What is the formula of a transistor?

The "formula of a transistor" is best understood as the relationships between the currents flowing through its terminals.

The key relationships, based on the reference, are:

• Relationship between Emitter and Collector Currents:

  The current at the emitter (I_E) and collector (I_C) terminals are related by the formula:

  I_C = α_FE I_E

  where α_FE is a constant (forward current transfer ratio in common-emitter configuration).

• Relationship between Collector and Base Currents:

  The current at the collector (I_C) and base (I_B) terminals are related by the formula:

  I_C = β_FE I_B

  where β_FE is the current gain of the transistor (also represented as h_FE in datasheets). β_FE can be calculated from α_FE using:

  β_FE = α_FE / (1 − α_FE)

These formulas describe the fundamental current relationships within a transistor. It's important to remember these are simplified models; real-world transistor behavior can be more complex.