**5. Conclusions**

In this work, we propose the p-n junction-based Si biochips with gold ring top electrodes and unstructured platinum bottom electrodes, which o ffer a novel possibility for online monitoring the biomass during the cultivation with considerable low cost. The promising reproducible impedance characteristics of two types of p-n junction-based Si biochips, i.e., PS5 and BS5, with di fferent implantation ion types and ion energy but the same ion fluence, are discussed and demonstrate the significant change between the impedance spectra before and after adding the bacteria suspension *Lysinibacillus sphaericus* JG-A12 in the top electrode region. With the help of the developed two-phase electrode structure, the equivalent circuit is designed for the p-n junction-based Si biochips, four modeling parameters for PS5 (Rp1, Cp1, Rp3, and Cp3) and three modeling parameters for BS5 (Cp1, Rp3, and Cp3) reveal the linear dependence relationship with the bacterial concentration. Such linear dependent parameters are useful for the quantitative measure of the bacteria concentration. The limitation of the bacteria detection utilizing p-n junction-based Si biochips strongly depends on inner and outer diameters of the ring top electrode. Thus, for detecting a smaller number of bacteria, the inner and outer diameters should be reduced. In the future, the p-n junction-based Si biochips by adding di fferent live cells or tissues with di fferent cell dimensions will be studied and their further potential in medicine and biology applications will be explored.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6374/9/4/120/s1. Table S1: The impedance magnitude for boron-doped biochip BS5 with di fferent amounts of analyte (1–5 μL bacteria) at frequencies 40 Hz, 400 Hz, and 4 kHz. This table corresponds to the experimental results in Figure 8a. Table S2. The impedance magnitude for the biochip PS5 with di fferent amounts of analyte (1–5 μL bacteria) at frequencies 40 Hz, 400 Hz, and 4 kHz. This table corresponds to the experimental results in Figure 8b.

**Author Contributions:** Conceptualization: H.S. and J.R.; methodology: H.S., N.D., and D.B.; software: M.K. and N.D.; validation: M.K.; formal analysis: M.K., N.D., and H.S.; investigation: M.K. and N.D.; resources: U.H., M.V., I.S., and D.B.; data curation: M.K.; writing—original draft preparation, M.K.; writing—review and editing, N.D. and H.S.; visualization: J.R.; supervision: H.S. and N.D.; project administration: H.S., S.E.S., and O.G.S.; funding acquisition: H.S., S.E.S., and O.G.S.

**Funding:** Funding by Sächsische Aufbaubank (SAB) is gratefully acknowledged (PolCarr-Sens project, gran<sup>t</sup> number: 100260515).

**Conflicts of Interest:** The authors declare no conflict of interest.
