$($a$)$ Explain how the following defects will behave in pure silicon $($i$.$e$.$ are they donors, acceptors or else, and why?$)$:

$($i$)$ Lithium interstitials

$($ii$)$ Silicon vacancies

Data: Atomic numbers: $Z_{Li}=3,Z_{Si}=14$

$($b$)$ Name and discuss two requirements for a semiconductor material to be used in electronic industry.