A method in which an oxide layer is formed on material defining and surrounding an emitter window. The technique comprises depositing a non-conformal oxide layer on the surrounding material and in the emitter window, whereby the thickness of at least a portion of the oxide layer in the emitter window is smaller than the thickness of the oxide layer on the surrounding material outside the emitter window; and removing at least a portion of the oxide layer in the emitter window so as to reveal at least a portion of the bottom of the emitter window whilst permitting at least a portion of the oxide layer to remain on the surrounding material. The technique can be used in the manufacture of a self-aligned epitaxial base BJT (bipolar junction transistor) or SiGe HBT (hetero junction bipolar transistor).