Structure of Kerr Black Hole

For a Kerr (rotating) black hole, the stationary surface is outside the event horizon and the region between is called the ergosphere. The horizon has radius r_g = 0.5 r_s so has half the size of the Schwarzschild horizon. Since the last stable circular orbit is at r_g, whereas for the Schwarzchild hole it was at 6r_g (= 3 r_s), an accretion disk can form much closer to the hole, and therefore can liberate much more energy.