2. STOCK-BASED COMPENSATION (Tables)

9 Months Ended

Feb. 28, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Compensation costs related to the Company's stock-based compensation

    The following table summarizes the stock-based compensation expense for the three and nine months ended February 28, 2018 and 2017 (in thousands):

	Three Months Ended				Nine Months Ended
	February 28,				February 28,
	2018		2017		2018		2017
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Cost of sales	$	28	$	22	$	107	$	69
Selling, general and administrative		162		187		530		575
Research and development		52		48		185		147
Total stock-based compensation	$	242	$	257	$	822	$	791

Assumptions for options valuation model

    Fair Value. The fair value of the Company’s stock options granted to employees for the three months ended February 28, 2018 and nine months ended February 28, 2018 and 2017 were estimated using the following weighted average assumptions in the Black-Scholes option valuation model:

	Three Months Ended			Nine Months Ended
	February 28,			February 28,
	2018			2018			2017
Expected term (in years)		4			4			4
Volatility		0.73			0.77			0.81
Risk-free interest rates		2.31	%		1.81	%		1.02	%
Weighted average grant date fair value	$	1.53		$	2.17		$	1.09

Stock option and RSU transactions

    The following table summarizes the Company’s stock option and RSU transactions during the three and nine months ended February 28, 2018 (in thousands):

	Available
	Shares
Balance, May 31, 2017		2,169
Options granted		(224	)
RSUs granted		(64	)
Balance, August 31, 2017		1,881
Options granted		(41	)
Balance, November 30, 2017		1,840
Options granted		(19	)
RSUs granted		--
Shares cancelled		13
Plan shares expired		(2	)
Balance, February 28, 2018		1,832

Stock option transactions

    The following table summarizes the stock option transactions during the three and nine months ended February 28, 2018 (in thousands, except per share data):

	Outstanding Options
				Weighted
	Number			Average		Aggregate
	of			Exercise		Intrinsic
	Shares			Price		Value
Balances, May 31, 2017		3,074		$	1.73	$	8,763
Options granted		224		$	3.93
Options exercised		(189	)	$	1.23
Balances, August 31, 2017		3,109		$	1.92	$	4,612
Options granted		41		$	3.46
Options exercised		(132	)	$	1.46
Balances, November 30, 2017		3,018		$	1.96	$	2,230
Options granted		19		$	2.76
Options cancelled		(13	)	$	2.71
Options exercised		(141	)	$	0.89
Balances, February 28, 2018		2,883		$	2.01	$	1,486
Options fully vested and expected to vest at February 28, 2018		2,849		$	2.01	$	1,480

Options outstanding

    The options outstanding and exercisable at February 28, 2018 were in the following exercise price ranges (in thousands, except per share data):

		Options Outstanding						Options Exercisable
		at February 28, 2018						at February 28, 2018
Range of Exercise Prices		Number Outstanding Shares		Weighted Average Remaining Contractual Life (Years)		Weighted Average Exercise Price		Number Exercisable Shares		Weighted Average Remaining Contractual Life (Years)		Weighted Average Exercise Price		Aggregate Intrinsic Value
$	0.59-$0.97		324		1.02	$	0.66		324		1.02	$	0.66
$	1.09-$1.36		605		1.88	$	1.28		605		1.88	$	1.28
$	1.68-$2.06		486		4.48	$	1.74		295		3.89	$	1.79
$	2.10-$2.81		1,204		3.79	$	2.45		1,012		3.74	$	2.47
$	3.46-$3.93		264		6.41	$	3.86		46		6.45	$	3.79
$	0.59-$3.93		2,883		3.43	$	2.01		2,282		2.93	$	1.83	$	1,351