How Does the Depletion Region Influence Diode Behavior in Forward Bias?

[azizlwl]

At depletion layer of p region, there more negative ions thus negatively charged region.
Likewise at n region, it is positively charged.

Then why in equivalent forward bias the diode behave as a battery of .7v with positive at depletion region of the p region where it is negatively charged?

 

[Kholdstare]

Diode never behaves like a battery.

 

[azizlwl]

I'm reading Electronic Devices by Floyd 7edition page31- Diode Models.

In a forward bias, the complete diode model is depicted as a 0.7V battery with + polarity facing the + Vbias , a resistor and a switch all in series.

In reverse bias, only a switch which is connected parallel to a resistor.

 

[Kholdstare]

Its an approximate model of a diode. Even below 0.7V the diode current is not zero in reality. But that current is very low and can be ignored. The model has 0.7V battery so that at 0.7V the current becomes zero and increases linearly onward. This is not a physical phenomena.

 

[azizlwl]

http://img829.imageshack.us/img829/9049/new2qm.jpg

My question is why is it that it has positive polarity even though the depletion layer of p region is negatively charged.
To my assumption it should be negative polarity.

 

[Kholdstare]

http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_3.htm
refer to fig 4.3.1 c , That is a plot of vacuum potential across the diode (at 0V bias). It is about 0.1V-0.3V for a normal pn junction and is called built-in potential. It is different from electro-chemical potential (which is what we call voltage/voltage drop for the sake of brevity). At 0V bias the voltage across the diode is always 0.

Cut-in voltage (0.7V) of a diode has no relation with built-in potential or electro-chemical potential. It is a model parameter (just like a transistor has a model parameter, beta). It just indicates the voltage below which we can very well ignore the current. Thus one can ask which equivalent circuit can follow this model? And we get the circuit with battery and all. They do not exist inside the diode in reality.