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

12 Months Ended

May 31, 2021

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, 2021, 2020 and 2019 (in thousands, except per share data): 

	Year Ended May 31,
	2021		2020		2019
Stock-based compensation in the form of stock options, RSUs, and ESPP purchase rights, included in:
Cost of sales	$	70	$	80	$	104
Selling, general and administrative		816		631		545
Research and development		215		199		256
Net effect on net loss	$	1,101	$	910	$	905
Effect on net loss per share:
Basic	$	0.05	$	0.04	$	0.04
Diluted	$	0.05	$	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 2021, 2020 and 2019 were estimated using the following weighted average assumptions in the Black-Scholes option valuation method: 

	Year Ended May 31,
	2021			2020			2019
Expected term (in years)		6			5			5
Volatility		72.0	%		71.5	%		71.9	%
Risk-free interest rates		0.44	%		1.56	%		2.83	%
Weighted average grant date fair value	$	1.12		$	0.95		$	1.33

Fair value assumption of the ESPP Purchase Rights

The fair value of our ESPP purchase rights for the fiscal 2021, 2020 and 2019 was estimated using the following weighted average assumptions: 

	Year End May 31,
	2021	2020	2019
Expected term (in years)	0.5 – 2.0	0.5 – 2.0	0.5 – 2.0
Volatility	74% – 88%	62% – 77%	48% – 78%
Risk-free interest rates	0.04%–0.17%	0.14%–1.81%	2.33%–2.82%
Weighted average grant date fair value	$1.03	$0.79	$1.14

Stock option and RSU transactions

The following tables summarize the Company’s stock option and RSU transactions during fiscal 2021, 2020 and 2019 (in thousands): 

	Available
	Shares
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
Options granted		(297)
RSUs granted		(340)
RSUs cancelled		1
Shares withheld for taxes and not issued		9
Options terminated		455
Options expired		(341)
Balance, May 31, 2021		1,137

Stock option transactions

The following table summarized the stock option transactions during fiscal 2021, 2020 and 2019 (in thousands, except per share data): 

	Outstanding Options
			Weighted
	Number		Average		Aggregate
	of		Exercise		Intrinsic
	Shares		Price		Value
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 granted		297	$	1.78
Options terminated		(455)	$	2.31
Options exercised		(229)	$	1.54
Balances, May 31, 2021		2,766	$	2.16	$	807
Options fully vested and expected to vest at May 31, 2021		2,732	$	2.16	$	795

Options outstanding

The options outstanding and exercisable at May 31, 2021 were in the following exercise price ranges (in thousands, except per share data): 

		Options Outstanding						Options Exercisable
		at May 31, 2021						at May 31, 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		6.05	$	1.27		56		6.15	$	1.29
$	1.64-$1.86		1,040		4.80	$	1.70		625		4.33	$	1.69
$	2.03-$2.46		1,035		3.09	$	2.21		814		2.68	$	2.20
$	2.65-$2.81		358		0.46	$	2.72		357		0.45	$	2.72
$	3.46-$3.93		200		3.16	$	3.86		193		3.16	$	3.85
$	1.22-$3.93		2,766		3.54	$	2.16		2,045		2.94	$	2.26	$	492