Fig. 1.
Schematic drawing of the three photodiode structures. (a) P+/Nwell. (b) Nwell/Psub. (c) P+/Nwell/Psub.
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Chaoqun Xu, Ying Sun, Yan Han, Dazhong Zhu. Double junction photodiode for X-ray CMOS sensor IC[J]. Journal of Semiconductors, 2014, 35(7): 074011. doi: 10.1088/1674-4926/35/7/074011
C Q Xu, Y Sun, Y Han, D Z Zhu. Double junction photodiode for X-ray CMOS sensor IC[J]. J. Semicond., 2014, 35(7): 074011. doi: 10.1088/1674-4926/35/7/074011.
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Double junction photodiode for X-ray CMOS sensor IC
doi: 10.1088/1674-4926/35/7/074011
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Abstract
A CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of P+/Nwell, Nwell/Psub and P+/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show P+/Nwell/Psub photodiode has larger photocurrent than P+/Nwell photodiode and Nwell/Psub photodiode, and its spectral response is more in accordance with CsI(Tl) fluorescence spectrum. Improved P+/Nwell/Psub photodiode detecting CsI(Tl) fluorescence was designed in CSMC 0.5 μm CMOS process, CTIA (capacitive transimpedance amplifier) architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8×8 pixel array and pixel pitch is 100×100 μm2. Testing results show the dark current of the improved P+/Nwell/Psub photodiode (6.5 pA) is less than that of P+/Nwell and P+/Nwell/Psub photodiodes (13 pA and 11 pA respectively). The sensitivity of P+/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of P+/Nwell/Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI(Tl) in an indirect X-ray sensor. Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 V·m2/W or 5097e-/pixel@8.05 keV considering the pixel size, integration time and average energy of X-ray photons. -
References
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