Extremal graphs without exponentially-small bicliques

The Tur\'an problem asks for the largest number of edges in an $n$-vertex graph not containing a fixed forbidden subgraph $F$. We construct a new family of graphs not containing $K_{s,t}$, for $t= C^s$, with $\Omega(n^{2-1/s})$ edges matching the upper bound of K\"ov\'ari, S\'os and Tur\'an.