Can you add the graph, please?

I have no idea what is Eg, or with which method you are calculating it. I have calculated activation energy from slope of a graph that was negative also. but until you provide the mathematical formula for Eg, it is difficult to talk about it.

the slope of the graph, between (alphaE)2 and E, gives the value of  {(alpha)2E}. just ignore negative sign, it is due to the fact that supply Eg (I think it is energy, but u knw it wel) to activate the parameter u want............... more talk can be if you share details...

Waqee ur Rehman bhai Eg is the optical band gap value the graph (alphaE)2 and E had two linear parts when extrapolated, one line had +ve while other had -ve slope. that's confusing. its equation is called Tauc relation. the intercept of the extrapolate line on horizontal axis gives the value of Eg.

can you share your graph?

your graph like this?

intercpt is negative and slop is positive either oposite?

no my graph contains a hump like structure

Hafeez Ullah slope of the line is negative but intercept is positive