- Multi-junction concentrator solar cells incorporate tunnel diodes that undergo a transition from high- conductance tunneling to low-conductance thermal diffusion behavior, typically at threshold current densities of the order of 10 2 -10 3 mA/mm 2 . We present experimental evidence of a prominent heretofore unrecognized dependence of threshold current density on the degree of localized irradiation, in different solar cell architectures. We also show that solar cells with non-uniform metallization can exhibit an additional spatial dependence to the tunnel diode threshold current density. These previously undiscovered phenomena - which should be observable in all non-uniformly irradiated photovoltaic tunnel diodes - are interpreted as being derived from the lateral spreading of excess majority carriers (analogous to current spreading in light-emitting diodes (LEDs)). The consequences for concentrator photovoltaics are addressed.