FET is named Field Effect Transistor while field effect is also producing in bjt. What are the factors/reasons that FET is called field effect while bjt is not? I have read textbook but it didn't explain it.
A electric "field" is produced whenever and wherever there is a voltage difference. Actually the field is always everywhere, but becomes non-zero when voltage differences are non-zero. Since non-zero voltages are fundamental to how every electrical device works, all such devices can be said to work based on or somehow related to the "field".
Field effect transistors (FETs) work on a pricipal that uses the field produced by the gate in a way that makes the channel conduct more or less. In bipolar transistors, the current that can be carried between the collector and emitter is modulated by the base current. Of course there are electric fields inside a bipolar transistor. The FET was named such because the of how the static field, as apposed to current, plays a central role in the operation of the device.
It's a naming scheme, which is not meant to be a detailed description of how the device works.
You are correct. There are fields inside of a BJT. The fields are not what is driving current. Current is actually delivered by the diffusion of minority carriers in the device. The fields in this case just ensure that the carriers go the correct direction across the junctions, and only effect how they cross junctions, nothing else. (But not always and there's some more fuzzy math to go around, taking into account tunneling and other quantum effects. However, that's way out of scope.)
Where as in FETs, the field either creates (MOSFETs) or removes (JFETs) a region with majority carriers that acts as a conducting connector. How conductive this region is, effects the current. So the current is directly dependent on the strength of the field. Thus the electric Field Effects the transistor.