Purple signal is the voltage at the transistor base, referenced to the common ground
Yellow signal is the voltage at the transistor emitter, referenced to the common ground
Notice the spike in the voltage when the transistor is turned off. This is due to an inductive component (DC motor) acting like a current source when the magnetic field is collapsing. With the current trying to force its way through the transistor, high voltage builds at the emitter.
Mitigating inductive spike with a diode (Snubber)
A diode can provide a conductive path for the current from the motor. This way, a lower voltage is sustained.
The diode in this circuit is a common 1N4007, and it is connected parallel to the motor.
