SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Risk (Details)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenue benchmark | Customer concentration risk | Intel
Concentration Risk [Line Items]
Concentration risk, percentage	15.00%	14.00%
Revenue benchmark | Customer concentration risk | Samsung Group (Samsung)
Concentration Risk [Line Items]
Concentration risk, percentage	11.00%	11.00%	18.00%
Revenue benchmark | Customer concentration risk | Taiwan Semiconductor Manufacturing Co. (TSMC)
Concentration Risk [Line Items]
Concentration risk, percentage			12.00%
Revenue benchmark | Customer concentration risk | SK Hynix Inc.
Concentration Risk [Line Items]
Concentration risk, percentage			10.00%
Net accounts receivable | Credit concentration risk | Intel
Concentration Risk [Line Items]
Concentration risk, percentage	8.50%	8.10%
Net accounts receivable | Credit concentration risk | Samsung Group (Samsung)
Concentration Risk [Line Items]
Concentration risk, percentage	7.00%	5.50%