For a NPN, where P is base, N+ is emitter, connect P and N+ accordingly to be used as a simple PN-junction diode. N as collector can be (1) left disconnected or connected to (2) base or (3) emitter. 1 and 2 configure NPN into a single diode, 3 configures NPN into 2 parallel diodes. All methods are valid, afterall Vbe is still 0.7V at room temperature. The net output current is the same as the input current to base in any configuration. 2 creates a special flywheel diode to protect it from inductive or/and capacitive energy discharge at the input end to base.
For PNP, P+ as emitter, N as base. Connected emitter and base accordingly as a simple PN-junction diode. If you connect collector to emitter, it's the same as having 2 parallel diodes with net current to base equates to input current. If you connect collector to base, this creates a special flywheel diode to protect the output end to the base. Or you can let collector disconnected.
Connecting collector to base or collector to emitter are commonly used, especially collector to base to have additional protection.