p型重掺杂应变Si_(1-x)Ge_x基区内建电场的物理机制

Physical Mechanism of the Built-In Electric Field for SiGe HBT with Heavy p-Doping

采用解析的方法计算了在基区掺杂为高斯分布,Ge组分分布为三角形分布和矩形三角形分布时基区内建电场的变化情况.重新拟合了价带有效态密度公式,并在计算内建电场时考虑了导带有效态密度的影响.发现加入Ge组分后引起的导带有效态密度变化、价带有效态密度变化以及禁带宽度变窄量变化对基区内建电场的影响要大于掺杂对内建电场的影响.Ge组分为三角形分布时,在总的Ge组分一定的条件下,内建电场从发射结到集电结逐渐变大.在任一给定位置x处,内建电场随着Ge组分的增加而增大.当Ge组分分布为矩形三角形分布时,对于给定的Ge组分转折点x1,基区内建电场从发射结到集电结缓慢地增大.在Ge组分恒定的区域,内建电场变化甚微,在Ge组分为线性缓变区域的同一位置x处,内建电场随Ge组分转折点x1的增大而缓慢地增大.此外,在x1附近内建电场变化有一个很大的陡坡.

A model for the built-in electric field in the base is developed and calculated analytically. The model is suitable for a base with a Gaussian doping distribution and different Ge profiles （e. g. ,triangle,box-triangle）. A new formula for the va-lence band effective states is obtained, and the effect of the conduction band effective states on the built-in electric field is considered for the first time. It is found that the effect of the Ge fraction on the built-in electric field is larger than that of impurities. When the Ge fraction profile is triangular,the built-in electric field increases gradually from emission to collector with the total Ge fraction. At a given location x, the built-in electric field becomes larger and larger with the increasing of the total Ge fraction. When the Ge fraction profile is box-triangular,the built-in electric field increases from emission junc-tion to collector for a given x1 and a given Ge fraction Yc. For an invariable Ge fraction profile, the built-in electric field is nearly invariable. For a linearly graded Ge fraction profile,the built-in electric field becomes larger and larger with the in-creasing of x1. In addition,there is a sharp drop near x1.