11. STOCKHOLDERS' EQUITY AND STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	May 31, 2021	May 31, 2020	May 31, 2019
Stock-based compensation in the form of stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 1,101	$ 910	$ 905
Effect on net loss per share, basic	$ .05	$ 0.04	$ 0.04
Effect on net loss per share, diluted	$ .05	$ 0.04	$ 0.04
Cost of Sales
Stock-based compensation in the form of stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 70	$ 80	$ 104
Selling, General and Administrative
Stock-based compensation in the form of stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	816	631	545
Research and Development
Stock-based compensation in the form of stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 215	$ 199	$ 256