Impact-ionization and noise characteristics of thin III-V avalanche photodiodes

It is, by now, well known that McIntyre\u27s localized carrier-multiplication theory cannot explain the suppression of excess noise factor observed in avalanche photodiodes (APDs) that make use of thin multiplication regions. We demonstrate that a carrier multiplication model that incorporates the effects of dead space, as developed earlier by Hayat et al. provides excellent agreement with the impact-ionization and noise characteristics of thin InP, In/sub 0.52/Al/sub 0.48/As, GaAs, and Al/sub 0.2/Ga/sub 0.8/As APDs, with multiplication regions of different widths. We outline a general technique that facilitates the calculation of ionization coefficients for carriers that have traveled a distance exceeding the dead space (enabled carriers), directly from experimental excess-noise-factor data. These coefficients depend on the electric field in exponential fashion and are independent of multiplication width, as expected on physical grounds. The procedure for obtaining the ionization coefficients is used in conjunction with the dead-space-multiplication theory (DSMT) to predict excess noise factor versus mean-gain curves that are in excellent accord with experimental data for thin III-V APDs, for all multiplication-region widths

Gain-bandwidth characteristics of thin avalanche photodiodes

The frequency-response characteristics of avalanche photodiodes (APDs) with thin multiplication layers are investigated by means of a recurrence technique that incorporates the history dependence of ionization coefficients. In addition, to characterize the autocorrelation function of the impulse response, new recurrence equations are derived and solved using a parallel computer. The mean frequency response and the gain-bandwidth product are computed and a simple model for the dependence of the gain-bandwidth product on the multiplication-layer width is set forth for GaAs, InP, Al/sub 0.2/Ga/sub 0.8/As, and In/sub 0.52/Al/sub 0.48/As APDs. It is shown that the dead-space effect leads to a reduction (up to 30%) in the bandwidth from that predicted by the conventional multiplication theory. Notably, calculation of the power-spectral density of the photocurrent reveals that the presence of dead space also results in a reduction in the fluctuations in the frequency response. This result is the spectral generalization of the reduction in the excess noise factor in thin APDs and reveals an added advantage of using thin APDs in ultrafast receivers

Intrasubband and Intersubband Electron Relaxation in Semiconductor Quantum Wire Structures

We calculate the intersubband and intrasubband many-body inelastic Coulomb scattering rates due to electron-electron interaction in two-subband semiconductor quantum wire structures. We analyze our relaxation rates in terms of contributions from inter- and intrasubband charge-density excitations separately. We show that the intersubband (intrasubband) charge-density excitations are primarily responsible for intersubband (intrasubband) inelastic scattering. We identify the contributions to the inelastic scattering rate coming from the emission of the single-particle and the collective excitations individually. We obtain the lifetime of hot electrons injected in each subband as a function of the total charge density in the wire.Comment: Submitted to PRB. 20 pages, Latex file, and 7 postscript files with Figure

Carrier relaxation in doped quantum wells

The relaxation time of an electron in a quantum well is derived within the random-phase approximation including full multi-subband and frequency dependent screening. The resulting expression encompasses both electron-electron and electron-phonon scattering taking into account the mutual interactions of the electrons and phonons. The intersubband relaxation time of an electron is numerically evaluated considering electron-electron and electron-phonon (bulk LO-phonon) scattering in a GaAs quantum well. It is shown that the intersubband relaxation time is significantly influenced by the electron density in the well. It is also shown that at room temperature it is necessary to use the finite temperature dielectric function to accurately determine the intersubband relaxation time. Scattering due to the coupled system of electrons and phonons is compared with the decoupled scattering where both electron-electron and unscreened electron-phonon scattering are considered separately. In addition, the above theory of carrier relaxation is applied to quantum well lasers. The gain saturation coefficient, c:, of InxGat-xAs/ Alo.2Gao.8As strained layer quantum well lasers (SL-QWLs) is calculated as a function of strain from carrier intrasubband relaxation times. The intrasubband relaxation times are calculated within the RPA including carrier-carrier as well as carrier-polar optical phonon interactions at a temperature of 300 K. The band structures are determined from the Luttinger-Kohn Hamiltonian and a multiband effective mass equation. It is demonstrated that the gain saturation coefficient increases with compressive strain in the active layer of the quantum well due to a corresponding increase of the intrasubband relaxation time. From this, a direct connection between strain and laser switching speed can be deduced.U of I Onlydissertatio