= Solution
\b[X is an aluminium-doped $p^+$ <back surface field> region.] Annealing alloys the rear <aluminium> with <silicon>; on cooling, silicon regrowth adjacent to the contact incorporates aluminium as an acceptor. This creates a more heavily p-type region between the p-type base and rear metal contact.
The resulting $p/p^+$ doping gradient creates a field that reduces the minority-electron population near the recombination-active rear metal interface. It therefore reduces rear-surface recombination and improves collection of photogenerated carriers. Heavy doping also helps form a low-resistance majority-carrier contact. This is a rear isotype doping transition, not the front n–p junction and not an electrically insulating blocking layer.
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