Net Income (Loss) Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Numerator:
Net income (loss) attributable to AXT, Inc.	$ 3,425	$ (178)
Less: Preferred stock dividends	(44)	(44)
Net income (loss) available to common stockholders	$ 3,381	$ (222)
Weighted-average number of common shares outstanding:
Denominator for basic net income (loss) per share - weighted-average common shares	41,004,000	39,812,000
Effect of dilutive securities:
Denominator for dilutive net income (loss) per common shares	42,726,000	39,812,000
Net income (loss) attributable to AXT, Inc. per common share:
Basic	$ 0.08	$ (0.01)
Diluted	$ 0.08	$ (0.01)
Preferred stock, shares issued (in shares)	883,000		883,000
Preferred stock, shares outstanding (in shares)	883,000		883,000
Preferred stock, par value (in dollars per share)	$ 0.001		$ 0.001
Preferred stock, value	$ 3,532		$ 3,532
Cumulative annual dividend rate (as a percent)	5.00%		5.00%
Liquidation preference over common stock (in dollars per share)	$ 4		$ 4
Options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted net income (loss) per share as the impact is anti-dilutive (in shares)	21,000	2,763,000
Restricted Stock Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from diluted net income (loss) per share as the impact is anti-dilutive (in shares)	57,000	941,000
Options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	1,082,000
Restricted Stock Awards
Effect of dilutive securities:
Effect of dilutive securities (in shares)	640,000