Fig. 5

The G _btt for different fields in absorption layer with different γ. The values of γ is 1.0 (black star), 1.1 (black down-pointing triangle), 1.2 (black diamond), 1.3 (black triangle), 1.4 (black square), 1.5(black circle). Presents the calculated results of G _btt by Formula 13. When the field of absorption exceeds 19 kV/cm, G _bbt gradually increases. It also can be found that G _bbt adapts the same order of magnitude for the charge layer doping level while γ is restricted to 1~1.5