11. STOCKHOLDERS' EQUITY, COMPREHENSIVE INCOME AND STOCK-BASED COMPENSATION (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	May 31, 2020	May 31, 2019	May 31, 2018
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 910	$ 905	$ 996
Effect on net (loss) income per share, basic	$ .04	$ 0.04	$ 0.05
Effect on net (loss) income per share, diluted	$ .04	$ 0.04	$ 0.04
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 80	$ 104	$ 148
Selling, General and Administrative
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	631	545	592
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights included in:
Total stock-based compensation	$ 199	$ 256	$ 256