The photovoltaic effect turns radiant energy, usually sunlight, straight into electrical energy. Unlike a heat engine, it involves no thermal step. A PV cell is the simplest device that shows this effect. One manual describes cells as large-area photodiodes. In a silicon cell, phosphorus doping makes an n-type layer with free electrons and boron doping makes a p-type layer with holes. Where the two layers meet, a depletion region with a built-in electric field forms. Light that reaches the depletion region creates electron-hole pairs. The junction field pushes the electrons and holes to opposite sides, which builds up a voltage between the n and p regions. When a load is connected, electrons flow through it and recombine with holes in the p region, so the cell delivers direct current. Real cells use a thin, heavily doped top n layer and a thick, lightly doped p layer. This widens the depletion region and lets more light reach it, so the cell produces more current. A single cell gives only around 0.5 volt. Cells are therefore connected in series and parallel inside a module to reach usable voltage and current, and an inverter converts the DC to AC for the grid.