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BJT characteristic curves----confusing point

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farooq026

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bjt characteristic curves

In BJT characteristic curves the linear(active region) is shown to start at 0.1,0.2 or 0.3 volts.If we install no resistance in collector circuit(Rc=0) then for these values of Vce, the collector base junction will be forward biased and Ic will have a small value in the negative direction. But in characteristic curves,at these values of Vce, Ic is shown to have a +ve value in miliamps. Is not there a contradiction?
 

bjt curves

Generally, characteristic curves are displaying voltage and currents observed at the transistor terminals. Thus they don't depend on an external resistor.

It may be the case, that some curves are drawn inaccurately, but I didn't find any. Can you please show an example, that is contradicting empirical transistor behaviour, indicating positive IC where it should be negative?
 

collector characteristic curve

Yes, you are right. For Vce=0 the base-collector junction is forward biased and there is a collector current which is opposite to the "normal" direction (µamps).
You can see this by simulation, However, as this property is not very important for applying the BJT in the "normal" region, the characteristic curves in the data sheet do not show explicitely this negative current. It is also a matter of the scale which is applied to scetch Ic vs. Vce .
 

bjt characteristic curve

When for a npn transistor, collector base junction i forward biased the voltage Vbc = +ve but smaller than the base emitter voltage Vbe because of moderte carrier density of collector. This leads to the saturation condition resulting in collector current smaller than the same in linear region for the same collector current. The actual collector current and the base current depends on the load line. Here as there is no resistance in the collector loop, Vce=Vcc, and Ic=0 (only some leakage collector current flows from collector to base)
 

bjt collector current

It's misleading to talk of a leakage current in case of a forward biased junction. The negative collector current with Uce = 0 also isn't actually small, it's only small in a regular Ic scale. As an empirical measured example: A total base current of 3 mA with BC547 results in Ie=-0.5 mA and Ic=-2.5 mA. A similar behavior can be expected with most transistors due to the unsymmetric design.

It has been known since long, that a bipolar transistor can work as an analog switch for a limited voltage range (e.g. 100 mV), but it should be operated in inverse mode (emitter and collector exchanged) to reduce the offset voltage.
 

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