Base-emitter voltage
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Under base-emitter voltage is meant the voltage between the base terminal and the emitter terminal of a bipolar transistor decreases. As soon as there is sufficient voltage at the base, the transistor becomes "permeable" and an electric current can flow through the collector-emitter path. This base-emitter voltage is usually referred to as U BE in data sheets .
With an npn transistor, the voltage must be positive and with a pnp type negative with respect to the emitter. The base-emitter voltage changes slightly with the flowing current, but for many applications it is sufficient to assume it to be constant. The transistor then acts like a switch that switches from open (non-conductive) to closed (conductive) when the voltage rises (NPN) or falls (PNP) at this voltage value. The base-emitter voltage from which a bipolar transistor becomes "permeable" depends on the semiconductor material used and on the temperature . For silicon at room temperature the value is around 600–700 mV, for germanium it is around 200–300 mV.
If the base-emitter voltage has the opposite polarity to the case just mentioned, then the base-emitter diode blocks, and the entire transistor remains blocked. It should be noted, however, that for most transistor types only a comparatively low permitted base-emitter reverse voltage is specified (often in the range of around 5 V).