Why can't I make an and gate with 1 NPN transistor?
If I understand your question correctly, you’d like to use a single NPN transistor as a 2-input AND gate like this:







simulate this circuit &ndash; Schematic created using CircuitLab

One thing you may not know is that BJT have a somewhat symmetric structure. You probably already know that it behaves like a diode between base et emitter, but it also behaves like a diode between base and collector.

Hence, if you have input1 low and input2 high, the B-C diode will short the circuit between input1 and input2 and something will fry.

You may want to prevent this by adding a diode between input1 and Q1’s collector. It would still not work when input1 is low and input2 is high because the B-E diode would the make output high.