TAILIEUCHUNG - MOSFET MODELING FOR VLSI SIMULATION - Theory and Practice Episode 2

Tham khảo tài liệu 'mosfet modeling for vlsi simulation - theory and practice episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16 2 Basic Semiconductor and pn Junction Theory Fig. Energy band diagram of a semiconductor silicon separate bands of allowed energies called the valence band and the conduction band. The energy levels in the valence bands are mostly filled with electrons forming the covalent bonds. The energy levels in the conduction bands are nearly empty. Electrons which occupy the energy levels in the conduction band are called free electrons or conduction electrons . The very closely spaced energy levels in the valence and conduction bands are often separated by a energy range where there are no allowed quantum states or energy levels known as the energy gap Eg or band gap . This energy gap between the two allowed energy bands is often referred to as the forbidden band or forbidden gap. Although the energy is a complex function of momentum in three dimensions and there are so many energy levels and so many electrons the energy band picture will be tedious to draw out if all the energy levels are shown. Thus only the edge levels of each of the allowed energy bands are shown in the energy band diagram see Figure . The electron energy is considered a positive guantity and is plotted upward on the energy-band diagram. If Ec and E are the energy levels for the lower edge of the conduction band and upper edge of the valence band respectively then the band gap Eg is 1 Eg Ec-Ev eV . 1 Note that the unit of Eg is electron-Volt eV or qV . By definition an electron-Volt is the energy an electron of charge q acquires when it moves through a potential difference of 1 Volt V . When Eg in eV is divided by q in units of electron charge not Coulombs the charge q of the electron cancels out and the result is Volts V . Thus Eg and Eg q have the same numerical value but different units. In general any physical quantity expressed in eV can be converted into Volts by simply dividing the quantity by the charge q and vice versa. If Eg is in Joules then Eg q is in Volts where q X .

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