What is the role of diode that connect between the base and the emitter of the transistor?
(anode to the base and cathode to the emitter)
I know diode permits current flow in only one direction. Are there more?

 

If you are talking about the intrinsic characteristics of the base-emitter junction, the "diode" is an inherent result of the base-emitter junction that forms part of the BJT structure. It is an artifact of the transistor design that looks like a diode when measured by itself but has no intentional "role" as a diode during normal transistor operation.

 

Are you referring to the P-N junction that is inherent at the base-emitter junction of an NPN BJT (bipolar junction transistor)?

On its own it looks and behaves like a diode. In a transistor it plays a much more active role in that it controls the electron-hole flow within the transistor itself causing an increase of the collector-emitter current.

 

Are there more?

This intrinsic diode usually has a reverse breakdown voltage between 4 volts and 6 volts. (It is not a very powerful diode.)

 

I need to study more detail!

 

Thank!
I need to study more detail!