Traps release time

However, when Tis less than T., the exponent in Eq. (3.6) is positive, so that the integral diverges and the average trap release time becomes infinite. The drift mobility should consequently drop abruptly to zero. To see that this is not what actually happens, consider the median release time of the carriers, defined by... 

The approximate expression applies when < p . Thus the drift mobility is thermally activated with the energy of the traps. When there is a distribution of trap energies, N E), the drift mobility reflects the average release time of the carriers, which is given by. 

The reason for the divergent Xj but finite x is that the average release time from the traps is dominated by the exponentially small density of states with very large trap energies. In any measurement that involves a finite number, n, of trapping and release events, the average carrier will not fall into a trap deeper than where... 

With the experimental results in mind we retium to the analysis of the trap-limited transport. The time-dependent decrease in the apparent mobility is obviously consistent with our earlier argument that the average trapping time will increase with the number of trapping events for an exponential band tail. Scher and Montroll (1975) were the first to point out this property of a very broad distribution of release times and to associate the effect with transport in disordered semiconductors. They analyzed the random walk of carriers with such a distribution and... 

Thermal emission measurements are generally based on electrical transport and in most cases involve the effects of a trapped space charge, Q. The two main ways of extracting information about the density of states distribution are from the release time of the charge from the traps... 

Let us consider the retarding effect of traps and how this effect changes the measured kinetic constants [51]. In the presence of traps, the time-dependent conservation equation for free carriers, contains an additional term, due to the net capture and release by traps, which results in a modified concentration of localized electrons rij ... 

For the case of PPV the dispersion of the photocurrent is too large for transit time determination. It can be concluded that even at room temperature the release times from the deepest traps, which control the transport properties, are in the range of 1 s or less. This corresponds to an effective mobility of less than 10 cm A s [69]. Based on TSC measurements from different groups [70], we conclude that this behaviour is primarily caused by the existence of grain boundaries. 

Trapping. Migrating charges can be temporarily or permanently immobilized in shallow or deep traps. The terms shallow and deep are only relative, and refer to the release times. Shallow traps are those from which carriers are released in the time of the experiment. 

Diffusion Pump System. After the pump line and trap have been shut off, a large valve is opened slowly enough that the mass flow of gas from the chamber through the valve into the od-diffusion pump system does not dismpt the top jet of the diffusion pump (DP) (Fig. 4). When the Hquid nitrogen is replenished after the trap has been operated for some time, release of previously trapped gas must be avoided. The so-caded ionization-gauge response pips at the start of the Hquid-nitrogen replenishment are an indication of trap ineffectiveness. 

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