![]() The current due to the reverse biased CB junction is given by -Ĭlearly these currents are opposite to each other. The current due to the forward biased EB junction is given by. The Base-Emitter junction is forward biased and the Base-Collector junction is reverse biased. Let us first assume that the BJT is in the active mode. ![]() In case of the Common-Base mode, the output characteristics look like this. I would like to take you through a different approach. Your question could also be stated as "what does 'pinch-off' mean?" Websites are one thing, but I wonder if any current textbook$$ feature this wrong BJT diagram?Īnd just for future reference, below is a good diagram for FETs, showing the gate-controlled channel-resistivity effect seen below pinchoff (at low drain voltage:) Also: your original question does apply to FETS, where there really is an odd threshold effect. Until the forward junction voltage approaches ~0.7V (for silicon at 300K,) a PN junction remains turned off, with large depletion region and low average conductivity. Yes, a junction maintains a significant depletion layer at zero current. When no current is flowing through the transistor, is there the presence of depletion layer? (A more idealized model would use zero-ohm silicon, and wouldn't have any tilted ramp at saturation, just a vertical drop at the Y-axis where Vce hits zero volts.) So, why does collector current go to zero when collector voltage is zero? (Heh, you already should know that one.)īetter question: why is there a knee in the curve? It's because everything stops when the net internal e-fields are gone when Vce=0.Īlso, at the left edge, why doesn't the VI curve at Vce=0 drop straight down, why is it instead tilted like a resistor curve? Because it's a resistor curve, the bulk silicon isn't a superconductor. ![]() Instead ask your question about the actual characteristic curves of a BJT: (There are even a few sites swinging the other way, depicting curves for a BJT but labeled as FETs!) Google Images finds many, many websites with this mistake. For BJTs there is no ohmic or 'variable resistor' region at low Vce as shown above. The above diagram shows the set of curves for an FET, but with labeling for BJT. ![]() Note that above graph is wrong, and there is no such phenomenon. ![]()
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