Question
The figure below provides similar ionization energy information (in eV) for germanium.
Te Cu Au Li Sb Se 0093 0096 012 013 14 27 26 28 GAP CENTER .29 31 .33 15 095 12 087 .09 06 01 011-011 =.02 Al Ga Be Zn Cr Cd Hg Co Ni Mn Fe Pt As in Figure 4-9 in the textbook, the levels drawn below the dashed line, which marks the energy gap center, are measured from the top of the valence band. Similarly, the levels drawn above the dashed line are measured from the bottom of the conduction band. The levels drawn below the dashed line are acceptor levels unless they are marked with a "D" to indicate they are donor levels. The levels drawn above the dashed line are donor levels unless they are marked with an "A" to indicate they are acceptor levels. For example: Gold in Ge has three acceptor levels and one donor level Figure from S.M. Sze, Physics of Semiconductor Devices, 2nd Edition, Wiley, New York NY, p2.1 (1981) Determine the energy required to cause a selenium impurity in Ge to donate a first electron to the conduction band a) b) c) d) Determine the energy required to cause the selenium ion of part a) to donate a second electron to the conduction band. Determine the energy required to cause an iron impurity in Ge to accept a first electron from the valence band Determine the energy required to cause an iron impurity in Ge to accept a second electron from the valence band
Explanation / Answer
a) Band Gap (Eg) of Germanium = 0.7 eV.
Inpurity Energy Level Difference from Conduction Band Ec-Ed= 0.14 eV and 0.28 eV. More easily the electron can be donated from 0.14 eV. Therefore Energy required to donate first electron to conduction band is 0.14 eV.
b) Selenium has two donar electrons in the outermost shell. Therefore second electron will come from Energy Level at 0.28 eV.
c) Energy required to accept first electron from valence band is 0.31 eV.
d) Energy required to accept first electron from valence band is Eg/2 + (Ei- Ea) = 0.7/2 + (0.35-0.27) = 0.35+0.08= 0.43 eV.
