IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 12, DECEMBER 2003 2573 (0.4% of saturation) for the PD and LPT arrays, respectively. While it is possible that the value for the PD sensor is anomalously high owing to a few defective columns, the FPN for the LPT sensor remains sig- nificantly less than that for the PD. It appears therefore that the reduction of FPN gained by operating the current mirrors at higher currents due to the gain of the LPT outweighs the effects of LPT base-width mismatch in the present design. As seen in Fig. 4(a) and (b), the FPN includes significant column-to-column variations. Since the column-based current mirrors are designed to be large enough that mismatch is negligible, this FPN is attributed to nonuniformity of the column-based sample-and-hold circuits used to perform current-to-voltage conversion, and is thus not inherent to the current mode design of the sensor. The sample images captured by the image arrays are presented in Fig. 4(c) and (d). Again, the column-based noise is clearly apparent in both images. V. CONCLUSION This brief has reported current amplification schemes at both the photodetector level and at the pixel circuit level, for applications in current-mode image sensors. First, a CMOS compatible lateral bipolar phototransistor is used as the sensor device, where the current gain is achieved through its transistor action. In addition, current amplifica- tion is achieved through the use of current mirrors in the pixel readout circuit. A prototype image sensor chip is designed and fabricated in a stan- dard 0.18- m CMOS process, it contains a photodiode sensor array and a LPT sensor array, both employ the amplifying current mirrors in the readout circuit. From experimental results, the use of lateral bipolar phototransistors and current mirrors improves the overall photosensi- tivity and conversion efficiency. Due to the n-well/p-substrate deple- tion layer, the spectral response of the LPT is higher in the red re- gions compared to the photodiode. Sample images have been success- fully captured by both image arrays. On the other hand, fixed pattern noise and dark signal are amplified along with the signal itself. Power consumption can potentially be reduced in future implementations by adding switches that disable current sources when not in use. The cur- rent-mode readout scheme with amplifying current mirrors proposed in this brief can be included as the front end to additional image pro- cessing circuits in either integration-mode or continuous-time image processing schemes. ACKNOWLEDGMENT The authors gratefully acknowledge funding from Betacom Inc. and NSERC Canada, and fabrication and support services from the Cana- dian Micro electronics Corporation. REFERENCES [1] E. R. Fossum, “CMOS image sensors: Electronic camera-on-a-chip,” IEEE Trans. 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Teuner, “CMOS photosensor arrays with on-chip signal processing,” in Proc. ESSCIRC, Southampton, UK, Sept. 16–18, 1997, pp. 236–239. Current Transport Modeling in Quantum-Barrier-Enhanced Heterodimensional Contacts Gregory B. Tait and Bahram Nabet Abstract—A physical model for electron and hole current transport is formulated in a novel heterodimensional contact that incorporates a barrier-enhancement region between a two-dimensional optically active InGaAs/ InAlAs quantum well and a three-dimensional metal contact. Developed for easy inclusion in fully self-consistent numerical device simulators, these quantum-mechanical-transmission boundary conditions are useful to investigate important carrier transport effects such as carrier accumulation and thermionic and tunneling emission in heterodimen- sional contacts. dc current simulations of the quantum-barrier-enhanced structure are compared with simulated currents in a structure with direct metal contact to the InGaAs quantum well. Results indicate a reduction in dark current of nearly three orders of magnitude, making these contact structures attractive for low-noise photodetector applications. Addition- ally, simulation of the transient current response of a photodetector with 1- m interdigitated contact spacing indicates an electrical bandwidth of 50 GHz. Index Terms—Heterodimensional contact, InGaAs photodetector, quantum barrier, transport simulation. I. INTRODUCTION Efforts to develop a comprehensive study of the theory and mod- eling of reduced dimensionality systems in semiconductor devices are presently driven by the continuing need to improve speed performance, transport efficiency, device density, and power management. These quantum-confined structures, however, are contacted from three-di- mensional systems in order to allow information to be transferred to and from the macroscopic world. The implementation of device designs that rely on structures of different physical dimensions, a het- erodimensional system, has as its basic goal the desire to maximize the Manuscript received June 2, 2003; revised August 6, 2003. This work was supported in part by the National Science Foundation under Grant ECS-0 117 073. The review of this brief was arranged by Editor S. Datta. G. B. Tait is with the Electrical Engineering Department, Virginia Common- wealth University, Richmond,VA 23284 USA (e-mail: gbtait@vcu.edu). B. Nabet is with the Department of Electrical and Computer Engineering, Drexel University, Philadelphia, PA 19104 USA (e-mail nabet@ece.drexel.edu). Digital Object Identifier 10.1109/TED.2003.820128 0018-9383/03$17.00 © 2003 IEEE