Investments in Privately-Held Raw Material Companies - Minority Investment Entities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AXT, Inc. Stockholders' Equity
Summarized income information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Net revenue	$ 6,252,000	$ 5,458,000	$ 8,549,000
Gross profit	1,504,000	558,000	1,675,000
Operating (loss)	504,000	(700,000)	(778,000)
Net income (loss)	111,000	(1,876,000)	(1,080,000)
Five Minority Investments
Summarized income information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Net revenue	20,049,000	18,991,000	33,212,000
Gross profit	4,907,000	2,013,000	6,457,000
Operating (loss)	1,957,000	(2,266,000)	(3,152,000)
Net income (loss)	1,014,000	(3,000,000)	(4,750,000)
Summarized balance sheet information of all the minority investment entities that are not consolidated and accounted for under the equity method [Abstract]
Current assets	24,136,000	22,144,000
Noncurrent assets	11,339,000	11,990,000
Current liabilities	12,502,000	13,726,000
Minority investment entities
Entity (loss) excluding impairment	100,000	(1,900,000)	(1,100,000)
Minority Investment Entities [Member]
Minority investment entities
Dividends received	0	362,000	$ 0
Undistributed retained earnings	$ 1,300,000	$ 1,200,000