Working: When plate potential (V_p) is positive, plate current ( I_p) flows in the circuit (because some emitted electrons reach to plate). If +  V_p enhances I_p  also increases and finally becomes maximum (saturation).

Space charge: If  V_pis zero /negative, then electrons collecting around the plate as a cloud which is known as space charge. Space charge reduces the emission of electrons from the cathode.

Characteristic curve of a diode: A diagram represents the variation of  I_p with  V_p at a given filament current ( I_f) is called as characteristic curve.