(3.8) The measured multiplied photocurrent and estimated primary current of an AlAsSb p-i-n structure (full characterisation presented in Appendix A) is shown in . The electric field at low voltage is not high enough to trigger impact ionisation. The primary photocurrent is due to the photocarriers generated in the cladding layer, some of the minority carriers recombined with majority carriers and the others will diffuse into the depleted region and being swept by the high electric field. As the applied reverse bias increases, the edge of depletion width will slowly move towards the light absorption surface and the collection efficiency is slowly increasing with bias. shows a model explaining the changes of depletion edge with bias on injected photocurrent. Woods showed the collected primary photocurrent can be et al[] described as (3.9) where is the electron charge, is the generation rate of photocarriers at the illuminated q G0 surface, is the distance between the illuminated surface and the depleted edge and is the L Ld minority diffusion length. Since (L0 and are illustrated in ), equation 3.9 L=L0-W(V) W(V) can be rewritten as (3.10) when >>L, can be simplified to be linear with as Therefore, Ld Ipr W(V), Ipr mW(V)+k0. ≈ the primary photocarriers can be considered to be increasing linearly with applied voltage and the bias dependent primary photocurrent can be derived by linear extrapolating the photocurrent measured at low biases. The gain was obtained by taking the of the multiplied photocurrent to the primary current using equation 3.8. Figure 3.5 Model to explain the voltage dependence of inject photocurrent Figure 3.6 Multiplied photocurrent and primary current of AlAsSb PIN2 under 442 nm illumination. The photocurrent is converted into voltage through a small resistance
(3.8) The measured multiplied photocurrent and estimated primary current of an AlAsSb p-i-n structure (full characterisation presented in Appendix A) is shown in . The electric field at low voltage is not high enough to trigger impact ionisation. The primary photocurrent is due to the photocarriers generated in the cladding layer, some of the minority carriers recombined with majority carriers and the others will diffuse into the depleted region and being swept by the high electric field. As the applied reverse bias increases, the edge of depletion width will slowly move towards the light absorption surface and the collection efficiency is slowly increasing with bias. shows a model explaining the changes of depletion edge with bias on injected photocurrent. Woods et al[] showed the collected primary photocurrent can be described as (3.9) where q is the electron charge, G0 is the generation rate of photocarriers at the illuminated surface, L is the distance between the illuminated surface and the depleted edge and Ld is the minority diffusion length. Since L=L0-W(V) (L0 and W(V) are illustrated in ), equation 3.9 can be rewritten as (3.10) when Ld >>L, Ipr can be simplified to be linear with W(V), as Ipr ≈ mW(V)+k0. Therefore, the primary photocarriers can be considered to be increasing linearly with applied voltage and the bias dependent primary photocurrent can be derived by linear extrapolating the photocurrent measured at low biases. The gain was obtained by taking the of the multiplied photocurrent to the primary current using equation 3.8. Figure 3.5 Model to explain the voltage dependence of inject photocurrent Figure 3.6 Multiplied photocurrent and primary current of AlAsSb PIN2 under 442 nm illumination. The photocurrent is converted into voltage through a small resistance