Tom,

The back-side (bottom) of virtually all devices is conductive.  It is
usually, the bulk silicon, but it is conductive.

Most device back-sides should be connected to the most negative potential in
the circuit to prevent inadvertent forward biasing of junctions.

On some occasions, the device designer provides 'grounding' of the bulk
semiconductor via the ground connections (wire bonds) on top.

Probably on the order of 95% of the silicon devices we put down, are on
'grounded' pads.

Steve Creswick CTS RF Integrated Modules

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