Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 169 (d) Fig. 11. Measured spectral responses of photodiodes under different reverse biased voltages in (a) n-/p-sub, (b) p + /n-/p-sub,(c) n-/p-epi/p+sub and (d) p + /n (a) (b) Fig. 12. Variations in positions of the space-charge regions of (a) n-/p-sub photodiode and (b) p + /n- photodiodes, at reverse bias voltages from 0V to -5V (the dimensions of each layer in this structure do not represent actual dimensions). Advances in Photodiodes 170 spectral response. Figure 13(b) shows the simulated spectral responses of n-, space-charge, p-substrate regions, and the total spectral responses at reverse biased voltages from 0V to - 5V when the reflection coefficient is zero. The variation of the spectral response for this photodiode increases with the reverse biased voltage more significantly than those in the other three photodiodes. (a) (b) Fig. 13. Simulated spectral responses in n-type and p-type semiconductors and in space- charge region under different reverse biased voltages ranging from 0V to -5V when the reflection coefficient being zero for (a) n-/p-sub and (b) p + /n- photodiodes. Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 171 4. Design methodology for color CMOS pixels without color filters As the abovementioned, we conclude that the color filter technology is still a good choice for color separation presently. In fact, some specific modifications for the semiconductor process or signal processing circuits are applied to color CMOS image sensors without color filters [15]-[17]. In this work, an equation based on the CMOS photodiode model is derived to determine the peak wavelength of the spectral response. The detail of the derivation procedure is illustrated in Appendix. Here, some solutions for obtaining different color spectral responses are briefly sketched. Additionally, the approaches to enhance the capability of separating the color spectral responses are discussed. 1. Reducing the spectral response in the long wavelength region: Generally, the thickness of the substrate is as thick as several hundreds of micrometers. Consequently, the spectral response is dominated by the induced photocurrent generated in the substrate region. Since the peak wavelength of the spectral response of substrate is generally located at the infrared region, the peak wavelength of the total spectral response tends to occur at the long wavelength region. There are two approaches to reduce the spectral response in the long wavelength region. a. The spectral responses in the long wavelength region can be effectively decreased by shortening the p-n junction in the deep region [16]. The depth of diffusion affects the photodiode to absorb wavelengths of incident light. Referring to the absorption length in Fig. 7, the light with a longer wavelength penetrates to the deeper junction so that the incident light with a longer wavelength can excite electron-hole pairs at the deep region. However, to become photocurrents, the electron-hole pairs should reach to the boundary edges of the space-charge region successfully such that they would be absorbed and transformed to the photocurrent. In other words, the photodiode has a greater response toward the incident light with a longer wavelength at a deeper region whereas for a shallower region it has a better response toward the incident light with a shorter wavelength. Additionally, to prevent CMOS circuits from latch-up, p-substrate is generally connected to the lowest potential in the system. To keep the potential of p-substrate in the lowest level and the photodiode under reverse biased voltages, a connection manner depicted in Fig. 14 is employed to solve the problem of the voltage drop between p and n nodes in the photodiode. Figure 15 shows the simulated results utilizing the recipes in Fig. 14. It clearly reveals that the peak wavelength increases with the depth of the p + layer. Fig. 14. Connection manner, recipes and structures obtaining three color spectral responses. Advances in Photodiodes 172 (a) (b) Fig. 15. Structures in Fig. 14 being simulated to yield (a) spectral responses of three recipes for red, green and blue photodiodes and (b) spectral responses of p + depth varying from 0.1μm to 2.1μm. b. The spectral response in the long wavelength region can be also lowered by reducing the thickness of the substrate layer to decrease the region for collecting excess minority carriers. Figure 16 depicts the n-/p-sub photodiode with thin p- substrate of which the thickness is only several micrometers. Figure 17 displays the simulated results by utilizing the corresponding recipes in Fig. 16. It is apparent that the spectral response in the long wavelength region is decayed. Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 173 p-substrate : 1X10 15 cm -3 n- : 1X10 15 cm -3 Red Photodiode 6.5um p + n + 5.8um n- : 1X10 15 cm -3 Red Photodiode n + 3.5um n- : 1X10 15 cm -3 Blue Photodiode n + 0.7um Fig. 16. Structures in Fig. 16 being simulated to yield (a) spectral responses of three recipes for red, green and blue photodiodes and (b) spectral responses of n﹣depth varying from 0.7μm to 5.8μm. (a) (b) Fig. 17. Simulated results employing the structures in Fig. 16 under different recipes. Advances in Photodiodes 174 (a) (b) Fig. 18. Simulated spectral responses of the n-/p-epi/p+sub photodiode in (a) p-epitaxial doping concentration of 1×10 15 cm -3 and p-epitaxial thickness ranging from 5 to 15 um um and (b) p-epitaxial doping concentration ranging from 1×10 15 cm -3 to 1×10 19 cm -3 and p- epitaxial thickness of 10 um . Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 175 2. The spectral response in the long wavelength region can be decreased by heavy doping substrate associated with the p-epitaxial layer. By adjusting the depth of the epitaxial layer, the desired spectral response can be obtained. Figure 18 depicts the simulated spectral responses of the n-/p-epi/p+sub photodiode under different thicknesses and doping concentrations of the epitaxial layer. According to this figure, the thickness and doping concentration of the epitaxial layer apparently affect spectral responses. In practice, some researchers proposed the approach of selective epitaxial growth to obtain various color spectral responses by changing the recipe of the epitaxial layer [20], [21]. 5. Conclusion Adaptive photodiode structures, of which design approach aiming at making the photo- response having a peak value at a specific wavelength, that are realized by the photodiodes with color-selective mechanisms under the condition of without extra color filters is proposed. Moreover, the influences of color filters, photodiode structures, recipes and reverse biased voltages on spectral responses are investigated. Measurement results illustrate that the color filters affect the spectral responses more significantly than the others. The spectral response varies with the reverse biased voltages slightly. The approach of implementing color pixels using the standard CMOS process without color filters is also proposed. This work clearly paves the way for designers to realize color-selective pixels in CMOS image sensors. Appendix: Derivation for peak wavelength of the spectral response The n-/p-sub photodiode as shown in Fig. A.1 is employed to illustrate how the proposed model is used to derive the peak wavelength of the spectral response. Fig. A.1 n-/p-sub photodiode. The total current density generated by the n-/p-sub photodiode is Advances in Photodiodes 176 ()() () 2 12 1 1 11 0 22 11 () () 1 total n photo p photo drift x p n photo n sub p sub photo x xx xx x x pp p p pp p p p pp p p p pp JJ J J qD dp x dx qD dn x dx q G dx DS D exx sh S ch LL L L L qD G L D xx Ssh ch LL α α τ α −− −−− == − =++ =+ + ⎛⎞ ⎛⎞ + ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ = − ⎛⎞ ⎜⎟ + ⎜⎟ ⎝⎠ ∫ ()() () () 1 23 23 2 21 2 2 23 23 0 22 0 1 x p xx xx x nsub nsub nsub nsub nsub xx e L xx xx qL G e e L Coth e Csch LL L qe e α αα α αα α α α φ − −+ + −− −− − −− ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ + ⎜⎟ ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎛⎞ ⎛⎞ ⎛⎞ ⎛⎞ −− ⎜⎟ +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ + − +− .(A1) The absorption coefficient α can be simplifily represented as a function of the incident light wavelength, i.e. ( ) f α λ = , and then Eq. (A1) can be modified to () () () ( ) () () () () 1 1 2 11 0 2 2 11 23 0 1 fx pp p p ppp p p pp fx total p p p pp p xf nsub nsub nsu fDS D exx sh S ch LLL L L qD G J fe Lf D xx Ssh ch LL L xx qL G e f L Coth L λ λ λ λ τ λ λ λ − − − −− − ⎛⎞ ⎛⎞ ⎛⎞ + ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎜⎟ ⎝⎠ =+ ⎜⎟ ⎛⎞ ⎛⎞ ⎛⎞ − ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ + ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ − + + () () ( ) () () ( ) 3 21 23 2 2 0 1 xf bnsub nsub fx fx xx eCsch L Lf qe e λ λλ λ φ − − − −− ⎛⎞ ⎛⎞ ⎛⎞ ⎛⎞ − ⎜⎟ − ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ − +− . (A2) In Eq. (A2), the surface generation rate 0 G is ( ) 0 in in fP P G Ahc Ahc λ λ αλ == . (A3) Additionally, A and P in in Eq. (A3) represent the unit area and unit incident light power, respectively. Hence, Eq. (A2) can be represented as follows. Color-Selective CMOS Photodiodes Based on Junction Structures and Process Recipes 177 () () ( ) () () ( ) () () () () ( ) () () 1 1 2 11 2 2 11 2 2 1 1 fx pp p p ppp p p pp fx total p p p pp p xf nsub n nsub fDS D exx sh S ch LLL L L qD f J fe D hc L f xx Ssh ch LL L qL f efL hc L f λ λ λ λ τλλ λ λ λλ λ λ − − − − − − ⎛⎞ ⎛⎞ ⎛⎞ + ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎜⎟ ⎝⎠ =+ ⎜⎟ ⎛⎞ ⎛⎞ ⎛⎞ − ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ + ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ + − () () () ( ) 3 21 23 23 0 xf sub nsub nsub fx fx xx xx Coth e Csch LL qe e λ λλ φ − −− −− ⎛⎞ ⎛⎞ ⎛⎞ ⎛⎞ −− ⎜⎟ +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎝⎠ +− (A4) The peak wavelength of the spectral response can be obtained by taking partial differential of Eq. (A4) by the variable of λ . () () () ( ) () () () () () ( ) () () ( ) () () ( ) () () () () () () () () ( ) () () () () ( ) ( ) 1 22 22 2 2 2 11 2 22 2 2 1 111 1'2 1 1'2 111 total fx pppp pppp pppp pp ppppppp p p J e Lf f L f DS f f DS f LS xx Lf DCosh LSSinh LL x LCosh f f L fDS f fD fxf L S fxf fx L qL hc λ λ λλ λ λλ λ λ λ λλ λ λλ λ λ λ λ λ λ − ∂ = ∂ −+− ++ ⎛⎞ ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ −+ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎛⎞ ⎛ ⎜⎟ −−++ + −+−++ ⎜⎟ ⎝ ⎝⎠ =+ () ( ) () () ( ) () () () () () () () ( ) () () () ( ) () () ( ) 2 2 2 11 2 22 22 2 2 2 1 11 1 2 2 2 11 1 1'11 21 1 pppp pp pp pp p p pp p p pppp pp xx L f DCosh LSSinh LL x Sinh f f L D f L S f D f x f L f x f L S f x f L L xx Lf DCosh LSSinh LL λ λλ λ λλ λ λ λ λ λ λ λ ⎞ ⎜ ⎟ ⎠ ⎛⎞ ⎛⎞ ⎛⎞ ⎜⎟ ⎜⎟ ⎜⎟ −+ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝⎠ ⎛⎞ ⎛ ⎞ ⎛⎞ ⎜⎟ −−++−++++ − ⎜⎟ ⎜ ⎟ ⎜⎟ ⎝⎠ ⎝ ⎠ ⎝⎠ + ⎛⎞ ⎛ ⎜⎟ −+ ⎜⎟ ⎝⎠ ⎝ () () () () () () () ( ) () () () () () 22 3 22 2 23 23 2 2 23 1 '' fx fx fx nsub nsub nsub nsub fx fx nsub n nsub xx xx Lf e fe Coth fe Csch LL Lf xx L ffe fe Coth L qL hc λλ λ λλ λλ λ λ λλ λ λ λ −− − − −− − −− − − ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎛⎞ ⎞ ⎜ ⎟ ⎜⎟ ⎜⎟ ⎜ ⎟ ⎜⎟ ⎜⎟ ⎜ ⎟ ⎠ ⎝⎠ ⎝ ⎠ ⎛⎞ ⎛⎞ −− +− ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ − − + + + () () () ( ) () () () () () () () () () () () () ( ) () () () () 3 22 3 2 23 2 2 2 23 23 22 3 2 2 3 3 ' 1 '''' 1 2'2 fx sub n sub nsub fx fx fx nsub nsub nsub nsub nsub fx nsub xx fe Csch L Lf xx xx L ffe Lxf fe xffCoth xffe Csch LL Lf Lffe f λ λλ λ λ λλ λ λλ λ λλ λ λλ λ λλ λ λ λλ λ λλ − −− − −− − −− −− − − − ⎛⎞ ⎛⎞ − − ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ − ⎛⎞ ⎛⎞ −− −−+ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ + − + − () () () () () () ( ) ()( ) () () () () () () ( ) () 2 3 12 2 1 22 22 23 23 2 2 2 12 '2' 1 '1 '1 fx fx nsub nsub nsub nsub nsub fxx fx fx xx xx fLe Coth f fLe Csch LL Lf q eefxefx hc λλ λλ λ λλλλ λ λλ λλ −− −− −− − −+ ⎛ ⎞ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎛⎞ ⎛⎞ −− ⎜ ⎟ − ⎜⎟ ⎜⎟ ⎜ ⎟ ⎝⎠ ⎝⎠ ⎜ ⎟ ⎜ ⎟ − ⎜ ⎟ ⎝ ⎠ +−+− (A5) The calculation result represents the slope of Eq. (A4). When Eq. (A5) equals to 0, the corresponding λ is the peak wavelength of the spectral response. Equation (A5) is a complex non-exact differential equation. Accordingly, some assumptions are employed to simplify the solution for Eq. (A5). The spectral response induced in the Advances in Photodiodes 178 space-charge region is generally too small to be neglected. Additionally, diffusion lengths of the minority carriers in n- and p-substrate are as long as several hundred micrometers owing to low-doped concentrations, and thus wavelengths in the visible region are much smaller than the diffusion lengths. Moreover, there exist the following assumptions () () 22 22 1 pp Lf Lf λ λ −≅ , (A6) () () 22 22 1 nsub nsub Lf Lf λ λ −− −≅ , (A7) and 1p Lx>> . (A8) Eq. (A5) can be simplified as follows. () () () () () () () () () () () () () () () () () () () () () () () () () () () () () () 11 1 1 1 2 2 44 3 22 2334 1 2 23 4 11 22 2 4 4 1 2' ' '2''' ''' 1 1 pp fx fx fx fx p ppp pp p fx pp pp pp p fx pp pp pp fx nsub SS ffe fe ffe fe D LLD SS S fe f f f f ffxf DD LD SS S ffx f f f fx f e LD LD LD fe Lf f λλ λ λ λ λ λ λλ λ λλλ λλ λ λλ λ λλ λλλ λ λλ λλ λ λλ λ λλ λ λ λ λ −− − − − − − − ++ − − −++++− ++ + + () () () () () () () () () () () () () () () () () () () 3 2 2 3 22 33 23 23 22 23 23 34 2 1 ' 11 '' '' fx fx nsub nsub nsub fx fx n sub n sub n sub n sub fx fx xx xx Coth f e Csch f f e LLL xx xx ffe Coth ffe Csch LL LL ffe ffe xf λλ λλ λλ λλλλ λ λλ λ λλ λ λλ λ λλ λ λλ −− −−− −− −− −− −− ⎛⎞ ⎛⎞ −− −+ ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎛⎞ ⎛⎞ −− +− ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ +− − () () () () () () () () () () () () () () () 3 2 2 3 4 23 2 43 23 3 22 23 23 1 ' 1 '2' 11 2' 2' 0 nsub nsub fx fx n sub n sub fx fx nsub nsub nsub nsub xx fx Coth Lf L xx ffe x Csch ffe Lf L xx xx ffe Coth ffe Csch LL LL λλ λλ λ λ λλ λ λλ λ λ λλ λ λλ λ −− −− −− −− −− −− ⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎛⎞ − ⎜⎟ ⎜⎟ ⎝⎠ ⎛⎞ − +− ⎜⎟ ⎜⎟ ⎝⎠ ⎛⎞ ⎛⎞ −− −+ = ⎜⎟ ⎜⎟ ⎜⎟ ⎜⎟ ⎝⎠ ⎝⎠ ⎝ ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ ⎠ (A9) 6. Acknowledgement This work was partially supported by National Science Council, Taiwan, under the contract number of NSC 97-2221-E-194-060-MY3. 7. References [1] B. E. Bayer, “Color Imaging Array,” U. S. Patent 3971065, July 1976. [2] W. J. Liu, Oscal T C. Chen, L. K. Dai, P. K. Weng, K. H. Huang and Far-Wen Jih, “A CMOS photodiode model,” Proc. of IEEE International Workshop on Behavioral Modeling and Simulation, pp. 102-105, 2001. [3] K. C. Chang, C. Y. Chang, Y. K. Fang and S. C. Jwo, “The amorphous Si/SiC heterojunction color-sensitive phototransistor,” IEEE Electron Device Letters, vol. 8, no. 2, pp. 64-65, 1987. [...]... electron-hole pair photogeneration profile (Hinckley et al., 2000) Back illuminated 182 Advances in Photodiodes CMOS pin ultra-thin (75 μm) photodiodes have found application in medical imaging, particularly making x-ray, high quality, real time imaging possible (Goushcha et al., 20 07) However, compared to front illumination, the backwall orientation is disadvantaged in crosstalk, speed and quantum efficiency... 2 Bring the SCR closer to the photo-carrier envelope near the pixel backwall by, a Thinning the pixel (Goushcha et al., 20 07) b Widening the SCR by, i Increasing the reverse bias to the PN junction, and ii Decreasing the doping on the substrate side of the PN junction, or iii Having no doping (intrinsic Silicon) between the P and N regions, making a pin “junction” (Goushcha et al., 20 07) c Extending... generated, having a width W, and an internal equilibrium potential, V0, across the junction The SCR width is more affected by lowering the substrate doping concentration than by increasing the reverse voltage bias Typical SCR width for 2 volt reverse bias is 6 μm, constrained by a 1014 cm-3 doping minimum Lowering the substrate doping to the intrinsic level, 1.5 x 1010 cm-3, (using an intrinsic substrate)... doping of 1014 cm-3 ( 17/ 14) or 1015 cm-3 ( 17/ 15), and an n-well doping of 10 17 cm-3 Back illuminated relative crosstalk generally decreases with increase in wavelength, because the absorption length increases This generates more carriers closer to the SCR, resulting in better pixel carrier capture efficiency The reverse is true for the front illuminated pixels (Hinckley & Jansz, 2005) 1 87 Extrinsic... would have passed through a thinner pixel, now generates carriers in a larger pixel volume, increasing its carrier capture and so benefiting sensitivity Below 650 nm, the light absorption length in silicon is less than the depth of the thinnest pixel (1.5 μm epilayers = 9 μm total pixel depth), resulting in all of the illumination being absorbed and generating carriers in close proximity to the SCR... Frontwall (FW) and Backwall (BW) illuminated StaG-R dependence on lateral inter-ridge gap thickness for 633 nm illuminations inside (60μm, 70 μm and maximum QE positions) the central pixel (Hinckley & Jansz 20 07) Illuminations falling outside the nested ridges (70 μm & Max QE) produced absolute QE responses that were affected minimally by a variation in lateral inter-ridge gap thickness Here the carrier... capture efficency: crosstalk increasing across the given wavelength band For any given epilayer thickness, front illumination crosstalk increasing while back illumination slightly decreases, and both responses level off at the same wavelengths The increase or decrease is proportional to the increase in absorption length with wavelength increase This is due to Silicon being an indirect band gap semiconductor:... illuminated, was compared to the QE response of two doping versions of the conventional photodiode (Fig 3) with the following doping (well/substrate) regimes Both versions had the same well doping as the flat-StaG, 10 17 cm-3 One version ( 17/ 15) had a substrate doping of 1015 cm-3 while the other ( 17/ 14) had an order of magnitude lower substrate doping of 1014 cm-3 (Hinckley & Jansz, 2005) 186 Advances. .. higher maximum QE in both modes compared to both conventional photodiodes (PD) The back illuminated StaG maximum QE is superior to the other geometries, for the depth of well (1 μm) For the shorter absorption length illuminations (λ < 70 0nm), minority hole generation in the well is significant in front illumination causing significant hole diffusion, suppressing sensitivity Back illumination is absorbed... changes within the nested ridges does not connect with the associated carrier envelope Noted is the decreasing trend for the closest illumination position (60μm) which intersects the nested ridge (Fig 7) The thinner the gap between nested ridges the larger the potential gradient (Fig 3) and drift coefficient resulting in more carriers being reflected into the pixel’s capture volume, resulting in greater . profile (Hinckley et al., 2000). Back illuminated Advances in Photodiodes 182 CMOS pin ultra-thin (75 μm) photodiodes have found application in medical imaging, particularly making x-ray,. reverse bias is 6 μm, constrained by a 10 14 cm -3 doping minimum. Lowering the substrate doping to the intrinsic level, 1.5 x 10 10 cm -3 , (using an intrinsic substrate) can expand the. Decreasing the doping on the substrate side of the PN junction, or iii. Having no doping (intrinsic Silicon) between the P and N regions, making a pin “junction” (Goushcha et al., 20 07) . c.