We propose a hybridizable discontinuous Galerkin (HDG) finite element method to approximate the solution of the time dependent drift-diffusion problem. This system involves a nonlinear convection diffusion equation for the electron concentration u coupled to a linear Poisson problem for the the electric potential φ. The non-linearity in this system is the product of the ∇φ with u. An improper choice of a numerical scheme can reduce the convergence rate. To obtain optimal HDG error estimates for φ, u and their gradients, we utilize two different HDG schemes to discretize the nonlinear convection diffusion equation and the Poisson equation. We prove optimal order error estimates for the semidiscrete problem. We also present numerical experiments to support our theoretical results. the above model, for example including additional particle transport equations (for example, for holes) and recombination terms. However the above system contains the principle difficulty from the point of view of proving convergence: the term ∇ · (u∇φ).Theoretical and numerical studies for this type of partial differential equation (PDE) have a long history. For the theoretical analysis of the drift-diffusion system; see [5,6,33,34,46,56] and the references therein. Computational studies started in the 1960s [28,38] and many discretization methods have been used for the drift-diffusion system in the past decades. For an extensive body of literature devoted to this subject we refer to, e.g., the finite difference method [30,39,50,55], the finite volume method [3,4,[11][12][13], the standard finite element method (FEM) [35,52,62], and mixed FEM [36,40]. Furthermore, there are many new models in which the drift-diffusion equation coupled with other PDEs; such as Stokes [41], Navier-Stokes [61] and Darcy flow [31]. However these extensions are outside the scope of this paper.The product of the gradient of the electric potential, ∇φ with electron concentration u in (1.1a) can cause a reduction in the convergence rate of the solution if the numerical schemes for the two equations are not properly devised. In [62], they obtained an optimal convergence rate in H 1 norm but a suboptimal in L 2 norm by the standard FEM. To overcome the convergence order reduction, a new method was proposed to discretize the system (1.1); mixed FEM for Poisson equation (1.1b) and standard FEM for (1.1a),. This scheme provides optimal error estimates for u and φ in both H 1 or H(div) as appropriate as well as in the L 2 norm. Very recently, the authors in [36] obtained an optimal convergence rate by using mixed FEM for both (1.1a) and (1.1b).In the drift-diffusion model, typically, the magnitude of ∇φ is huge (see [8]). Therefore, it is natural to consider the discontinuous Galerkin (DG) method to discretize the system (1.1). In [51], a local DG (LDG) method was used to study a 1D drift-diffsuion equation, they obtained an optimal convergence rate by using an important relationship between the gradient and interface jump of the numerical solution with the independen...