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Diode characteristics - help needed

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belww

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Hi

Please help me to solve the following problem:

There are three points given on diode characteristic (1N4148 diode)

Q1=(50uA, 0.6V)
Q2=(500uA, 0.7V)
Q3=(2mA, 0.75V)

I have to calculate:

Is - reverse bias saturation current
n - emission coefficient

I suppose I have to use Shockley equation
\[ I_F = I_S ( e^\frac{U_F}{nU_T} - 1)\]

but I don't know what to do with these three points.
Should I choose 1 point and put \[I_F\] and \[U_F\] of that point into Shockley equation and then solve for Is and n?
 

I guess so. You need to solve Is, n, and UT.
 

Hey,
UT is nothing but cut in voltage (0.6 to 0.7V) of the diode.
Well, what you can do is...Put two sets of IF and UF(say Q1 & Q2) and UT. So you will get two equations for IS and n.
then solve it for Is and n (two variable and two equations).
 

UT = 20mV

but solving for Is and n seems to be very complicated

\[
n= \frac{U_F}{ln(\frac{I_F}{I_S} +1) U_T }

\]

\[
I_S=\frac{I_F}{e^\frac{U_F}{nU_T}-1}
\]

Are above equations correct?
 

Correct or not the top equation (to find n) has an Is term in it, to find Is's value you must solve equation ll which has a n term in it ! :-(
I would try a rearrange the original equations, then take logs of each side, this way constants fall out and e^z term becomes z X log10 e . draw a graph to find the slope and the offset, substitute them in the original equations and work out the various constants.
Frank
 
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