Electrical curriculum: What is Voltage?
Getting any closer Claude?
It's is the electric field that causes the semi conductor effect. That is all I'm saying.
The currents CAN NOT exist without the bias voltages to produce the semi conductor effect to alter the conductivity of the material that allow the currents to exist.
Again let me state that in words that may clearly illustrate my point. NO CURRENT can flow until those electric field potentials are met. The charge transfer (current flow) required to establish those voltages again have no relation to the fact that the voltage field itself is what causes the transistor to change conduction states.
The currents cannot exist w/o bias voltage? I've stated that same thing repeatedly. Every electrical device known requires I & V. I can never exist w/o V. We both know that. But, hear me out, V cannot exist w/o I. Neither can exist independently of the other under dynamic conditions.
How does one establish Vbe? Since charges must be displaced in order to get a depletion layer, motion of charges occurs before the voltage builds. Ib chronologically precedes Vbe. But Ib is not the cause of Vbe, nor vice-versa.
Let's look at a simple p-n 2 terminal device, an LED. Is an LED a current-controlled light source, or voltage-controlled? Both I & V are indispensable, as the LED will not emit light w/o both of them. Conservation of energy requires that for every mW of optical output, at least 1 mW of electrical power input is needed, plus that to cover losses. The I*V product has to be non-zero so that power is non-zero. Fair enough?
But how shall we drive the LED? Let's say we wish to operate at 10 mA, and the forward drop at 10 mA w/ 25C temp is 1.8V, for a red LED. Shall we directly connect a 1.800V constant voltage source across the LED? If we do, then a temp increase takes place, resulting in a larger Is, reverse saturation current. Then If increases, resulting in a further Is increase. We have thermal runaway. Driving an LED w/ constant voltage is not done w/o a resistance in between.
With a constant current source, the Vf is incidental. As long as the current source has more than 1.8V of compliance, it works. Just as If = Is*[(exp(Vf/Vt))-1] it is also true that Vf = Vt*[ln((If/Is)+1)]. If we force 10 mA, Vf becomes 1.80V at 25C temp. When powered on, the temp will increase, so Is increases. But Is going up results in Vf going down. Thus thermal stability is achieved. An LED is inherently stable thermally when driven from constant current w/ the voltage being incidental. That is all I mean by "current controlled". It has nothing to do with causality.
If a power supply with constant voltage is employed to power said LED, if we insert a resistor, we get results like that of constant current drive. If the source is 12V, and the resistor is 1.0 kohm, then at 25C, the current If is (12.0-1.80)/1.0k = 10.2 mA. Should the temp increase, and Is increase, any increase in If results in an increase in the drop across the 1.0k resistor. Thus the LED forward drop Vf decreases. Thermal stability is achieved.
With a constant voltage source plus a large enough resistor, we get stability like that of constant current drive.
The same issues apply for all devices. Some non-linear devices work better when driven from a CVS (constant voltage source), while others are best used with a CCS. That is all implied here.
Does this help at all?