# Help on understanding NPN transistor

## Main Question or Discussion Point

Hey,

I would like help understanding this scenario:

(1) base-collector voltage, V_BC, is reverse-biased
(2) base-emitter voltage, V_BE, is forward-biased
(3) collector current, I_C, is negative

Normally, if I_C was positive, I would assume the transistor to be in the forward-active region. However, can you still consider it in the forward-active region if the collector current is negative? Does this scenario make sense?

Thanks,

Mark

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Never heard of a negative current.. It is only a convention wrt some current flow direction.

Basically electrons fire across the forward biased emitter-base and then about 99% of them carry on across the reverse biased base-collector. A small base current controls a much larger collector current.

Defennder
Homework Helper
Did you instead meant to say that the collector current is flowing out of the collector junction? Hmm, that sounds as though the BJT is working in the reverse active mode. The gain for this is much for smaller though due to disproportionate doping for the collector and emitter regions. Check your textbook for details on that mode of operation.

If the BE junction is forward biased, and the CB junction is reverse biased, shouldnt Ic be positive? The transistor should work in the active region.

uart
Hey,

I would like help understanding this scenario:

(1) base-collector voltage, V_BC, is reverse-biased
(2) base-emitter voltage, V_BE, is forward-biased
(3) collector current, I_C, is negative

Normally, if I_C was positive, I would assume the transistor to be in the forward-active region. However, can you still consider it in the forward-active region if the collector current is negative? Does this scenario make sense?

Thanks,

Mark
Well Mark that is the one "quadrant" of collector operation that is physically impossible to obtain. All three other possiblities at the collector can indeed occur. I'll summarize them (NPN).

1. I_c +ive, BC junction reverse biased = active region

2. I_c +ive, BC junction slightly forward biased = saturation region.

3. I_c -ive, BC forward biased = reverse operation.

In modes 1 and 2 above the BE junction should be forward biased. In mode 3 it either operates as an inefficient reversed NPN (that is, with C and E functions interchanged) if the BE junction is reverse biased, otherwise it pretty much degenerates into two diodes with a common annode (that being the base).

BTW. I should note that the definition of +ive collector current is when the current flows into the device. This is a standard definition. Perhaps you were unaware of this and were mistakenly thinking negative Ic corresponded to the case where Ic crosses the BC junction in the direction from N to P? In this case then that's actually the positive direction for Ic and you would be looking a simple active region operation.

hey, thx everyone. it helped improve my conceptual understanding of bjt transistors.

-Mark