A n-p-n transistor conducts when(a) both collector and emitter are negative with respect to the base(b) both collector and emitter are positive with respect to the base(c) collector is positive and emitter is negative with respect to the base(d) collector is positive and emitter is at same potential as the base
Answers
In a transistor, if the collector-emitter junction is reverse biased, how does the collector attract electrons?
A very valid and logical question you have asked and I like to answer such questions as they give you many concepts regarding transistors.
The reverse biasing of collector emitter terminal does not play a role in the working of a transistor and hence this question needs to be modified as
In a transistor, if the collector-base junction is reverse biased, how does the collector attract electrons?
See, a transistor is nothing but a switch where the base current controls the flow of current from collector to emitter.
(this is the reason it is known as current controlled device)
In NPN transistor when operating in active region
The base emitter junction is “forward biased” as shown in the above diagram (positive to positive, negative to negative)
where as
The base to collector junction is reverse biased (positive to negative , negative to positive)
In a transistor, if the collector-emitter junction is reverse biased, how does the collector attract electrons?
A very valid and logical question you have asked and I like to answer such questions as they give you many concepts regarding transistors.
The reverse biasing of collector emitter terminal does not play a role in the working of a transistor and hence this question needs to be modified as
In a transistor, if the collector-base junction is reverse biased, how does the collector attract electrons?
See, a transistor is nothing but a switch where the base current controls the flow of current from collector to emitter.
(this is the reason it is known as current controlled device)
In NPN transistor when operating in active region
The base emitter junction is “forward biased” as shown in the above diagram (positive to positive, negative to negative)
where as
The base to collector junction is reverse biased (positive to negative , negative to positive)
When we increase Vbe ≥ 0.7 Volts (the value 0.7 V is a typical value of potential barrier voltage) the transistor is forward biased and the forward current will start flowing.
Now large number of electrons in emitter layer is repelled by negative terminal of Vbe and they flow towards b-e junction.
They cross the junction and enter into small base layer.
Here some electrons combine with holes (in the base) and are attracted by positive terminal of Vbe and remaining maximum number of electrons flow into collector layer, crossing the second junction i.e. collector-base junction.
The electrons in the collector region are repelled by these (guest) electrons and thus, then all the electrons are present in collector layer are attracted by positive terminal of Vcb.
Thus, all these electrons complete their journey back into emitter layer and produce conventional currents in the transistor as shown in the above circuit.
Thus, as per Kirchhoff Current Law, we can write, Ic + Ib = Ie.
Now when Vbe is still increased, more electrons are repelled by negative terminal of Vbe. So base-emitter junction is more and more forward biased. Thus the base current (Ib) increases, which in turn increases Ic.
Hence, we can say that collector current (Ic) is the function of base current (Ib).
But there is a typical value of Vbe for each transistor, at which the collector current Ic no longer remains the function of base current Ib.
Also collector current is directly proportional to the base current.
In all this process, maximum number of electrons from emitter layer flow into collector layer. So collector current is ALMOST EQUAL to emitter current. Hence we say that, collector current is proportional to emitter current.
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As per your question, the base-collector junction is reverse biased but still the current will flow as the electrons from the emitter are forcefully injected into the collector region due to negative polarity at emitter terminal (of Vbe).
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sorry for the lengthy answer as this question cannot be answered in short and if explained shortly you will still get a lot more questions you currently have since, transistor theory is very interesting and more fun to study.
Hope this helps :)
Thanks for reading
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