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Document and Entity Information - shares
 3 Months Ended
Sep. 30, 2018
Oct. 25, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name ALPHA & OMEGA SEMICONDUCTOR Ltd  
Entity Central Index Key 0001387467  
Current Fiscal Year End Date --06-30  
Entity Filer Category Accelerated Filer  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q1  
Amendment Flag false  
Entity Common Stock, Shares Outstanding   23,787,936