Lecture_11

Lecture_11 - PN Junctions Summary (1) Semiconductor...

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1 Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Semiconductor transport theory • Current density equations • Drift • Diffusion Recombination mechanisms • Radiative • Traps • Surface PN Junctions Summary (1) Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Two different kinds of processes can lead to currents of electrons or holes in a semiconductor Applying an electric Feld will cause carriers to move by drift Particles also tend to move from regions of high density to regions of lower density-- diffusion . Current density equations
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2 Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Field-induced transport At low electric Felds, electrons or holes move at a drift velocity that is proportional to electric Feld. Microscopically, electrons or holes are accelerated by the Feld, but keep colliding with other particles or with phonons. These collisions randomly "reset" the velocities in the direction of the Feld. At high Felds, particles are accelerated more before a collision, so they have a higher average velocity. The coefFcient relating average "drift" velocity to applied electric Feld is mobility, μ e for electrons and h for holes Hence the drift currents resulting from an electric Feld, E J drifte = ne e E J drifth = pe h E (4.8) (4.9) and Current density equations (2) Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Because current is proportional to electric Feld in the low-Feld "mobility” limit, conduction is linear, i.e., ”Ohmic" The conductivity, σ , for n -doped material is = ne e = pe h ρ = 1 = 1 ne e = 1 = 1 pe h (4.10) (4.11) (4.12) (4.13) with resistivity for p-doped material and resistivity ±or a material with both carrier types (i.e. intrinsic material, or when photoexcited), the conductivities should be added . Linear ”Ohmic" transport
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3 Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Mobility is limited by scattering mechanisms, such as scattering from phonons or ionized impurities The smaller the effective mass, the larger the mobility for most of these mechanisms Hence, GaAs can have a relatively large electron mobility compared to Si Mobilities fall off at higher doping densities because of ionized impurity scattering Impurity density dependence of mobility Prof. J. S. Harris 6 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Solid line-calculation including alloy scattering Dashed line -Calculation neglecting alloy scattering. Experimental points clearly illustrate that alloy scattering is dominating the mobility for p-type AlGaAs Mobilities in alloy materials depend on the alloy fraction for at least two possible reasons (1) scattering due to alloy Fuctuations (random local variations in the alloy composition) in the crystal p-type Al x Ga (1-x) As Mobilities in alloy semiconductors µ (cm 2 /Vs
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Lecture_11 - PN Junctions Summary (1) Semiconductor...

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