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The mechanisms of electric-field- and temperature-dependent effective drift mobility and impact ionization coefficient of both holes and electrons in amorphous selenium (a-Se) are investigated in this paper. An analytical model for the microscopic mobility, momentum relaxation mean free path, and hence the effective drift mobility and impact ionization coefficient of carriers, is proposed in this paper by considering the density of states distribution, field enhancement release rate from the shallow traps, and carrier heating. The results of the model are fitted with the published experimental results on effective mobility and impact ionization coefficient with wide variations of the applied electric field and temperature. A better fitting considering thermally activated tunneling for the field-enhancement release rate indicates that the effective drift mobility at extremely high fields is mainly controlled by the neutral defect states near the band edges. The density of state function near the band edges, consisting of an exponential tail and a Gaussian peak, can successfully describe the electric-fieldand temperature-dependent effective drift mobility characteristics in a-Se. The momentum relaxation mean free path decreases with increasing field and decreasing temperature, which is required to describe the electric-field- and temperature- dependent behaviors of impact ionization coefficient in a-Se. PACS Nos.: 72.20.Jv, 72.20.Ht, 71.23.An. Nous etudions ici les mecanismes de dependance en champs electriques et en temperature de la mobilite de derive efficace et du coefficient d'ionisation par impact des trous et des electrons dans le selenium amorphe (a-Se). Nous tenons compte de la distribution de densite d'etats, de la liberation du renforcement du champ des pieges peu profonds et du rechauffement des porteurs, afin de proposer un modele analytique pour la mobilite microscopique, le libre parcours moyen de relaxation d'impulsion et ainsi la mobilite de derive efficace et le coefficient d'ionisation par impact. Les resultats du modele sont ajustes avec les resultats experimentaux publies sur la mobilite efficace et le coefficient d'ionisation par impact avec de grandes variations du champ electrique et de la temperature. Un meilleur ajustement qui tient compte d'un effet tunnel dans la liberation de renforcement du champ indique que la mobilite de derive aux valeurs elevees de champ est surtout controlee par les etats defauts pres des bords de bande. La fonction de densite d'etats pres du bord de bande qui consiste en un pic gaussien et une queue exponentielle peut decrire avec succes les caracteristiques de la dependance sur le champ electrique et la temperature de la mobilite de derive dans le a-Se. Le libre parcours moyen de relaxation de l'impulsion diminue lorsque le champ croit et que la temperature baisse, ce qui est necessaire pour decrire correctement la dependance en temperature du coefficient d'ionisation par impact dans a-Se. [Traduit par la Redaction], 1. Introduction Amorphous selenium (a-Se) based multilayer detector structures are currently used in conventional direct-conversion X-ray imaging (X-rays are directly converted to free electron and hole pairs in the photoconductor [...]

A numerical model is developed to describe the electron-hole pair (EHP) creation energy in amorphous selenium (a-Se) for high energy photon excitation by solving the three dimensional coupled continuity equations of electrons and holes considering carrier drift, diffusion and bimolecular recombination between non-geminate electrons and holes. The numerical model is fitted with the published experimental results on widely varying X-ray energy, electric field and temperature. The model shows a good agreement. The results of this paper indicate that the columnar recombination model is capable of describing the electric field, temperature, and photon energy dependent EHP creation energy in a-Se for high energy photons. [ABSTRACT FROM AUTHOR]

An analytical model is developed to study the mechanisms of X-ray generated free Electron-hole pair (EHP) creation energy in amorphous selenium (a-Se) at high electric fields. The model is presented to show the electric field and temperature dependence of the charge extraction yield limited by the columnar recombination for the materials that have widely unequal drift mobility for electrons and holes, such as a-Se. The model is compared with Jaffe's columnar recombination model with widely varying field and temperature. In addition, the free EHP creation energy is calculated by incorporating the initial charge extraction yield and the charge collection efficacy of the free carriers. Also, the results of this model are compared with the recently published experimental results on EHP creation energy. The analysis of the results confirm that the proposed model gives the best possible explanation to the columnar recombination mechanisms in a-Se and the free EHP creation mechanisms at diagnostic X-ray exposures can be described by the columnar recombination. [ABSTRACT FROM AUTHOR]