Geiger Mode Avalanche Photodiode with CMOS Transimpedance Amplifier for Optical Data Link Applications A.M. Moloneya, A.P. Morrisona, J.C. Jacksonb, A. Mathewsonb and P.J. Murphyb Department of Electrical and Electronic Engineering, University College Cork, Ireland b National Microelectronics Research Centre, ‘Lee Maltings’, Prospect Row, Cork. Ireland a Abstract – Simulations of the first realisation of a detector receiver combination based on the Geiger mode avalanche photodiode (GMAP), which is an avalanche photodiode compatible with a 1.5 m CMOS and SOI CMOS process, are presented. The receiver design and simulations are achieved using a 1.5 m CMOS process. The simulations incorporate ADS Level 3 model sets. Two receivers are simulated a dynamic and a nonadjustable both receivers are shown to work well to 1 Gb/s. 1 INTRODUCTION High volumes of optoelectronic integrated circuits (OEICs) will be needed in the coming years for optical interconnect, optical storage systems (CD-ROM, DVD), optical data links for use in Local Area Networks (LANs), home networking, Fibre-to-the-Home (FTTH) and backbone interconnect. To be competitive with copper technology for links and bus applications these OEICs must be affordable therefore the high-speed, high-sensitivity III-V material systems which have long been used for high-performance long-haul communication systems are not an option. The growing need for such OEICs along with the low-cost of silicon and its potential for very large scale integration has prompted a search for a monolithically integrated silicon photoreceiver suitable for gigabit-rate data transmission. The most challenging aspect of fabricating a high-performance monolithic silicon photoreceiver is the integration of a highspeed, high-sensitivity photodetector and an amplifier without adding complexity to the existing transistor fabrication process [1]. Many different photodiodes have been successfully integrated with silicon electronic circuitry to date. Schow et al have reported a lateral interdigitated p-i-n photodiode integrated with an NMOS transimpedance preamplifier on bulk silicon this reached a speed of 1 Gb/s [2] while on SOI substrate the receiver operated at 2 Gb/s [1]. Heide integrated vertical p-i-n photodiodes in a twin-well 1.0 m CMOS process the OEICs fabricated handled a data rate of 622 Mb/s with a single supply voltage of 3.3 V [3]. A CMOS photodiode consisting of a standard CMOS N-well with an interdigitated network of Pdiffusion fingers was used by Woodward to form a monolithically integrated photoreceiver with a sensitivity of -6.3 dBm at 1 Gbit/s [4]. A spatially modulated light detector (SMLdetector) with sense-amplifier receiver in standard 0.8 m CMOS operating at 180 Mb/s was presented by Kuijk et al [5], while Fang et al integrated an amorphous silicon p-i-n photodiode with a crystalline silicon n-p-n transistor and formed a photoreceiver [6]. This paper proposes that Geiger mode avalanche photodiodes that are CMOS and siliconon-insulator (SOI) CMOS compatible, have high gain and high responsivity are viable candidates for use in gigabit, monolithically integrated, low-cost, high-reliability, silicon photoreceivers. Simulations of transimpedance amplifiers incorporating the GMAPs are presented here. 2 PHOTODETECTOR The photodetector used in the simulations is the Geiger-mode avalanche photodiode (GMAP). The GMAPs shown in Figure 1 with 20 m junction active areas and different overlap dimensions were fabricated. Details of the fabrication of the GMAPs have been reported previously [7]. The anode doping was tailored to provide the low (< 30 V) breakdown voltage. The fabricated GMAPs have shown high responsivity (0.04 A/W at 5 V reverse bias, 0.4 A/W at 27 V reverse bias), high gain (10 at 27 V, 100 at 27.25 V and in excess of 1000 at 27.5 V reverse bias) and low dark current (5 pA at 5 V reverse bias 33 pA at 27.5 V reverse bias corresponding to 7 fA/m2 and 47 fA/m2 respectively). The s-parameters for a 20 m diameter GMAP with a 3 m junction overlap were used to develop a small signal equivalent circuit - from which a bandwidth of 5.95 GHz for the photodetector was calculated [8]. A n ode C ath ode C ath ode A n ode O xide P+ N + P P+ O verlap A ctiveA rea P-Ep i P+S u bstrate Figure 1Virtual guard ring GMAP structure showing active area and overlap region The GMAP was implemented in the simulations using a capacitor. A GMAP has three associated capacitances, bondpad capacitance, metal contact capacitance and depletion capacitance. Since the simulations are of a monolithically integrated photoreceiver that is the photodiode and receiver are on the same silicon substrate, there will be no metal contact and bondpad capacitance and the capacitance of the GMAP is simply the depletion capacitance. The capacitance was calculated using the SPICE level 3 (geometric junction model) equation for depletion capacitance [9]. The depletion capacitance is modelled by junction bottom and junction periphery capacitances. The formula for bottom area (Equation 1) and periphery (Equation 2) capacitances is similar except each has its own model parameters. The model parameters (Cj, Cjsw, mj, mjsw, j) have been previously extracted [7]. Each GMAP consists of two bottom area capacitances (active area and overlap regions) and one periphery capacitance (where N+ and P-epi meet) which are all in parallel and may be summed. Calculated capacitances for a reverse bias voltage of –27 V are shown in Table 1 and Table 2. All the capacitances are low (< 200 fF) and will not limit the speed of the amplifier excessively. Carea (V ) C per (V ) Cj V 1 j mj C jsw P V 1 j m jsw (1) (2) where: Cj = zero-bias junction capacitance per unit junction bottomwall area A = junction area Cjsw = zero-bias junction capacitance per unit junction periphery P = junction perimeter j = area junction contact potential mj = area junction grading coefficient mjsw = periphery junction grading coefficient V = magnitude of the bias voltage Table 1 Calculated GMAP capacitances with active area diameter = 20 m Table 2 Calculated GMAP capacitances with active area diameter = 10 m 3 RECEIVER The three basic preamplifier topologies commonly used for integrated photoreceivers are: 1) the simple resistor terminated design, 2) the high-impedance (HZ) design and 3) the transimpedance design. The transimpedance architecture has been chosen for this work since it has been demonstrated as being the most suitable solution for a preamplifier for front-end optical receiver circuits. TZ amplifiers present advantages of simplicity, high-bandwidth and large dynamic range [10]. The f3dB of a TZ amplifier with a single dominant input pole is given by Equation 2. f3dB Av 1 2 R fb (Cin C fb (1 Av )) (3) where: Av = open-loop voltage gain of the voltage amplifier Rfb = feedback resistance Cfb = parasitic shunt capacitance across feedback resistor Cin = input capacitance to the voltage amplifier (FET gate capacitance + photodiode capacitance) The feedback resistance Rfb must be kept large for low Johnson noise and hence good receiver sensitivity therefore all capacitance must be as low as possible particularly that of the photodetector (the dominant capacitance) to ensure high bandwidth. Vdd Cpd M2 V1 M1 6.0/1.5 V2 2.0/1.5 2.0/1.5 2.0/1.5 M1 Mfn 2.0/1.5 Vdd Cpd M4 Vout V1 6.0/1.5 V2 2.0/1.5 2.0/1.5 2.0/1.5 M2 M3 M4 Vout M3 Mfn 2.0/1.5 Vt Mfp Mfp 2.0/1.5 2.0/1.5 Figure 2 (a) Circuit schematic of dynamic photoreceiver. (b) Circuit schematic of non-adjustable photoreceiver The HP ADS (Advanced Design System) simulator was used to establish the CMOS large signal model and to evaluate the performance of the photoreceiver. ADS level 3 MOSFETs were built using extracted SPICE level 3 model sets for the 1.5 m CMOS process. Figure 2 shows the circuit schematics for the dynamic and non-adjustable receivers respectively. The photoreceiver preamplifiers are similar in design to those reported by A.V. Krishnamoorthy et al [11] for optoelectronic-very-large-scale integration (OE-VLSI). The GMAP is modelled by an AC-current source and capacitor. The receivers consist of transimpedance front-ends followed by a decision stage. The transimpedance stage (M1, M2, Mfn and Mfp) incorporates a CMOS inverter (M1 and M2), the large signal transfer characteristic of which shows a gain of 10.2 approximately (see Figure 3). For operation as an inverting small-signal voltage amplifier, the inverter is biased in the high-slope region of the transfer characteristic using the transimpedance feedback resistance. Feedback in the dynamic circuit is achieved using a parallel combination of a common-gate NMOSFET and a saturated PMOSFET. The gate voltage of the NMOSFET can be adjusted to give a feedback resistance which allows optimum performance at a given bit-rate and optical power. Feedback is accomplished in the nonadjustable receiver using a diode connected NMOSFET and a parallel saturated PMOSFET. The second stage of the photoreceivers (M3 and M4) is an inverter which acts as a decision stage with a switching voltage of 2.1 V, this stage generates a logic level output. 5 Output (V) 4 3 2 1 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Input (V) Figure 3 Large-signal transfer characteristic of CMOS inverter (M 1 and M2) 4 Output Input Voltage (V) 3 V1 2 Vout 1 0 0 2 4 6 8 10 Time (ns) Figure 4 Simulation of the dynamic transimpedance photoreceiver at 1Gb/s with peak input current of 40 A. Photodiode capacitance is 75 fF, Vt=3.0 V and Vdd=5.0 V. 4 Output Input Voltage (V) 3 V1 2 Vout 1 0 0 2 4 6 Time (ns) 8 10 Figure 5 Simulation of the non-adjustable transimpedance photoreceiver at 1Gb/s with peak input current of 40 A. Photodiode capacitance is 75 fF and Vdd=5.0 V. Figures 4 and 5 show the simulated transient characteristics of the transimpedance receivers at 1 Gb/s (500 MHz). The photodiode capacitance for both receivers was taken to be 75 fF since simulations showed that to reach gigabit speed the capacitance had to be 75 fF (it can be seen from Table 2 that this capacitance can be obtained for an active area diameter of 10m). At this bit-rate the feedback resistance of the dynamic receiver was 20.1 k and of the non-adjustable was 20.6 k these resistances were calculated using Equation 4 where Id is the photodetector current. V2 V1 Id (4) 4 CONCLUSION Simulations of the first monolithic detector/receiver in a CMOS process based on the GMAP detector are presented showing stable operation to 1 Gb/s. Much higher bit rate operation of receivers based on the GMAP can be expected with the use of a submicron CMOS process. Many optoelectronic applications can profit from this novel photoreceiver concept. 5 ACKNOWLEDGEMENTS The authors wish to acknowledge the National Microelectronics Research Centre (NMRC) Ireland for the use of the Spice level 3 model sets and Dr. Kevin McCarthy, UCC for advice on the 1.5 m CMOS process. REFERENCES [1] R. Li, J.D. Schaub, S.M. Csutak and J.C. Campbell “A High-Speed Monolithic Silicon Photoreceiver Fabricated on SOI,” IEEE Photon. Technol. Lett., 12(8):1046-1048, August 2000. [2] C.L. Schow, J.D. Schaub, R. Li, J. Qi and J.C. Campbell, “A 1 Gb/s Monolithically Integrated Silicon NMOS Optical Receiver,” IEEE J. Sel. Top. Quantum Electron., 4(6):1035-1039, November/December 1998. [3] T. Heide, A. Ghazi, H. Zimmermann and P. Seegebrecht, “Monolithic CMOS Photoreceivers for Short-Range Optical Data Communications,” Electron. Lett., 35(19):1655-1656, September 1999. [4] T.K. Woodward and A.V. Krishnamoorthy, “1 Gbit/s CMOS Photoreceiver with Integrated Detector Operating at 850 nm,” Electron. Lett., 34(12):1252-1253, June 1998. [5] M. Kuijk, D. Coupee and Rr. Vounckx, “Spatially Modulated Light Detector in CMOS with Sense-Amplifier Operating at 180 Mb/s for Optical Data Link Applications and Parallel Optical Interconnects Between Chips,” IEEE J. Select Top. Quantum Electron., 4(6):1040-1045, November/December 1998. [6] Y.K. Fang, C.R. Liu, K.H. Chen and C.H. Lin, “A Low Cost and High Current Gain a-Si/c-Si Heterojunction Photoreceiver for Large Area Optoelectronics Integrated Circuit Applications,” IEEE Electron. Dev. Lett., 16(5):190-192, May 1995. [7] J.C. Jackson, A.P. Morrison, P. Hurley, W.R. Harrell, D. Danjanovic, B. Lane and A. Mathewson, “Process Monitoring and Defect Characterisation of Single Photon Avalanche Photodiodes,” Proc. IEEE 2001 Int. Conference on Microelectronic Test Structures, March 2001. [8] A.M. Moloney, A.P. Morrison, J.C. Jackson, A. Mathewson and P.J. Murphy, “Small Signal Equivalent Circuit for Geiger-Mode Avalanche Photodiodes,” Electron. Lett., 38(6):285-286, March 2002. [9] Avant Star-HSPICE Manual. www.ece.orst.eedu/moon/hspice98/, 1998. [10] C-W Kuo, C-C Hsiao, S-C Yang and Y-J Chan, “2 Gbit/s Transimpedance Amplifier Fabricated by 0.35 m CMOS Technologies,” Electron. Lett., 37(19):1158-1160, September 2001. [11] A.V. Krishnamoorthy, T.K. Woodward, K.W. Goosen, J.A. Walker, A.L. Lentine, L.M.F. Chirovsky, S.P. Hui, B. Tseg, R. Leibenguth, J.E. Cunningham and W.Y. Jan, “Operation of a Single-Ended 550 Mbit/s, 41 fJ, Hybrid CMOS/MQW Receiver-Transmitter,” Electron. Lett. 32(8):764-765, April 1996.