4. For heterojunctions in the GaAs-AlGaAs system, the direct bandgap difference is accommodated approximately in the conduction band and in the valence band. The bandgap of AlGaAs is 1.85 eV if Al composition is 0.3. Draw the band diagrams of two heterojunctions: (a)As on n-GaAs (b)As on n-GaAs (20%)