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On what parameters does the Ic collector current of a transistor depend upon, base emitter voltage or base current?

I know that BJT is a current controlled device. But still we have graphs that show that Ic changes when Vbe changes.

Which one should I consider?

winny
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Freshman
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    Have you read: https://en.wikipedia.org/wiki/Bipolar_junction_transistor? When Vbe changes, will Ib stay constant or change as well? What does that mean for Ic? It is not a matter of considering one **or** the other, they're all related. Remember that the BE junction is a **diode**. – Bimpelrekkie Jan 27 '19 at 13:05
  • Yes - they are related. However - the question is legitimate: Which quantity is the physical cause of Ic control? My answer: The voltage Vbe only. Or do you think that - for example - two charged carriers (base current change) can cause the release of 1000 additional charged carriers from the emitter (assuming a beta value of 500)?. – LvW Jan 27 '19 at 13:12
  • Indeed, few charged carriers in base can lead on average to (beta +1) times as much carriers to be released from the emitter, beta times of which are swooped up by the collector. Why? Because when you introduce a carrier of one type in the base, the emitter will have to supply one carrier of opposite type to neutralize it. But this carrier does not have a map to reach the offending carrier - it will cross the forward biased junction and hope for the best. The probability it will (contribute to) neutralize the carrier is small, because most carriers diffuse rapidly and end up in the collector – Sredni Vashtar Sep 25 '19 at 14:13
  • You can find a description of this role of the base current, among the others, on Streetman, "Solid State Electronic Devices", and in Levinshtein & Simin, "Transistors: from crystals to integrated circuit". Note: this does not imply that the BJT is *exclusively* current controlled, since as many other have said, voltage and current are concomitant and not one the cause of the other. – Sredni Vashtar Sep 25 '19 at 14:18

2 Answers2

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You can use one or the other. The base current changes when Vbe changes.

Generally it's more convenient to consider it a voltage controlled device when you're looking at small-signal behavior, where there is a transconductance gm.

For simple switching applications, current controlled is more convenient.

Spehro Pefhany
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  • Yes - I agree, For practical purposes we have the choice either to rely on voltage or on current control. For this purpose, we have two different small signal equivalent circuits (beta control resp. gm-control). But this - more practical related - approach must not be confused with physical effects which cause the collector current to change... – LvW Jan 27 '19 at 13:16
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    And then at a device physics level, it's sometimes useful to think of it as charge controlled. – Hearth Jan 27 '19 at 13:21
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    @LvW I actually like the parsimonous way Spehro wrote this out. For small-signal behavior, the BJT is operated in active mode, it's collector acts more like a current source/sink, and \$g_m\$ matters. For switch behavior, the BJT is operated in saturated mode, it's collector acts more like a voltage source, and sufficient recombination current is the main concern. – jonk Jan 28 '19 at 04:16
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consider this plot of I_V for a bipolar

enter image description here

analogsystemsrf
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