Gallium Arsenide is Germane to Solar Cells

Gallium arsenide, a simple combination of two elements, interconverts light and electricity; GaAs lasers turn electricity into light and GaAs solar panels convert light back into electricity. There are alternative combinations of elements for these tasks, but each has its limits. What strikes me is how gallium and arsenic bookend germanium:

I need a name for "binary combination of elements which brackets and mimics another element." The term isoelectronic is close but doesn't cut it for me. There is a mathematical symmetry about GaAs in view of Ge and it goes like this: (31 + 33)/2 = 32 or, in chemical logic symbols: (Ga + As)/2 = Ge.

Like gallium arsenide, germanium is a photovoltaic material. Google "germanium solar cell" and you will find cutting edge research involving blends of gallium arsenide with germanium. I'm glad there is on-going research into new materials because I am not sure we should be putting arsenic on every rooftop much like we're putting mercury in every lightbulb.

A similar "bookend relation" occurs a couple rows up in the Table between boron, carbon, and nitrogen. Look how boron and nitrogen bracket carbon:

Once again, (5 + 7)/2 = 6. And just like carbon, boron nitride (BN) has both graphite- and diamond-like structures. One type of BN is even harder than diamonds: link

I see a pattern here: the centrality of the carbon group, C, Si, Ge, etc. to the family of main group elements: