Measurement of depletion layers

The depletion layer profile contains information about the density of states distribution and the built-in potential. The depletion layer width reduces to zero at a forward bias equal to and increases in reverse bias. The voltage dependence of the jimction capacitance is a common method of measuring W V). Eq. (9.9) applies to a semiconductor with a discrete donor level, and 1 is obtained from the intercept of a plot of 1/C versus voltage. The 1/C plot is not linear for a-Si H because of the continuous distribution of gap states-an example is shown in Fig. 4.16. The alternative expression, Eq. (9.10), is also not an accurate fit, but nevertheless the data can be extrapolated reasonably well to give the built-in potential. The main limitation of the capacitance measurement is that the bulk of the sample must be conducting, so that the measurement is difficult for undoped a-Si H. 

The depletion layer profile on undoped a-Si H can be obtained by transient photoconductivity. A pulse of light excites electron-hole pairs very near the contact. As in the time-of-fiight experiment, holes are immediately collected by the contact and electrons drift down the internal field of the depletion layer, giving transient conductivity of 

9 eV and 1.2 eV, which agree reasonably well with the other measurements. 

The carriers drift under the influence of the internal field to the edge of the depletion layer and then stop in the field-free region. The charge collected in the external circuit is 

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