Stockholders' Equity

6 Months Ended

Jun. 30, 2021

Equity [Abstract]

Stockholders' Equity

Stockholders’ Equity

Shares Reserved for Future Issuance

The following shares of the Company’s common stock were reserved for future issuance as of June 30, 2021 (in thousands):

Shares reserved for issuance upon conversion of Convertible Senior Notes	2,554
Shares underlying outstanding warrants	251
Shares underlying outstanding stock options	5,691
Shares underlying unvested restricted stock units	453
Shares authorized for issuance pursuant to awards granted under the ESPP	1,289
Shares authorized for future equity award grants	1,440
	11,678

Common Stock Warrants

Warrants outstanding to purchase shares of the Company's common stock as of June 30, 2021 were as follows:

Issue Date	Exercise Price Per Share		Warrants Outstanding	Expiration Date
October 2017	$	3.50	29,700	October 2022
March 2017	$	23.50	193,788	March 2027
August 2011 - August 2012	$	73.73	27,704	August 2021 - August 2022
			251,192

Each warrant allows the holder to purchase one share of the Company's common stock at the exercise price per share of the respective warrant. The Company issued 522 and 125,522 shares of its common stock, respectively, upon the exercise of warrants during the three and six months ended June 30, 2021, and 32,574 and 34,689 shares of its common stock, respectively, upon the exercise of warrants during the three and six months ended June 30, 2020.

Stock Plans

The Company’s Amended and Restated 2013 Stock Incentive Plan (2013 Plan) was originally approved by the Company’s board of directors in October 2013. Under the 2013 Plan, the Company may grant stock options, stock appreciation rights, restricted stock and restricted stock units to individuals who are then employees, officers, directors or consultants of the Company.

The ESPP enables eligible employees to purchase shares of the Company’s common stock using their after-tax payroll deductions, subject to certain conditions. Generally, offerings under the ESPP consist of a two-year offering period with four six-month purchase periods which begin in May and November of each year.

Stock-Based Compensation

Common Stock Options

The Company granted options to purchase 239,608 and 355,008 shares of common stock, respectively, during the three and six months ended June 30, 2021, and options to purchase 621,045 and 850,956 shares of common stock, respectively, during the three and six months ended June 30, 2020. These options have an exercise price equal to the closing price of the Company’s common stock on the applicable award date, and generally vest as to 25% of the underlying shares on the first anniversary of the award, with the balance of the options vesting monthly over the following three years.

The Company estimates the fair value of stock options using the Black-Scholes option-pricing model on the grant date. The assumptions used in the Black-Scholes option-pricing model for common stock options were as follows:

	Stock Options
	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Weighted average grant date fair value (per share)	$54.92		$52.50		$56.89		$50.04
Risk-free interest rate	1.1	%	0.4	%	1.0	%	0.6	%
Expected dividend yield	0.0	%	0.0	%	0.0	%	0.0	%
Expected volatility	74.9	%	75.0	%	75.1	%	74.3	%
Expected term (in years)	6.1		6.0		6.1		6.0

Restricted Stock Units (RSUs)

The Company granted 339,386 and 342,594 restricted stock units (RSUs), respectively, during the three and six months ended June 30, 2021, and 131,848 RSUs during the three and six months ended 2020. These RSUs have a grant value equal to the closing price of the Company’s common stock on the award date, and generally vest based only on service as to 25% of the underlying shares on the first anniversary of the award, with the balance of the RSUs vesting quarterly over the following three years. The Company also granted 25,674 performance-based RSUs during the three months ended June 30, 2021. The performance-based RSUs have a grant value equal to the closing price of the Company’s common stock on the award date, and vest upon the Company’s actual performance relative to predefined performance metrics and subject to the awardee’s continuing service through the December 31, 2024 measurement date. The weighted average grant date fair value of RSUs granted were as follows:

	Restricted Stock Units
	Three Months Ended June 30,		Six Months Ended June 30,
	2021	2020	2021	2020
Weighted average grant date fair value (per share)	$82.31	$82.33	$82.41	$82.33

Employee Stock Purchase Plan

The Company records stock-based compensation expense associated with the ESPP using the Black-Scholes option-pricing model. Valuations are performed on the grant date at the beginning of the purchase period, which generally occurs in May and November of each year. The assumptions used in the Black-Scholes option-pricing model for the ESPP were as follows:

	ESPP
	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Weighted average grant date fair value (per share)	$29.24		$36.18		$29.24		$36.18
Risk-free interest rate	0.1	%	0.2	%	0.1	%	0.2	%
Expected dividend yield	0.0	%	0.0	%	0.0	%	0.0	%
Expected volatility	47.5	%	65.7	%	47.5	%	65.7	%
Expected term (in years)	1.3		1.3		1.3		1.3

The following table summarizes the allocation of stock-based compensation expense included in the condensed consolidated statements of operations for all stock-based compensation arrangements (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of sales	$	1,591	$	2,162	$	3,067	$	4,326
Selling, general & administrative	10,881		11,707		20,292		23,210
Research and development	2,505		2,552		4,565		4,750
Total stock-based compensation expense	$	14,977	$	16,421	$	27,924	$	32,286

The total stock-based compensation expense capitalized as part of the cost of the Company’s inventories was $0.8 million at June 30, 2021, and $0.6 million at December 31, 2020.