Exciton BEC: In an electron-hole bilayer, electrons and holes in separate layers bind into excitons via Coulomb attraction across the gap d. Below T_c, they Bose-Einstein condense.
KT transition: In 2D, true BEC impossible — system undergoes Kosterlitz-Thouless transition via vortex binding. ρ_s(T_KT) = 2k_BT_KT/π.
Signatures: Counterflow superfluidity, Josephson-like interlayer tunneling, quantized Hall drag.