2. STOCK-BASED COMPENSATION - Compensation costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2016	Nov. 30, 2015	Nov. 30, 2016	Nov. 30, 2015
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	$ 215	$ 254	$ 534	$ 573
Cost of Sales
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	23	20	47	42
Selling, General and Administrative
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	141	186	388	424
Research and Development
Stock-based compensation in the form of employee stock options, RSUs and ESPP shares included in:
Total stock-based compensation	$ 51	$ 48	$ 99	$ 107