The temperature dependence of the critical current in planar high‐TcJosephson junctions fabricated in YBa2Cu3O7thin films by focused electron irradiation has been studied. It is shown that in the range of critical current densities spanning more than five orders of magnitude and temperature range 0.1≤T/Tc≤1, the critical current densityjcvaries as (1−T/Tc)2. TheTdependence of the critical current, however, is affected by the transition from the narrow junction to the wide junction limit asjcincreases. An expression for the Josephson penetration depth in thin‐film coplanar structures is derived, and magnetic field penetration depth in junction banks is extracted from theIc(T) dependences. ©1996 American Institute of Physics.