Stock Plans (Tables)

12 Months Ended

Dec. 31, 2023

Stock Plans

Schedule of share-based compensation expense

	For the year ended December 31,
	2023		2022		2021
	(in thousands)
Cost of sales	$	4,913	$	4,551	$	2,373
Research and development		8,994		6,682		3,850
Selling, general, and administrative		14,651		11,761		9,026
Total	$	28,558	$	22,994	$	15,249

Summary of unrecognized share-based compensation costs

	Unrecognized		Weighted
	Share-Based		Average Period
	Compensation		Expected to be
	Costs		Recognized
	(in thousands)		(in years)
Restricted stock units	$	19,428	2.1
Restricted stock awards		9,967	1.2
Performance share units		7,875	1.8
Total unrecognized share-based compensation cost	$	37,270	1.8

Summary of stock option activity

		Weighted
	Number of	Average
	Shares	Exercise Price
	(in thousands)
Balance - December 31, 2020	730	$	35.26
Exercised	(2)		23.36
Expired	(285)		40.16
Balance - December 31, 2021	443		32.15
Expired	(266)		32.95
Balance - December 31, 2022	177		30.94
Exercised	(2)		30.47
Expired	(165)		30.53
Balance - December 31, 2023	10	$	37.42

Summary of stock options exercised

	Year ended December 31,
	2023		2022		2021
	(in thousands)
Cash received from options exercised	$	56	$	—	$	37
Intrinsic value of options exercised	$	56	$	—	$	6

Summary of non-vested restricted and performance shares activity

		Weighted
		Average
	Number of	Grant Date
	Shares	Fair Value
	(in thousands)
Balance - December 31, 2020	2,040	$	12.73
Granted	1,031		24.26
Performance award adjustments	159		18.38
Vested	(1,014)		15.50
Forfeited	(133)		15.08
Balance - December 31, 2021	2,083		17.33
Granted	1,253		29.12
Performance award adjustments	85		14.03
Vested	(844)		15.00
Forfeited	(81)		20.18
Balance - December 31, 2022	2,496		23.83
Granted	1,282		23.83
Performance award adjustments	183		10.59
Vested	(1,364)		17.47
Forfeited	(133)		29.29
Balance - December 31, 2023	2,464	$	26.19

Summary of valuation assumptions for performance awards

	Year ended December 31,
	2023			2022			2021
Weighted average fair value	$	32.25		$	45.28		$	27.81
Dividend yield		0	%		0	%		0	%
Expected volatility factor(1)		54	%		58	%		63	%
Risk-free interest rate(2)		3.84	%		2.13	%		0.34	%
Expected life (in years)(3)		3.0			3.0			3.0

(1) Expected volatility is measured using historical daily price changes of the Company’s stock over the respective expected term.

(2) The risk-free rate for periods within the contractual term is based on the U.S. Treasury yield curve in effect at the time of grant.

(3) The expected life is the number of years the Company estimates that the awards will be outstanding prior to exercise.

Summary of valuation assumptions for employee stock purchase plan

	Year ended December 31,
	2023			2022			2021
Weighted average fair value	$	5.77		$	6.00		$	5.90
Dividend yield		0	%		0	%		0	%
Expected volatility factor(1)		42	%		43	%		52	%
Risk-free interest rate(2)		5.03	%		1.73	%		0.07	%
Expected life (in years)(3)		0.5			0.5			0.5

(1) Expected volatility is measured using historical daily price changes of the Company’s stock over the respective expected term.

(2) The risk-free rate for periods within the contractual term is based on the U.S. Treasury yield curve in effect at the time of grant.

(3) The expected life is the number of years the Company estimates that the purchase rights will be outstanding prior to exercise.