Investigation of carrier transport through silicon wafers by photocurrent measurements

Abstract

Measurement of the ac photocurrent in metal/insulator/semiconductor capacitors can be used as a tool to measure minority-carrier diffusion and lifetime. The amplitude of the ac photocurrent generated at a silicon surface biased into inversion depends on the number of excess minority carriers present at that surface. By comparing this amplitude when intensity-modulated light is directed to each side of the same device, minority-carrier diffusion from the back to the front of the device can be characterized. An analytical model of this transport process predicts the dependence of the ac photocurrent on frequency and wafer thickness, and allows the determination of a value of the bulk lifetime free of the influence of surface recombination. Measurements under low-light intensity levels are presented on n-type silicon wafers with lifetimes in the 10–100 μs range. Lifetimes are found about a factor of 2 lower than those measured with noncontact photoconductive decay, at high-light intensity levels. This is expected due to the difference between high- and low-level minority-carrier injection. Fitting the data to the model also yields a value of 115 μm for the average depth at which carriers are generated and diffuse to the front with backside illumination at 940 nm.

Light-Directed Electrical Stimulation of Neurons Cultured on Silicon Wafers

“…Excluding the added radius due to passing fibers, the spatial resolution intrinsic to our technique is around 100 microns, which is in agreement with the expected bulk diffusion length assuming a sub-10-s minority carrier lifetime (Bousse et al 1994). One can imagine some possible ways of improving this.…”

Sensor Modeling

“…Finally, Figure 4.18 shows the output current of the LAPS at V g = ± 3 V (ie, well beyond the threshold voltage), as a function of the AC frequency. As expected [36], such a curve exhibits a maximum; the current decreases beyond such a maximum owing to the decreasing AC hole diffusion length. …”

Novel interfaces for light directed neuronal stimulation: advances and challenges

“…For p-type silicon, electrons, which are the minority charge carriers, generate photocurrent upon application of reversed bias. Neuronal stimulation, based on this effect, was demonstrated by several groups utilizing either single crystal, [95][96][97] ( Figure 1A and B) or hydrogenated amorphous silicon (a-Si:H) [98][99][100][101] ( Figure 1C and 1D). …”

“…94 The temporal and spatial resolution of silicon photoexcitation is limited by minority charge carrier lifetime and diffusion coefficient, which is approximately equal to the thickness of the silicon. 99 It was argued that this resolution can be improved by patterning the silicon surface, and thinning the chip. 94,100 Another possibility is using low conductivity silicon which can be achieved, for example, by doping the silicon with gold.…”