4. Answer the following questions about MOS capacitor. (a) Explain the flat-band voltage. (5%)( b) Consider a n⁺polysilicon-SiO₂-Si capacitor with a p-type silicon substrate doped to N_A = cm, a silicon dioxide insulator with a thickness of 300 . The metal-semiconductor work function diference is -0.98 V. The fixed charge within the SiO₂-Si interface Q/q = 4✖The dielectric constant of SiO₂ is 3.9. Calculate the flat-band voltage. (15%)