14. STOCK-BASED COMPENSATION (Tables)

9 Months Ended

Feb. 28, 2021

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 28, 2021 and February 29, 2020 (in thousands):

	Three Months Ended				Nine Months Ended
	February 28,		February 29,		February 28,		February 29,
	2021		2020		2021		2020
Stock-based compensation in the form of employee stock options, RSUs and ESPP purchase rights, included in:
Cost of sales	$	14	$	19	$	45	$	58
Selling, general and administrative		194		137		593		401
Research and development		63		51		160		152
Total stock-based compensation	$	271	$	207	$	798	$	611

Fair value assumptions for option valuation model

    Fair Value. The fair values of the Company’s stock options granted to employees for the three and nine months ended February 28, 2021 and February 29, 2020, 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,
	2020			2021			2020
Expected term (in years)		5			6			5
Volatility		0.72			0.72			0.71
Risk-free interest rate		1.57	%		0.39	%		1.85	%
Weighted average grant date fair value	$	1.37		$	1.09		$	0.98

Fair value assumption of the ESPP Purchase Rights

    The fair values of the ESPP purchase rights granted for the nine months ended February 28, 2021 and February 29, 2020 were estimated using the following assumptions:

	Nine Months Ended
	February 28,			February 29,
	2021			2020
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 rate		0.10%-0.14	%		1.56%-1.81	%
Weighted average grant date fair value	$	0.44		$	0.80

Stock option and RSU transactions

    The following tables summarize the Company’s stock option and RSU transactions during three and nine months ended February 28, 2021 (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
RSUs granted		(42	)
Shares withheld for taxes and not issued		4
Options cancelled		21
Balance, February 28, 2021		1,195

Stock option transactions

    The following table summarizes the stock option transactions during the three and nine months ended February 28, 2021 (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 cancelled	(21	)	$	1.86
Options exercised	(61	)	$	2.07
Balances, February 28, 2021	2,804		$	2.16	$	1,994
Options fully vested and expected to vest at February 28, 2021	2,769		$	2.17	$	1,967

Options outstanding

    The options outstanding and exercisable at February 28, 2021 were in the following exercise price ranges (in thousands, except per share data):

			Options Outstanding						Options Exercisable
			at February 28, 2021						at February 28, 2021
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		6.30	$	1.27		36		6.36	$	1.28
	$	1.64-$1.86		1,047		5.04	$	1.70		589		4.50	$	1.69
	$	2.03-$2.46		1,017		3.25	$	2.21		781		2.86	$	2.19
	$	2.63-$2.81		405		0.66	$	2.71		404		0.65	$	2.71
	$	3.46-$3.93		202		3.41	$	3.86		184		3.41	$	3.85
	$	1.22-$3.93		2,804		3.70	$	2.16		1,994		3.01	$	2.28	$	1,224