Document and Entity Information (USD $)	12 Months Ended
	Mar. 31, 2012	Jun. 01, 2012	Sep. 30, 2011
Document and Entity Information [Abstract]
Entity Registrant Name	SUPERTEX INC
Entity Central Index Key	0000730000
Current Fiscal Year End Date	--03-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 186,156,348
Entity Common Stock, Shares Outstanding		12,008,921
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Mar. 31, 2012