Constant Amplitude Technique



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Constant Amplitude Technique

In this approach all parameters of the trapezoidal gate pulses (, , , and ) are constant during the course of experiment. The reverse bias is constant as well and can be set to zero. The bottom level is varied to scan the interface while the top level is kept sufficiently high so that the whole interface becomes inverted during the top level (for -channel devices)gif. In this method, the interface is scanned along the characteristics where is the coordinate actually scanned. This characteristic is responsible for the part of the conventional curve, discussed in Section 3.5.1, from the maximum of this curve after of the channel region until the right falling edge representing of the channel region. Since the trap emission times are constant during the course of experiment, the trap density is given with a simple relationship

 

derived for symmetrical cases in Appendix G. A drawback of this approach is the variable oxide field while filling the traps with electrons during the gate top level. This effect can change the energy position of the interface traps as discussed in Section 3.1.3. The importance of this effect in measurements is, however, not clear at present. Experimental work is necessary here. If the experiments provide the same result by using the constant amplitude method assuming different gate amplitudes (and proportionally changed rise and fall times so that the slopes remain constant) the latter effect can be judged as irrelevant.

Figure 3.37 presents the charge-pumping characteristic calculated numerically assuming a nonuniform trap distribution within the source and drain junctions as shown in Figure 3.38. The trap distribution extracted by relationship 3.132 is in good agreement with the assumed distribution, Figure 3.38. When applying the constant amplitude method on the stressed devices expression 3.132 should be applied on the difference of currents in stressed and virgin devices. Note that in this technique the scanned energy interval in the band gap is variable during the course of experiment. The variations are, however, smaller than those in method II, where the emission levels also vary due to the variation of .

In the analysis presented in this section all differentiations are carried out by using the sin-convolution filter, since the numerical data is too noisy to be differentiated by

 

 

finite difference formulas. The procedure is clarified in Appendix G. By changing the filter with from to it has been confirmed that the differentiation by the sin-convolution does not influence the results significantly.

 

In the calculations, the relations or are used which are obtained for the virgin device. In the stressed devices a high density of localized traps change the local potential, thereby leading to an error in determining the exact position when for a virgin device is assumed. To analyze this error we calculate for virgin and stressed devices which correspond to the problem in Figure 3.34 (for the peak density of ). As obtained, the errors in shown in Figure 3.39 and its derivation are not significant in practice even for high trap densities.



next up previous contents
Next: 3.5.3 Charge-Pumping Characteristics of Up: 3.5.2 Extraction of the Previous: Other Large-Signal Charge-Pumping



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