A diode with low substrate current leakage and suitable for BiCMOS process technology. A buried layer is formed on a semiconductor substrate. A connection region and well contact the buried layer. Isolation regions are adjacent to two sides of the buried layer, each deeper than the buried layer. The isolation regions and the buried layer isolate the connection zone and the well from the substrate. The first doped region in the well is a first electrode. The well and the connection region are electrically connected, acting as a second electrode.