4a. Based on Fig. 4a, explain the fundamental working principle of a PN LED.

b. Fig. 4b shows phosphorus and boron doping of silicon. Using the diagram,explain how doping is achieved, the type of doping and the role of electrons and holes in the resultant material.

c. State the Stefan-Boltzmann law, explain its parameters and how it could be used as a basis for a passive infra-red (PIR) thermal imaging system.