To choose an acceptable transistor replacement, select one with similar polarity, material, breakdown voltage and functional type, belonging to the same package. Before replacing it in the circuit, check if it can take the required current and has a similar power dissipation. Although these are the most important parameters of the replacement transistor, certain other considerations are relevant to some circuits.
Choosing a suitable replacement transistor amounts to determining the specifications of the original and matching them with those of possible replacements. If a transistor data book which lists its specifications is not available, you may be able to find many details by examining its package or the circuit it is being used in. Use a transistor whose specifications overlap considerably with those of the original as a replacement, if an identical one is difficult to find. Ensure that the maximum and minimum ratings of the replacement, such as breakdown voltage, current rating and power dissipation, closely match those of the original.
Install the replacement transistor in the circuit, and check its performance. Satisfactory performance of the circuit is the only real way to ascertain that the replacement is adequate. If there is a problem, verify that the process of selecting the replacement contains no errors, and look for other parameters, such as frequency limit and package, that may affect the operation of the circuit.