During processing metalization layers, on metal paths is collecting electrostatic charge which could breakdown the gate oxide in transistors.
To avoid this effect a diodes are connecting to long metal paths.
when long length metal is connected to gate of a transistor, During the Fabrication Process the large amount of charge is induced*in plasma etching, ion implantation and in other processes.*these induced ions will hit the gate directly and damage the SIO2 layer, this may effect the transistor operation. this is known as antenna effect.
to overcome this effect we will put a reverse biased diode near the gate of a transistor, to discharge the ions produced during the plasma etching and other processes. this diode is known as antenna diode.