The use of degenerate (heavily doped) Si as an electrode material for electrochemical biosensors is demonstrated by a series of voltammetry and impedance studies. Cyclic voltammetry in solutions of NH4F and K3Fe(CN)(6)/K4Fe(CN)(6) exhibits reversible electrochemistry without illumination, which to the best of the authors' knowledge has not been previously reported for Si electrodes in an aqueous environment. The presence of F- prevents formation of the Si native oxide, while the use of a degenerate Si substrate makes its electrochemistry more like that of a metal. In addition, impedance detection of peanut protein Ara h 1 is demonstrated on a degenerate Si electrode following immobilization of the monoclonal mouse antibody to this protein. The variation in charge-transfer resistance with protein concentration is employed to estimate the dissociation constant of the surface-immobilized antibody-antigen complex as 0.25 nM. The advantages of degenerate Si relative to n- and p-type Si include the lack of photoeffects, a simpler equivalent circuit, easier electrical connection to the working electrode, and knowledge that the applied potential applied is dropped within the electrical double layer, not within the semiconductor space-charge layer. Most important, degenerate Si electrodes are easily incorporated into Si-based semiconductor devices. (C) 2008 The Electrochemical Society. [DOI: 10.1149/1.2988047] All rights reserved.