半导体物理与器件－42节.pptVIP

At thermal equilibrium, pn = ni2 , however, semiconductor devices operate under non-equilibrium conditions. many important If the carrier density is higher than that in thermal equilibrium, and if the source of excitation is removed, excess carriers given time will recombine to re-establish thermal equilibrium (np = ni2). On the other hand if the carrier density is lower than that in thermal equilibrium, additional carriers will be generated to re-establish thermal equilibrium. Non-equilibrium Carrier Carrier Generation At any T 0°K, some electrons in the valence band gain sufficient energy to jump into the conduction band. This leaves behind holes. In thermal equilibrium Gth = Rth Equilibrium low level Injection high level injection n ? Nd + Dn p ? ni2 / Nd + Dp Low Level : Dn Nd High Level : Dn 3 Nd Carrier Recombination Direct Recombination Direct Band-gap Semiconductor e. g. GaAs Indirect Band-gap Semiconductor e.g. Si, Ge An impurity (e.g., Au) is introduced which provides a trapping level or a set of allowed states at energy Et Recombination is accomplished by trapping an electron and a hole. Before After This is an extremely important result and is based upon the work of Shockley, Read and Hall. This theory is known as SRH recombination. 1. The driving force or rate of recombination is proportional to (pn - ni2), i.e., the deviation from equilibrium. 2. U is a maximum when Et = Ei , i.e., trapping levels near mid-band are the most efficient recombination centers. When Et ? Ec , electron capture is more probable but hole capture is less probable. When Et ? Ec, electron emission back to conduction band is highly probable and recombination is less probable. In N-type semiconductor, lots of electrons will be around for capture and hence the limiting factor in recombination will be the minority carrier (i.e., hole) lifetime tp. 2. P-type: p ?? n and p ?? ni Again, the limiting factor is the minority carrier (in this case, electron) lifetime. 3. If the tr