Probing sub-GeV dark matter requires designing low threshold detectors and understanding backgrounds. In this talk I will address these two issues. First, we point out several unexplored low-energy backgrounds in sub-GeV dark matter searches, which arise from high-energy particles of cosmic or radioactive origin that interact with detector materials. In this talk, I will focus on Cherenkov radiation and luminescence from electron-hole pair recombination. We demonstrate that these processes provide plausible explanations of the observed events at SENSEI and SuperCDMS HVeV. Second, we propose a new idea of probing light dark matter using doped semiconductors. Dopants in semiconductors form energy levels that are tens of meV below the conduction band or above the valence band. These materials can be new detector targets for dark matter scattering with a threshold of tens of meV, which can probe dark matter masses down to tens of keV.