Space charge limited currents level

An example of the difficulties encountered when trying to fabricate an ohmic electrode, able to sustain a space-charge-limited current, is the recent work of the Neher group [179]. The authors deposited barium as an electron injection cathode on top of an electron transporting polymer based on a naphthalene diimide core whose LUMO is as low as 4 eV below vacuum level. Although the Fermi level of barium should be above the LUMO of the polymer, the electron current is. 

Almost by definition, an electronic device has at least two contacts. Here we consider a single one therefore, it is assumed that the bulk conductivity of the material and the distance between contacts are large enough, so that over most of that distance the material s energy levels are not influenced by the contacts. This is not true in the case of space-charge-limited currents [215], especially in double injection, a situation that may occur in CP LEDs (see Section V.C). 

It should be emphasized that a carrier transport can only be described by Ohm s law (Eq. 1.35) if sufficient empty energy levels exist in the corresponding energy band and a minimum carrier density is present in the material. On the other hand, in the case of an intrinsic high bandgap semiconductor, the carrier density may be negligible so that only those carriers carry the current which are injected into the crystal via one contact. In this case we have a space charge limited current which is proportional to (Child s law). 

In the technique of multiphoton-ionization spectroscopy, two or more photons excite atoms or molecules from the ground state to an excited state which may be ionized by several methods (see Sect.8.2.5), e.g., field ionization, photoionization, collisional, or surface ionization. If the laser is tuned to multiphoton resonances, ionization signals are obtained if the upper level is ionized which can be, for instance, monitored with the setup shown in Fig.8.42. The ionization probe is a thin wire inserted into a pipe containing the atomic vapor. If the probe is negatively biased relative to the walls of the pipe, thermionic emission will lead to space-charge-limited current. Ions produced by the laser excitation partly neutralize the space charge, thereby allowing an increased electron current to flow (see Sect.8.2.4). 

The electrical current of a coplanar interdigilal gold/LPPP/gold device is space charge limited due to p-type charge earner traps localized in the bandgap [28]. This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels ,1 and ,", which can be calculated from the rise in current, /, below the peak temperature ... 

A true Fowler-Nordheim dependence according to Equation 57 was found for currents <10 A and for tip radii <0.5 pm (see Figure 22b). At higher levels of the emission current a space-charge limited behavior of the current is observed. With... 

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