Business Segments - Schedule of Business Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net revenues	$ 15,084	$ 15,227	$ 14,460	$ 20,692	$ 20,174	$ 21,003	$ 20,633	$ 23,225	$ 65,463	$ 85,035	$ 100,053
Operating income (loss)	(1,181)	$ 31	$ (972)	$ 1,637	1,653	$ 1,351	$ 1,248	$ 664	(485)	4,916	6,072
Capital expenditures									2,676	714	1,495
Depreciation and amortization									1,258	1,690	1,854
Identifiable assets	102,098				126,477				102,098	126,477
Continuing Operations
Segment Reporting Information [Line Items]
Capital expenditures									2,675	583	994
Depreciation and amortization									1,078	1,125	1,053
Discontinued Operations
Segment Reporting Information [Line Items]
Identifiable assets					22,755					22,755
Operating Segments | Semiconductor
Segment Reporting Information [Line Items]
Net revenues									54,516	66,455	80,163
Operating income (loss)									4,168	8,744	11,848
Identifiable assets	51,648				56,855				51,648	56,855
Operating Segments | Semiconductor | Continuing Operations
Segment Reporting Information [Line Items]
Capital expenditures									912	379	352
Depreciation and amortization									821	828	715
Operating Segments | SiC/LED
Segment Reporting Information [Line Items]
Net revenues									10,304	13,682	13,761
Operating income (loss)									684	3,641	3,672
Identifiable assets	12,717				7,779				12,717	7,779
Operating Segments | SiC/LED | Continuing Operations
Segment Reporting Information [Line Items]
Capital expenditures									1,724	171	603
Depreciation and amortization									197	136	136
Non-Segment Related
Segment Reporting Information [Line Items]
Net revenues									643	4,898	6,129
Operating income (loss)									(5,337)	(7,469)	(9,448)
Identifiable assets	$ 37,733				$ 39,088				37,733	39,088
Non-Segment Related | Continuing Operations
Segment Reporting Information [Line Items]
Capital expenditures									39	33	39
Depreciation and amortization									$ 60	$ 161	$ 202