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Question 4 (25 marks) A semiconductor photodiode is made of aluminium gallium arsenide (Alo.05Ga0.95As) which has a band-gap energy of 1.48 eV. When illuminated by light of wavelength 780 nm with light intensity of 11 W/m², the photodiode has a short-circuit current of 4.4 µA and an open-circuit voltage of 1.15 V. (a). It is given that the photodiode has an active area (i.e. the light-sensitive area) of 0.81 mm². Determine the responsivity of the photodiode in the condition specified above. (5 marks) (b). Using the result in (a), hence or otherwise, calculate the external quantum efficiency of the photodiode in the condition specified above. (5 marks) (c). Sketch a graph showing the I-V curves of the photodiode when illuminated with light intensity of 11 W/m² and 22 W/m² respectively. All axes and important features of the graph must be labelled properly. (8 marks) (d). Based on the result in (b) or otherwise, sketch a graph showing the responsivity curve of the photodiode assuming ideal behaviour (which does not mean 100% quantum efficiency). All axes and important features of the graph must be labelled properly. (7 marks)