Plasma formation and extraction processes in silicon n⁺np⁺ TRAPATT (TRApped Plasma Avalanche Triggered Transit) diodes were simulated. The drift-diffusion model was chosen for the simulation of the processes. This model is adequate for diodes under consideration with a total thickness of 6.5 μm. Two approximations of carrier diffusion coefficient dependence on the electric field above 20 kV/cm were used. A strong dependence of plasma density and oscillation period on n+n junction steepness was found in the case of a constant electron diffusion coefficient in the electric field range above 20 kV/cm. This behaviour depends on the impact ionization model in silicon. Two models were used. In one of them we included drift and diffusion current in the impact ionisation process. In the other model we included only the drift current in the impact ionisation process. In the second case the influence of the n+n junction steepness on the plasma formation process is much stronger. In the diodes with a highly abrupt n⁺n junction the TRAPATT mode is impossible. These results explain our experiments on TRAPATT diodes with an abrupt n⁺n junction.
Keywords: simulation, avalanche diodes, diffusion
PACS: 85.30.Mn

ELEKTRONŲ DIFUZIJOS ĮTAKOS PLAZMOS SUSIDARYMUI SILICIO TRAPATT DIODUOSE MODELIAVIMAS
J. Vyšniauskas^a, V. Klimenko^b, J. Matukas^a , V. Palenskis^a
aVilniaus universiteto Fizikos fakultetas, Vilnius, Lietuva
^bPuškovo Žemės magnetizmo, jonosferos ir radijo bangų sklidimo instituto Vakarų padalinys, Kaliningradas, Rusija

Darbe pateikti plazmos susidarymo ir išsiurbimo procesų n⁺np⁺ silicio TRAPATT (TRApped Plasma Avalanche Triggered Transit) dioduose kompiuterinio modeliavimo rezultatai. Procesų modeliavimui buvo pasirinktas dreifinis-difuzinis modelis. Šis modelis tinka minėtų diodų modeliavimui, kadangi jų ilgis yra 6,5 μm. Buvo naudojamos dvi krūvininkų difuzijos koeficiento priklausomybės nuo elektrinio lauko daugiau kaip 20 kV/cm aproksimacijos. Buvo aptikta stipri plazmos tankio ir virpesių periodo priklausomybė nuo n+n sandūros statumo, kai elektronų difuzijos koeficientas yra pastovus elektriniuose laukuose (per 20 kV/cm). Šis reiškinys priklauso nuo smūginės jonizacijos modelio silicyje. Buvo naudojami du modeliai. Viename iš jų buvo įvertinta dreifo ir difuzijos srovė skaičiuojant krūvio generacijos spartą. Kitame modelyje – įvertinta tik krūvininkų dreifo srovė smūginės jonizacijos procese. Antruoju atveju sandūros statumo įtaka plazmos formavimosi procesui yra daug didesnė. Dioduose su ypač stačia n+n sandūra TRAPATT moda negalima. Šie rezultatai paaiškina mūsų eksperimentus su stačios n+n sandūros TRAPATT diodais.

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