14. STOCK-BASED COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 271	$ 207	$ 798	$ 611
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	14	19	45	58
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	137	593	401
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 63	$ 51	$ 160	$ 152