The electron mobility rises with temperature. Thus, for VBE=const. the emitter produces more electrons and the current increases. On the other hand, for Ic=const (that is the condition to evaluate the Vbe temperature dependence!) a lower Vbe is necessary to produce the same current. Therefore the neg. temp-co for Vbe.
but i do not think so,
from the formula µ=µo*T^(-3/2), seems mobility decreases with temperature.
Yes, you are right. I didn't use the correct term. It's not the mobility of electrons but rather the ability of the emitter to emit more electrons. This can be verified easily by SHOCKLEY's equation that shows the interdependence between Vbe and Ie with temparature as parameter.