Traps trap limited drift mobility

For the simplest case of a single set of localized states sitnated at a particular energy Ei, the trap-limited drift mobility of carriers moving in extended states at is readily compnted from equation (3.3). If the effective density of extended states at Ep is Np and the trap concentration is N, then we may write... 

Recently the concept of trap limited drift velocity providing the explanation for the observed saturation have gained much currency (12,13). This has been shown to be inapplicable in explaining these results (3) and % ould any way require trap limited mobilities to be > 20 m /V/s. This ultra high value finds no explanation in such trap limited transport theories and would indicate that the intrinsic mobility was even higher I... 

In the case of material with a significant concentration of localized states, it is possible to assnme that transport of a carrier over any macroscopic distance will involve motion in states confined to a single energy. Here it is necessary to note that a particn-larly important departnre from this limiting situation is (according to Rose [4]) a trap-limited band motion. In this case, transport of carrier via extended states is repeatedly interrnpted by trapping in localized states. The macroscopic drift mobility for such a carrier is reduced from the value for free carriers, by taking into acconnt the proportion of time spent in traps. Under steady-state conditions, we may write... 

The residual potential is due to trapped electrons in the bulk of the specimen. The simplest theoretical model, which is based on range limitation and weak trapping (Vj drift mobility and lifetime r product) via the Warter equation [19] ... 

To explain the trap-limited transport it is useful to consider the model of a single trapping level of density, Nj., at energy E. below the conducting states, as illustrated in Fig. 3.10. The drift mobility is the free carrier mobility reduced by the fraction of time that the carrier spends in the traps, so that,... 

In some cases the drift of carriers may be seriously interrupted by capture at trapping sites in the solid. If the traps are energetically shallow, i.e. the depth of the potential well is comparable with thermal energies (kT), the carriers will soon escape again, and the only effect will be to reduce the apparent mobility. The trap-limited mobility fiT will be given by... 

Figure 6.18 Results from junction-recovery measurements on Sn02/Ti02/Au diodes. The electron drift mobility and the mobility-lifetime product are determined for various injection conditions and forward bias. The electron mobility is found to increase with increasing injection level, while the mobility-lifetime product remains approximately constant. These findings can be consistently explained in a transport model based on trap filling and a transport-limited recombination mechanism. An alternative explanation can, however, also be based on a tunnelling transport model (Konenkamp, 2000a).

Figure 2. Plots of the measured dc current density filled squares, steady state current density open circles, the current density computed from drift mobility measurements and Eq. 1 filled diamonds, the current density computed from the transient dark injection peak values. The contact under test is a carbon filled polymer coated with a transport layer (TPD/polycarbonate) that is known to support trap free hole transport. The insert shows a typical dark injection transient compared to a small signal TOF transient. Conformity of key features of the steady state and transient data with the theory of trap free space charge limited currents provides a self consistent demonstration of contact ohmicity.

More recent work [221-225] has not yet resolved the puzzle. It seems, however, that the small mobilities obtained initially were influenced by trapping, and the intrinsic value should be larger. Evidence for a sublinear increase in drift velocity with electric field and a tendency toward saturation has been found [224], but at a much higher field than proposed earlier. The lower limit of the low-field mobility would be about 103 cm2/V s. But since the chain length in the PDAs investigated to date is not known, the relative influence of intrachain transport and interchain hopping in this value is uncertain. It will be some time before values to be compared to a theory of transport in a CP are available. The high electron mobilities... 

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