14. STOCK-BASED COMPENSATION (Tables)

6 Months Ended

Nov. 30, 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 six months ended November 30, 2020 and 2019 (in thousands):

	Three Months Ended				Six Months Ended
	November 30,				November 30,
	2020		2019		2020		2019
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Cost of sales	$	15	$	19	$	31	$	39
Selling, general and administrative		194		135		399		264
Research and development		48		51		97		101
Total stock-based compensation	$	257	$	205	$	527	$	404

Fair value assumptions for option valuation model

    Fair Value. The fair value of the Company’s stock options granted to employees for the three and six months ended November 30, 2020 and 2019 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,
	2020			2019			2020			2019
Expected term (in years)		6			5			6			5
Volatility		0.73			0.72			0.72			0.71
Risk-free interest rate		0.43	%		1.60	%		0.39	%		1.85	%
Weighted average grant date fair value	$	0.91		$	1.05		$	1.09		$	0.98

Fair value assumption of the ESPP Purchase Rights

    The fair values of the ESPP purchase rights granted for the three and six months ended November 30, 2020 and 2019 were estimated using the following assumptions:

	Three and Six Months Ended			Three and Six Months Ended
	November 30, 2020			November 30, 2019
Expected term (in years)		0.5-2.0			0.5-2.0
Volatility		0.74-0.82			0.62-0.71
Expected dividend	$	0.00		$	0.00
Risk-free interest rates		0.10%-0.14	%		1.56%-1.81	%
Estimated forfeiture rate		0	%		0	%
Weighted average grant date fair value	$	0.42		$	0.80

Stock option and RSU transactions

    The following tables summarize the Company’s stock option and RSU transactions during the three and six months ended November 30, 2020 (in thousands):

	Available
	Shares
Balance, May 31, 2020		1,650
Options granted		(200	)
RSUs granted		(196	)
Options cancelled		188
RSUs cancelled		2
Options expired		(125	)
Balance, August 31, 2020		1,319
Options granted		(73	)
RSUs granted		(57	)
Options cancelled		185
Options expired		(162	)
Balance, November 30, 2020		1,212

Stock option transactions

    The following table summarizes the stock option transactions during the three and six months ended November 30, 2020 (in thousands, except per share data):

	Outstanding Options
				Weighted
	Number			Average		Aggregate
	of			Exercise		Intrinsic
	Shares			Price		Value
Balances, May 31, 2020	3,153		$	2.17	$	102
Options granted	200		$	1.86
Options cancelled	(188	)	$	2.18
Options exercised	(148	)	$	1.30
Balances, August 31, 2020	3,017		$	2.19	$	148
Options granted	73		$	1.44
Options cancelled	(185	)	$	2.42
Options exercised	(19	)	$	1.67
Balances, November 30, 2020	2,886		$	2.16	$	56
Options fully vested and expected to vest at November 30, 2020	2,850		$	2.16	$	54

Options outstanding

    The options outstanding and exercisable at November 30, 2020 were in the following exercise price ranges (in thousands, except per share data):

	Options Outstanding						Options Exercisable
	at November 30, 2020						at November 30, 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
$1.22-$1.34		133		6.55	$	1.27		17		6.46	$	1.25
$1.64-$1.86		1,092		5.28	$	1.70		574		4.64	$	1.69
$2.03-$2.46		1,048		3.48	$	2.21		771		3.02	$	2.19
$2.63-$2.81		411		0.90	$	2.71		410		0.88	$	2.71
$3.46-$3.93		202		3.66	$	3.86		174		3.66	$	3.85
$1.22-$3.93		2,886		3.95	$	2.16		1,946		3.14	$	2.29	$	9