def particle_current_density_drift(spec=None, mu=0,
                                   density=0, d_phi_d_z=0):
    """

    Parameters
    ----------
    spec : string, required
        specifies whether carriers are holes or electrons.
        The following arguments are accepted: "electrons","n","holes","p"
        The default is None.
    mu : float, required
        Electron or hole mobility. The default is 0.
    density : float, required
        Electron or hole density. The default is 0.
    d_phi_d_z : float, required
        Derivative of electrostatic potential with respect to z.
        The default is 0.

    Returns
    -------
    The drift particle current density

    """
    if(spec == "electrons" or "n"):
        j = mu*density*d_phi_d_z
        return j
    elif(spec == "holes" or "p"):
        j = -mu*density*d_phi_d_z
        return j
    else:
        raise ValueError("Error! Incorrect specifier given!")