14. STOCK-BASED COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2020	Nov. 30, 2019	Nov. 30, 2020	Nov. 30, 2019
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 257	$ 205	$ 527	$ 404
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	15	19	31	39
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	194	135	399	264
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 48	$ 51	$ 97	$ 101