What is leakage current of a pn-junction and how to measure it?
Submitted by SDRAM Technology on Fri, 10/02/2009 - 09:37
A leakage current is the current which is flowing thought a pn-junction when is inversely polarized. As the definition says in order to measure the leakage current of a pn-junction you simply have to apply a voltage in the reverse direction of the junction and measure the resulting current. A schematic picture of this process is presented in Figure 1.
Due to the fact that on an integrated chip, e.g. SDRAM chip, there are a huge number of pn-junctions, leakage currents are of a paramount importance and should be kept as small as possible in order to reduce the current consumption of the whole chip.
Figure 1: Leakage current of a pn-junction
The most important reason for leakage currents are defects in the substrate, which can appear during the implantation process of the ph-junctions. Such “implantation defects” can be eliminated by a suitable anneal after the implantation, however if the anneal procedure is not well optimized not all implantation defects will be eliminated, which in turn will increase the leakage current of the junction.
Another reason for high pn-leakage currents are the so called TiSi-grains. In order to make a contact to a pn-junction usually a thin layer of Titanium (Ti) is sputtered on/in the contact. After that the contact is annealed. During the annealing process Ti is reacting with Si to form TiSi. The interface between TiSi and Si-substrate is not smooth, but has a “grain”-like structure (see Figure 1), which can degrade the leakage current significantly if are not well controlled by the annealing process.