11. STOCKHOLDERS' EQUITY, COMPREHENSIVE INCOME AND STOCK-BASED COMPENSATION (Tables)

12 Months Ended

May 31, 2020

Equity [Abstract]

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

    The following table summarizes the stock-based compensation expense for the fiscal years ended May 31, 2020, 2019 and 2018 (in thousands, except per share data):

	Year Ended May 31,
	2020		2019		2018
Stock-based compensation in the form of stock options, RSUs, and ESPP purchase rights, included in:
Cost of sales	$	80	$	104	$	148
Selling, general and administrative		631		545		592
Research and development		199		256		256
Net effect on net (loss) income	$	910	$	905	$	996
Effect on net (loss) income per share:
Basic	$	0.04	$	0.04	$	0.05
Diluted	$	0.04	$	0.04	$	0.04

Fair value assumptions for Option Valuation Model

    The fair values of the Company’s stock options granted to employees in fiscal 2020, 2019 and 2018 were estimated using the following weighted average assumptions in the Black-Scholes option valuation method:

	Year Ended May 31,
	2020			2019			2018
Expected term (in years)		5			5			4
Volatility		0.72			0.72			0.77
Risk-free interest rates		1.56	%		2.83	%		1.95	%
Weighted-average grant date fair value	$	0.95		$	1.33		$	2.07

Fair value assumption of the ESPP Purchase Rights

    The fair value of our ESPP purchase rights for the fiscal 2020, 2019 and 2018 was estimated using the following weighted-average assumptions:

	Year End May 31,
	2020		2019		2018
Expected term (in years)		0.5 – 2.0		0.5 – 2.0		0.5 – 2.0
Volatility		0.62 – 0.77		0.48 – 0.78		0.56 – 0.81
Risk-free interest rates		0.14%–1.81%		2.33%–2.82%		1.92%–2.25%
Weighted-average grant date fair value	$	0.79	$	1.14	$	1.01

Stock option and RSU transactions

    The following tables summarize the Company’s stock option and RSU transactions during fiscal 2020, 2019 and 2018 (in thousands):

	Available
	Shares
Balance, May 31, 2017		2,169
Options granted		(338	)
RSUs granted		(64	)
RSUs cancelled		33
Options terminated		16
Options expired		(4	)
Balance, May 31, 2018		1,812
Options granted		(804	)
RSUs cancelled		8
Options terminated		195
Options expired		(64	)
Balance, May 31, 2019		1,147
Additional shares reserved		1,196
Options granted		(738	)
RSUs granted		(25	)
Shares withheld for taxes and not issued		6
Options terminated		457
Options expired		(393	)
Balance, May 31, 2020		1,650

Stock option transactions

    The following table summarized the stock option transactions during fiscal 2020, 2019 and 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		338		$	3.56
Options terminated		(16	)	$	2.72
Options exercised		(537	)	$	1.17
Balances, May 31, 2018		2,859		$	2.04	$	1,987
Options granted		804		$	2.19
Options terminated		(195	)	$	2.32
Options exercised		(361	)	$	0.85
Balances, May 31, 2019		3,107		$	2.20	$	283
Options granted		738		$	1.61
Options terminated		(457	)	$	1.98
Options exercised		(235	)	$	1.22
Balances, May 31, 2020		3,153		$	2.17	$	102
Options fully vested and expected to vest at May 31, 2020		3,115		$	2.17	$	101

Options outstanding

    The options outstanding and exercisable at May 31, 2020 were in the following exercise price ranges (in thousands, except per share data):

		Options Outstanding						Options Exercisable
		at May 31, 2020						at May 31, 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-$1.28		248		2.10	$	1.26		176		0.19	$	1.28
$	1.64-$1.77		945		5.35	$	1.66		487		4.66	$	1.68
$	2.03-$2.46		1,223		3.70	$	2.21		858		3.03	$	2.19
$	2.63-$2.81		511		1.29	$	2.70		510		1.28	$	2.70
$	3.46-$3.93		226		4.16	$	3.84		172		4.18	$	3.82
$	1.22-$3.93		3,153		3.71	$	2.17		2,203		2.85	$	2.25	$	67