2. STOCK-BASED COMPENSATION - Compensation costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2015	Nov. 30, 2014	Nov. 30, 2015	Nov. 30, 2014
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	$ 254	$ 277	$ 573	$ 474
Cost of Sales
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	20	16	42	30
Selling, General and Administrative
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	186	212	424	360
Research and Development
Stock-based compensation in the form of employee stock options and ESPP shares included in:
Total stock-based compensation	$ 48	$ 49	$ 107	$ 84