13. STOCK-BASED COMPENSATION (Details Narrative) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Feb. 28, 2019	Feb. 29, 2020	Feb. 28, 2019
Stock-based compensation costs capitalized as part of inventory	$ 0	$ 0	$ 0	$ 0
Intrinsic value of options exercised	92	160	$ 159	322
Weighted average remaining contractual life of the options exercisable and expected to be exercisable			3 years 9 months 11 days
Stock Option and RSU Transactions
Stock-based compensation expense related to stock options and RSUs	$ 166	$ 166	$ 480	$ 505
Restricted Stock Units granted (in thousands)	0	0	0	0
Restricted Stock Units vested (in thousands)	3	4	10	13
Restricted Stock Units unvested (in thousands)	13	34	13	34
Restricted Stock Units unvested intrinsic value	$ 27	$ 50	$ 27	$ 50
2016 Equity Incentive Plan
Unrecognized stock-based compensation	1,244		1,244
Estimated forfeitures of unvested stock based awards, amount	3		$ 3
Weighted average period for recognition of costs			2 years 8 months 12 days
Employee Stock Purchase Plan
Weighted average period for recognition of costs			9 months 18 days
Stock-based compensation related to the ESPP	41	$ 59	$ 131	$ 200
Compensation cost related to purchase rights under the ESPP but not yet recognized	$ 77		$ 77
ESPP purchase right granted (in thousands)	0	0	38	327
ESPP shares issued (in thousands)			71	64
ESPP shares available for issuance (in thousands)	299		299