A polycrystalline Si thin film transistor substrate having a self-aligned LDD and provided with a gate made of a Mo—W alloy having a W concentration not lower than 5% by weight and lower than 25% by weight and preferably a W concentration of 17 to 22% by weight, which is formed by a process comprising a wet-etching step using an etching solution having a phosphoric acid concentration of 60% to 70% by weight, has uniform characteristic properties and is excellent in productivity.