3.4.2 Carriers Emitted from the Interface States



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3.4.2 Carriers Emitted from the Interface States

Henceforward, we focus on the later effect which consists of transferring of the electrons emitted from the interface states towards the bulk. This effect is not consistent with the present understanding of the charge pumping in MOSFETs. It has been established in the literature that the minority carriers emitted from the interface traps into the conduction band are collected by the source and/or drain junctions. Therefore, they do not contribute to . This assumption is adopted for the minority carriers

The evolution of and in Figures 3.16 show, however, that some part of the electrons emitted from the interface states contributes to , thereby, it increases . Figures 3.17 and 3.19 demonstrate that As opposed to the component of the current which is dominant at short , the component of at long depends only slightly on the slope of the falling edge . It is moderately influenced by the channel length, as is shown in Figure 3.19 where the same amount of traps is positioned at the center of the channel of several identical devices with only different gate length. Note that the component of at short , which originates due to the remaining , is independent of the trap density. We conclude that the geometric current component in real MOSFETs occurs as a superposition of two independent effects:

An experimental evidence for these two processes is given in the sensitive measurements of the geometric current component in [98]. Fig.4 in [98] shows evidently that does not vanish for long , while it increases rapidly for smaller than some threshold value, in real MOSFETs. Note that the present calculations are even in a nearly quantitative agreement with these measurements.

 

The transfer of the emitted carriers towards the bulk occurs at the onset of the hole accumulation on the interface. If a significant portion of the traps recombine with the incoming holes the novel effect represents only a small correction of in the order of a few . However, this effect occurs near the onset of accumulation when the hole capture is still small as well. In these cases, the is comparable with . As a consequence, this effect modulates the falling edge of the characteristics, as is obtained in the calculations shown in Figure 3.18. In this example, the geometric component which originates from the due to fast turn-off is negligible ( for device). The finding that the novel effect produce a stretch-out of the falling edge of the curve in a lin-log scale has a minor practical importance, since this part of the characteristics is never used to extract some information.
In the three-level techniques, however, the novel effect can be relevant if the mid-level is sufficiently low. Particularly important is the so-called capture mode of the three-level measurements [395][11], which is employed to extract the cross-section of the capture process in addition to the common cross-section in the emission [397][154]. In the capture region of the operation during the mid-level , all conditions are fulfilled that the emitted electrons transfer towards the bulk, but not towards the junctions, as is adopted in the theoretical model of this technique. An example shown in Figure 3.20 demonstrates the modulation of the capture-side of the saturated- versus characteristics by the current . In the numerical calculation is assumed, which is long sufficiently to achieve the steady-state conditions at the end of the mid-level for all used. The same calculation is carried out for this device, but without any interface traps. The obtained which originates solely from the remained due to a short , is very small (less than in the range ) leading to the conclusion that the effect shown in Figure 3.20 does not occur as a consequence of the standard geometrical current component due to , but because of the emission from the traps.

 

 

The novel effect can also influence the pulsed-interface-probing (PIP) measurements [52][51]. In the PIP theoretical model it is assumed that all electrons (minorities) which are emitted from the interface traps move back to the junctions. The negative bulk current is measured which consists exclusively of the holes emitted from the interface states during the deep-depletion conditions at the interface (bottom-level duration). If some of the electrons emitted from the traps are injected into the bulk, they generate a positive bulk electron current introducing an error in measurements. Note that the bulk hole current is very small in this technique. Although the hole capture does not take place in this method, because is sufficiently high, the transient effect due to a small depletion-region shortening can cause the injection of some portion of the emitted electrons into the bulk, as is observed in our calculations.

 

Up to now we have considered the falling edge of the gate pulses for -channel MOSFETs or the rising edge for -channel devices. Regarding the rising edge for -channel devices and the falling edge for -channel devices, our study did not show any non-ideal behaviour, even at short switching times ().

Finally, a comment should be given on the geometric component in SOI devices [445][357]. The same effects which occur in the bulk MOSFETs produce the geometric current component in SOI devices as well. Note that in SOI gated-diodes (-- diodes) the effective channel length is twice the channel length in MOSFETs, causing that the effect is much more pronounced. The injection of the minorities into the bulk has also been employed for the operation of SOI MOSFETs in some applications [407][405]. In addition to the minority carriers, a finite time for the response of the majority carriers can be exposed in SOI devices, leading to the geometric current component due to majority carriers, as well. The majority-carrier effects are pronounced for both, the gated-diodes [445][357] and the SOI MOSFETs with a side-added bulk contact [445], when the devices are made in a thin film. Whether the geometric component is produced by the minority or the majority-carrier effects depends directly on the conditions holding on the back-interface (accumulation or inversion) [192].



next up previous contents
Next: 3.5 Hot-Carrier Degradation Analysis Up: 3.4 Geometric Current Component Previous: 3.4.1 Minority Carriers Remaining



Martin Stiftinger
Sat Oct 15 22:05:10 MET 1994