This paper presents a study of the three-dimensional (3D) Dirac semimetal Cd3As2, a well-known semiconductor with high carrier mobility. Based on first-principles calculations, the authors show that Cd3As2 is a symmetry-protected topological semimetal with a single pair of 3D Dirac points in the bulk and non-trivial Fermi arcs on the surfaces. It can be driven into various topological phases, such as a topological insulator, a Weyl semimetal, or a quantum spin Hall (QSH) insulator with a gap exceeding 100 meV by breaking symmetries or reducing dimensionality. The 3D Dirac cones in the bulk can support sizable linear quantum magnetoresistance (MR) even at room temperature. The crystal structure of Cd3As2 is complex, with two possible structures (Structure I and II) that differ in symmetry and vacancy ordering. The authors performed first-principles band-structure calculations using the density functional theory (DFT) method with the generalized gradient approximation (GGA) and the HSE method to improve the accuracy of the s-p band gap estimation. The results show that Cd3As2 exhibits band inversion around the Γ point, indicating non-trivial topology. The effective low-energy Hamiltonian for the 3D Dirac fermion is derived from the 8-band Kane model, which describes the electronic structure of Cd3As2. The resulting Hamiltonian shows that Cd3As2 has a pair of four-fold degenerate zero-energy Dirac points at the Fermi level. The surface states of Cd3As2 are calculated using a tight-binding model, revealing non-trivial surface states and Fermi arcs on the surfaces. The authors also investigate the quantum transport properties of Cd3As2, including the QSH effect in its quantum well structure and the linear quantum MR even at room temperature. The study concludes that Cd3As2 is a promising candidate for future transport studies due to its high carrier mobility and unique electronic structure. The paper also discusses the potential for superconductivity in Cd3As2, which could be related to topological superconductivity.This paper presents a study of the three-dimensional (3D) Dirac semimetal Cd3As2, a well-known semiconductor with high carrier mobility. Based on first-principles calculations, the authors show that Cd3As2 is a symmetry-protected topological semimetal with a single pair of 3D Dirac points in the bulk and non-trivial Fermi arcs on the surfaces. It can be driven into various topological phases, such as a topological insulator, a Weyl semimetal, or a quantum spin Hall (QSH) insulator with a gap exceeding 100 meV by breaking symmetries or reducing dimensionality. The 3D Dirac cones in the bulk can support sizable linear quantum magnetoresistance (MR) even at room temperature. The crystal structure of Cd3As2 is complex, with two possible structures (Structure I and II) that differ in symmetry and vacancy ordering. The authors performed first-principles band-structure calculations using the density functional theory (DFT) method with the generalized gradient approximation (GGA) and the HSE method to improve the accuracy of the s-p band gap estimation. The results show that Cd3As2 exhibits band inversion around the Γ point, indicating non-trivial topology. The effective low-energy Hamiltonian for the 3D Dirac fermion is derived from the 8-band Kane model, which describes the electronic structure of Cd3As2. The resulting Hamiltonian shows that Cd3As2 has a pair of four-fold degenerate zero-energy Dirac points at the Fermi level. The surface states of Cd3As2 are calculated using a tight-binding model, revealing non-trivial surface states and Fermi arcs on the surfaces. The authors also investigate the quantum transport properties of Cd3As2, including the QSH effect in its quantum well structure and the linear quantum MR even at room temperature. The study concludes that Cd3As2 is a promising candidate for future transport studies due to its high carrier mobility and unique electronic structure. The paper also discusses the potential for superconductivity in Cd3As2, which could be related to topological superconductivity.