Techniques are disclosed for reducing off-state leakage of fin-based transistors through the use of a sub-fin passivation layer. In some cases, the techniques include forming sacrificial fins in a bulk silicon substrate and depositing and planarizing shallow trench isolation (STI) material, removing and replacing the sacrificial silicon fins with a replacement material (e.g., SiGe or III-V material), removing at least a portion of the STI material to expose the sub-fin areas of the replacement fins, applying a passivating layer/treatment/agent to the exposed sub-fins, and re-depositing and planarizing additional STI material. Standard transistor forming processes can then be carried out to complete the transistor device. The techniques generally provide the ability to add arbitrary passivation layers for structures that are grown in STI-based trenches. The passivation layer inhibits sub-fin source-to-drain (and drain-to-source) current leakage.