Does a negative base current in BJT npn mean that the operation mode is Reverse Active?

Question

Given Vce = 2V, Ic = 1.7mA, and Ie = 1.6 mA, I used KCL to determine that Ib = -0.1 mA. Does this mean that the BJT npn is in reverse active mode?

Answer 1

In order to have a NPN transistor in the reverse-active region, you need to have Vc<Vb<Ve (for an NPN BJT). This means that Vce must be less than 0. Given that your calculated Vce is equal to 2 volts, I think you might've calculated your base current wrong since current will flow into the base terminal of a transistor regardless of mode of operation. I've made a simple circuit to show what exactly goes on when an NPN transistor is in the reverse-active region so that you can understand it better: https://everycircuit.com/circuit/5450644806434816

Answer 2

The situation you describe is this:

^{simulate this circuit – Schematic created using CircuitLab}

This is not reverse active, since \$I_C\$ is positive.

The only way to get the transistor to source current from its base in this way is to significantly reverse bias the base-emitter junction. That junction behaves like a zener diode, and would require \$V_B << V_E\$ to begin conducting. Typically we assume that the reverse breakdown voltage of the junction is 5V or more, so to make the junction source current, you would need to make the base at least 5V lower in potential than the emitter:

^{simulate this circuit}

_{Note: The simulator does not model a reverse-biased base-emitter junction; I've had to add everything in the dotted box to emulate that.}

However, in that state, emitter current is into the emitter. I'm not sure what the collector would be doing in this state, but I would hazard a guess that it would be sinking near zero current, as shown above.

In any case, regardless of collector current, the scenario you describe is infeasible.