The doping process for the drain and source necessitates high-temperature annealing methods exceeding 800°C. However, if aluminum (Al) were utilized as the gate material, it would melt at such elevated temperatures, given its melting point of around 660°C. On the other hand, polysilicon, commonly employed as a gate material, remains stable and does not melt under these high-temperature conditions, that is why we prefer polysilicon over metal for making gate.