expense	$ 658	$ 283	$ 1,408	$ 519