Transistor works with just base connected

Question

Currently I am trying to self learn electronics. After the diode I moved to the transistor.

I understood the transistor as an “electronic” switch with no moving parts. When the base is connected, then the current can flow from the collector to the emitter. Otherwise it cannot.

Though, when I put 5v across the base, with no voltage across collector nor emitter, an LED, connected to the emitter through a resistor, lights up. Is this normal? Am I missing something on the functions of a transistor?

Answer 1

Yes, it's normal, current into base flows out from emitter, regardless of what connections, if any, you have on the collector.

The base current controls the collector current. Both base and collector current come out of emitter.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1.

• (a) A transistor when tested with a diode tester appears to be a pair of PN (diode) junctions.
• (b) In this case Q2 is being used as a diode and performs the same as D4 would in the equivelant circuit.

Answer 3

The base-emitter junction is a PN junction which is broadly similar to (but obviously with important differences from) a diode's PN junction that also allow it to control the flow of charge carriers to/from the collector. Like @justme mentioned, current into the base exits through the emitter just as if you had connected a diode. The transistor's base connection is, however, not designed to carry large currents like a diode, so be sure to limit the current with a resistor.