13. STOCK-BASED COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 207	$ 225	$ 611	$ 705
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	19	24	58	83
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	137	137	401	421
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Total stock-based compensation	$ 51	$ 64	$ 152	$ 201