2. STOCK-BASED COMPENSATION - Compensation costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2017	Feb. 29, 2016
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	$ 257	$ 213	$ 791	$ 786
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	22	20	69	62
Selling, General and Administrative
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	187	145	575	569
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	$ 48	$ 48	$ 147	$ 155