transistor current is dependent on a factor known as Beta of the transistor. a darlington pair has a large Beta(10,000) , a small signal transistor such as the 2n3904 has a small beta of around 100. the beta of a transistor determines the amount of current that flows from collector to emitter ( bipolar transistors) for each amount of current that flows into the base you get a hundred fold increase between collector and emitter (2n3904), so you insert 1 ma (milliamp) into the base, you get 100 ma out the emitter. there is a doped region in the middle of the transistor that expands when current is applied to the base, this expansion allows more current to flow from collector to emitter (npn). there are many types of transistors but they all employ basically the same function. The mathematics involved is a bit more complicated but I speaking in general terms here to avoid getting someone lost.
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transistor either increase or decrease current bt capacitor stores the energy
The active region of a transistor is when the transistor has sufficient base current to turn the transistor on and for a larger current to flow from emitter to collector. This is the region where the transistor is on and fully operating.
emitter current
Recombination of negative and positive charge carriers
The total current in a transistor is the emitter current, which is the sum of the collector current and the base current. The ratio of collector current over base current, in linear mode, is beta-DC, or hFe. However, in order for linear mode to be true, the ratings of the transistor must not be exceeded, and the collector current must not be limited by the circuit. Note also that hFe is temperature dependent, and is expressed in the data sheet as a minimum, not an absolute value - it can vary from transistor to transistor.