12. STOCK-BASED COMPENSATION (Tables)

6 Months Ended

Nov. 30, 2019

Share-based Payment Arrangement [Abstract]

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

    The following table summarizes the stock-based compensation expense for the three and six months ended November 30, 2019 and 2018 (in thousands):

	Three Months Ended				Six Months Ended
	November 30,				November 30,
	2019		2018		2019		2018
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Cost of sales	$	19	$	23	$	39	$	59
Selling, general and administrative		135		136		264		284
Research and development		51		65		101		137
Total stock-based compensation	$	205	$	224	$	404	$	480

Fair value assumptions for option valuation model

    The fair value of the Company’s stock options granted to employees for the three and six months ended November 30, 2019 and 2018 were estimated using the following weighted average assumptions in the Black-Scholes option valuation model:

	Three Months Ended				Six Months Ended
	November 30,				November 30,
	2019		2018		2019		2018
Expected term (in years)		5		5		5		5
Volatility		0.72		0.70		0.71		0.72
Risk-free interest rate		1.60%		3.01%		1.85%		2.84%
Weighted average grant date fair value	$	1.05	$	1.21	$	0.98	$	1.38

   The fair values of the ESPP purchase rights granted for the three and six months ended November 30, 2019 were estimated using the following weighted-average assumptions:

	Three and Six Months Ended
	November 30, 2019
Expected term (in years)		0.5-2.0
Volatility		0.62-0.71
Expected dividend	$	0.00
Risk-free interest rates		1.56% - 1.81%
Estimated forfeiture rate		0%
Weighted average grant date fair value	$	0.80

Stock option and RSU transactions

    The following tables summarize the Company’s stock option and RSU transactions during three and six months ended November 30, 2019 (in thousands):

	Available
	Shares
Balance, May 31, 2019		1,147
Options granted		(527	)
Options cancelled		151
Options expired		(119	)
Balance, August 31, 2019		652
Options reserved		1,196
Options granted		(58	)
Options cancelled		280
Options expired		(256	)
Balance, November 30, 2019		1,814

Stock option transactions

    The following table summarizes the stock option transactions during the three and six months ended November 30, 2019 (in thousands, except per share data):

		Outstanding Options
					Weighted
		Number			Average		Aggregate
		of			Exercise		Intrinsic
		Shares			Price		Value
Balances, May 31, 2019		3,107		$	2.20	$	282
Options granted		527		$	1.64
Options cancelled		(151	)	$	1.50
Options exercised		(49	)	$	1.27
Balances, August 31, 2019		3,434		$	2.16	$	41
Options granted		58		$	1.77
Options cancelled		(280	)	$	2.19
Options exercised		(85	)	$	1.06
Balances, November 30, 2019	$	3,127		$	2.18	$	358
Options fully vested and expected to vest at November 30, 2019	$	3,091		$	2.18	$	354

Options outstanding

    The options outstanding and exercisable at November 30, 2019 were in the following exercise price ranges (in thousands, except per share data):

		Options Outstanding						Options Exercisable
		at November 30, 2019						at November 30, 2019
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
$	1.09-$1.28		260		0.57	$	1.28		260		0.57	$	1.28
$	1.64-$2.06		1,199		5.53	$	1.76		447		4.22	$	1.81
$	2.10-$2.81		1,439		3.13	$	2.43		1,146		2.48	$	2.43
$	3.46-$3.93		229		4.66	$	3.85		151		4.69	$	3.80
$	1.09-$3.93		3,127		3.95	$	2.18		2,004		2.79	$	2.25	$	219