Physical explantion of BJT ideality factor

[diarmuid]

Hello All,

Does the ideality factor physically represent anything in a BJT?

I have seen it to be dependant on Ice. Why is this?

Thanks,

Diarmuid

 

[dick_freebird]

You have fixed losses (recombination) in the diodes and this
is a "regressive tax" (accounts for all or most of base current
when applied current is low). I would not say that Ice is the
primary actor however. To me it's the ratio of the current
that sets up / modulates Ice, and the current that only goes
B-E without incrementally adding Ic.

 

[D.A.(Tony)Stewart]

A variety of effects occur in bipolar transistors, which are not included in the ideal transistor model. These include the base-width modulation effects and the current due to recombination in the depletion layers. High injection effects, base spreading resistance and emitter current crowding.



As for a p-n diode, high injection modifies the ideality factor of the collector current, making it approximately equal to 2. The ideality factor of the base current however remains unchanged since the minority carrier density in the emitter does not exceed the majority carrier density in the emitter until saturation.



The net effect is that the current gain decreases with increasing bias.

In most practical cases and especially when the base is thin and highly doped, high injection will not even be observed in a bipolar transistor, as the effect of series resistances will be dominant instead. Heterojunction bipolar transistors have a much higher base doping so that high-injection does not occur in such devices.

- also temperature dependence and breakdown mechanisms in BJTs are not included in ideal model.