According to a disclosed embodiment, a gas is supplied at a certain partial pressure for a chemical reaction with a top surface of a base (120) in a transistor (100). The top surface of the base (120) is heated to a certain temperature to promote the chemical reaction. For example, the gas can be oxygen, the base (120) can be an epitaxial single crystal silicon-germanium base of a heterojunction bipolar transistor ("HBT"), and the chemical reaction can be oxidation of the silicon in the top surface of the silicon-germanium base (120). In one embodiment of the invention, the partial pressure of oxygen is maintained at 0.1 atmosphere and the top surface of the base (120) is heated using rapid thermal processing ("RTP") to a temperature of 500 °C. The chemical reaction forms a dielectric layer on the top surface of the base (120). For example, using oxygen as stated above, a dielectric layer of silicon oxide ("interfacial oxide") is formed. Controlling the thickness and density of the interfacial oxide causes the gain of the transistor (100) to be as desired. For example, using oxygen in the silicon-germanium HBT at 0.1 atmosphere partial pressure, and RTP to heat the top surface of the base (120) of the HBT to 500 C, an interfacial oxide is formed with thickness approximately 9.0 to 13.0 Angstroms and area density in a range of approximately 1 * 10 to 4 *10 atoms per square centimeter, which causes the gain of the HBT to be the desired value of approximately 100.0.