The value of the gain ß of a transistor varies depending on the fabrication characteristics of the device itself. The exact value is not defined but lies in a range, with ß usually between 20 and 200. A typical (and ideal) value is 100.
The value of the gain also depends on the environment, being especially affected by thermal variations during the operations of the transistor. Biasing techniques that counter a variation of ß are usually preferred to one that left it unchecked. Read more