FinFETs in which a swelled material within the fin, typically an oxide of the fin semiconductor, causes strain that significantly increases charge carrier mobility within the FinFET channel. The concept can be applied to either p-type or n-type FinFETs. For p-type FinFETs the swelled material is positioned underneath the source and drain regions. For n-type FinFETs the swelled material is positioned underneath the channel region. The swelled material can be used with or without strain-inducing epitaxy on the source and drain areas and can provide greater strain than is achievable by strain-inducing epitaxy alone.