2. STOCK-BASED COMPENSATION - Compensation costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	$ 213	$ 219	$ 786	$ 693
Cost of Sales
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	20	17	62	47
Selling, General and Administrative
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	145	148	569	508
Research and Development
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	$ 48	$ 54	$ 155	$ 138