13. STOCK-BASED COMPENSATION (Tables)

9 Months Ended

Feb. 29, 2020

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 nine months ended February 29, 2020 and February 28, 2019 (in thousands):

	Three Months Ended				Nine Months Ended
	February 29,		February 28,		February 29,		February 28,
	2020		2019		2020		2019
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Cost of sales	$	19	$	24	$	58	$	83
Selling, general and administrative		137		137		401		421
Research and development		51		64		152		201
Total stock-based compensation	$	207	$	225	$	611	$	705

Fair value assumptions for option valuation model

    The fair value of the Company’s stock options granted to employees for the three and nine months ended February 29, 2020 and February 28, 2019, were estimated using the following weighted average assumptions in the Black-Scholes option valuation model: 

	Three Months Ended						Nine Months Ended
	February 29,			February 28,			February 29,			February 28,
	2020			2019			2020			2019
Expected term (in years)		5			5			5			5
Volatility		0.72			0.69			0.71			0.72
Risk-free interest rate		1.57	%		2.77	%		1.85	%		2.83	%
Weighted average grant date fair value	$	1.37		$	1.01		$	0.98		$	1.34

    The fair values of the ESPP purchase rights granted for the nine months ended February 29, 2020 were estimated using the following weighted-average assumptions:

	Nine Months Ended
	February 29, 2020
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 the three and nine months ended February 29, 2020 (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
Options granted		(6	)
Options cancelled		11
Options expired		(8	)
Balance, February 29, 2020		1,811

Stock option transactions

    The following table summarizes the stock option transactions during the three and nine months ended February 29, 2020 (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 granted		6		$	2.30
Options cancelled		(11	)	$	2.42
Options exercised		(98	)	$	1.34
Balances, February 29, 2020		3,024		$	2.21	$	544
Options fully vested and expected to vest at February 29, 2020		2,989		$	2.21	$	536

Options outstanding

   The options outstanding and exercisable at February 29, 2020 were in the following exercise price ranges (in thousands, except per share data): 

		Options Outstanding						Options Exercisable
		at February 29, 2020						at February 29, 2020
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	177			0.32	$	1.28	177			0.32	$	1.28
$	1.64-$2.06		1,183		5.30	$	1.76		520	4.28		$	1.80
$	2.10-$2.81		1,436		2.90	$	2.43		1,166		2.31	$	2.43
$	3.46-$3.93		228		4.41	$	3.85		162		4.44	$	3.81
$	1.09-$3.93		3,024		3.80	$	2.21		2,025		2.81	$	2.28	$	301