npn transistor

Hello there! So, to understand the transistor you need to comprehend what is inside the component. Since you're using a NPN transistor on your circuit I'll explain towards it. Basically a transistor is a silicon junction containing 3 sections, the first one is the collector, which is Doped with electrons. The middle section is called Base, which is doped with "holes" or we could say it lacks electrons in its atomic structure and at last the third section is the emitter, which is again doped with electrons (it is slightly less doped than the collector if I'm not mistaken). So we have 2 PN junctions containing barrier potentials. The barrier potentials are the reason why you need to rise the base voltage to make your transistor conduct. There are various forms to make a transistor conduct, those are called operation regions. In your circuit, the npn transistor is operating on the forward active region, where your Vcollector > Vbase > Vemitter. The minimum voltage to bias your Base-Emitter junction is around 600mV. You're using a potential 200mV higher than the necessary thus drawing the current from the 9V source to turn on the LED. To understand this phenomena I suggest you to read about PN junctions. If you're interested about the operating regions, it can be understood viewing the plot of the Collector-Emitter voltage versus Collector current, there you can find a proper Base current that will set your transistor at the desired point of operation. If you're interested in electronics I suggest reading Sedra | Smith Microelectronics. It is a really good book and you can find older editions online. I hope my explanation took your doubts away. Good studies!

Excellent explanation...👍