Hello
although you when you work in the forward region it seem to be that the Is is not here but still it considered in the diode forward current as you showed in the diode equation. at the high temperature the value of Is will be large, so you can not neglect it or consider it constant.
... I want to compare the Ic at 200K and 350K, can I say the difference of them are mainly come from e(Vbe/Vt) but not from Is0(T)?
The saturation current of a p-n junction (or a diode) is temperature dependent.
If SPICE model does not include this temperature dependence - that's a problem of the SPICE model.
According to Sze's book (Physics of Semiconductor devices):
Js ~ T^(3+gamma/2) * exp(-Eg/kT)
Here T is the temperature, gamma is some parameter (D/tau ~ T^gamma), and Eg is the bandgap of a semiconductor.
The dominant factor here is exponential temperature dependence.
It is determined by the fact that Js is proportional to ni^2, where ni is intrinsic concentration that is strongly temperature dependent: ni ~ exp(-Eg/2kT).
In reality, another effect is important - reverse current is not actually saturated with reverse voltage, it is increasing due to the expansion of the depletion region volume where the charges are thermally generated and extracted by the electric field in the depletion region.
Max
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Hi timof, you said Js ~ T^(3+gamma/2) * exp(-Eg/kT)
Do you really want to say Jc ~ T^(3+gamma/2) * exp(-Eg/kT)? And Js ~T^(3+gamma/2) ?
Could you give me an idea what this gamma roughly is. Then I can see the change of Js(T) will take effect on Jc(T).
Thank you!
Erikl -
I am pretty sure that SPICE calculates the temperature dependence of Js according to the formulas given above.
You are talking about a SPICE model parameter Is0, that is "fixed", i.e. temperature-independent, while we are talking about the real life temperature behavior of diode's reverse current.
You are talking about a SPICE model parameter Is0, that is "fixed", i.e. temperature-independent, while we are talking about the real life temperature behavior of diode's reverse current.
The original post (about log-amplifier application of diodes) and all related contributions have been talking about diode forward current.
I wanted primarly to clarify, that reverse bias current isn't the original topic of this thread.Would you think these are different saturation currents with unfortunately the same abbreviation?
P.S.: I forgot an important point about Is temperature dependency. With temperature indpendent Is, the forward current at constant Vd would have a negative temperature coefficient according to the Shockley equation. As everyone knows, the T.C. is positive, correponding to a Vd T.C. of about -2mV/K at constant Id. This means, Is must be stronger temperature dependent than the negative T.C. of exp(Vd/Vt).
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