## Copyright (C) 2004-2008  Carlo de Falco
  ##
  ## SECS1D - A 1-D Drift--Diffusion Semiconductor Device Simulator
  ##
## SECS1D is free software; you can redistribute it and/or modify
  ##  it under the terms of the GNU General Public License as published by
  ##  the Free Software Foundation; either version 2 of the License, or
  ##  (at your option) any later version.
  ##
## SECS1D is distributed in the hope that it will be useful,
  ##  but WITHOUT ANY WARRANTY; without even the implied warranty of
  ##  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  ##  GNU General Public License for more details.
  ##
  ##  You should have received a copy of the GNU General Public License
## along with SECS1D; If not, see <http://www.gnu.org/licenses/>.
##
## author: Carlo de Falco <cdf _AT_ users.sourceforge.net>

## -*- texinfo -*-
##
## @deftypefn {Function File}@
## {@var{R}} = Uscharfettergummel(@var{nodes},@var{Nnodes},@var{elements},@var{Nelements},@var{acoeff},@var{bcoeff},@var{v})
##
## Build the Scharfetter-Gummel matrix for the the discretization of
## the LHS of the Drift-Diffusion equation:
##
##  -(a(x) (u' - b v'(x) u))'= f
##
## @itemize @minus
## @item @var{nodes}: list of mesh nodes
## @item @var{Nnodes}: number of mesh nodes
## @item @var{elements}: list of mesh elements 
## @item @var{Nelements}: number of mesh elements
## @item @var{acoeff}: piecewise linear diffusion coefficient
## @item @var{bcoeff}: piecewise constant drift constant coefficient
## @item @var{v}: piecewise linear drift potential
## @end itemize
##
## @end deftypefn

function A = Uscharfettergummel(nodes,Nnodes,elements,Nelements,acoeff,bcoeff,v)

  h=nodes(elements(:,2))-nodes(elements(:,1));
  
  c=acoeff./h;
  Bneg=Ubernoulli(-(v(2:Nnodes)-v(1:Nnodes-1))*bcoeff,1);
  Bpos=Ubernoulli( (v(2:Nnodes)-v(1:Nnodes-1))*bcoeff,1);
  
  
  d0    = [c(1).*Bneg(1); c(1:end-1).*Bpos(1:end-1)+c(2:end).*Bneg(2:end); c(end)*Bpos(end)];
  d1    = [1000;-c.* Bpos];
  dm1   = [-c.* Bneg;1000];
  
  A = spdiags([dm1 d0 d1],-1:1,Nnodes,Nnodes);
  
endfunction