CMOS Phototransistors for Deep Penetrating Light P. Kostov, W. Gaberl, H. Zimmermann Institute of Electrodynamics, Microwave and Circuit Engineering, Vienna University of Technology Gusshausstr. 25/354, 1040 Vienna, Austria Tel.: 0043-1-58801-35401; e-mail: plamen.kostov@tuwien.ac.at, wolfgang.gaberl@tuwien.ac.at, horst.zimmermann@tuwien.ac.at Abstract In this work, we report on the design as well as the electrical and optical characteristics of several 100 100 μm 2 silicon pnp phototransistors built in a 0.6 μm OPTO ASIC CMOS process using a special starting material. This special starting material consists of a low doped epi-layer on top of the p-substrate. Responsivities up to 98 A/W for modulated light were achieved. Furthermore some phototransistors reach bandwidths up to 13.4 MHz. Electrical forward current gains up to 187 were achieved. The CMOS integration of these phototransistors paves the way for cheap optoelectronic integrated circuits (OEIC), where analog and digital circuitry can be implemented together with active optical detectors. Application examples are highly sensitive optical sensors, active pixel image sensors, light barriers, opto-coupler, etc. Keywords CMOS, Silicon, Phototransistors, Light detector, OEIC, NIR. I. INTRODUCTION Photodetectors are essential for the conversion of optical signals into electrical ones. In a standard CMOS process different types of photodetectors with different characteristics can be built. The mostly used photodetectors in silicon OEICs are photodiodes (PN-PD, PIN-PD), avalanche photodiodes (APDs) and phototransistors (PTs). 1) PN-Photodiode: By far the most used photodetector is the PN-photodiode. PN-PDs can be realized in two different structures. First, they can be built as a well/substrate junction. Second, the detector can be built as a p + /n-well junction (junction depth about 2μm). Both structures have similar properties. A major drawback of PN junctions is their thin space-charge region (SCR) of less than 2μm at typical supply voltages (<5V) due to high doping concentrations. For deep penetrating light, shallow SCRs lead to slow detectors since most of the charges are generated outside the SCR region (e.g.: NIR-light with 850 nm has a 1/e penetration depth of about 16.6 μm [1]). 2) PIN-Photodiode: PIN photodiodes overcome this limitation by a (thick) additional low doped epi-layer. The low doped epitaxial layer leads to a thick SCR and the PIN- PD gets faster for light with a deep penetration depth. Nevertheless, the responsivity is limited for optimum quantum efficiency to 0.65 A/W @ 850 nm and 0.55 A/W @ 650 nm [1]. 3) Avalanche Photodiodes: APDs and PTs achieve by internal amplification mechanisms higher responsivities than PDs. This amplification is important for detecting weak optical signals. A major drawback of APDs is their need for high voltages (e.g. up to 450 V in [2]). High voltages are hard to handle in system-on-chip (SoC) applications. A shallow APD with a responsivity of 4.6 A/W at 430 nm and a reverse bias of 19.5 V is reported in [3]. The above mentioned drawbacks can be avoided by the use of phototransistors. 4) Phototransistors: Phototransistors achieve current amplification without high voltages. This is the main benefit of PTs compared to APDs. A PT consists of a PD (base-collector junction) and an internal BJT for current amplification. Fig. 1 depicts the cross-section of a PT in a standard CMOS process. In [4], PTs in a standard-buriedcollector (SBC) BiCMOS technology with a responsivity of 2.7 A/W are reported. In our work we realized different designs of silicon integrated PTs with different characteristics. The characteristics of each PT can be adjusted by means of different layout designs of the base and emitter area. Cheap implementation in a CMOS process opens the opportunity for the production of cheap silicon based OEICs. Application examples are highly sensitive optical sensors, active pixel image sensors, light barriers, opto-coupler, etc. This work reports on the design and characterization of these PTs. II. PHOTOTRANSISTOR: STRUCTURES AND PROPERTIES Several 100 100 μm 2 pnp type silicon integrated PTs were built in a 0.6 μm OPTO ASIC CMOS technology. The only difference compared to a standard OPTO ASIC process was the use of a special starting material. The special starting material has a 15 μm thick low doped (2 10 13 cm -3 ) p-epitaxial layer and a 1 μm n-epitaxial layer (10 14 cm -3 ) on top of the p ++ -substrate. The p-epitaxial layer ensures a thick SCR resulting in a thick drift zone at the base-collector junction. Therefore this structure is well suited for the detection of deep penetrating light. Fig. 1: SCHEMATIC AND CROSS SECTION OF A PNP PT 46
A. Base Three different layouts of the PTs base area were designed. First, the base was formed only by the low doped n-epitaxial layer. Second, a higher doped base was formed by an n-well. Third, a mix of the both mentioned dopings was used. Stripes of n-well were implanted with different widths to adjust the effective doping concentration of the base (Fig. 2). The main geometrical parameters for the adjustable base are shown in Tab. 1. B. Emitter The emitter was also realized in three different layouts. First, a large emitter with a plane of 97 97 μm 2 was designed. Second, a striped emitter with 1.4 μm wide stripes was designed. Between the stripes are 8.4 μm wide gaps. Third, the emitter was built by a small emitter dot in the center or at the corner of the PTs, respectively. The center emitter has a size of 1.4 1.4 μm 2 and the corner emitter a size of 7.0 1.4 μm 2. Two emitter structures (stripped and center emitter) are depicted in Fig. 3. C. Collector The collector of each PT is formed by the substrate itself. The collector is connected via a large area ring of substrate contacts at the border of the photosensitive area. NW Stripes Resulting doping Width w Distance d concentration 100 μm - 100 % (NW full ) 2 μm 1 μm 66 % (NW 66 ) 1 μm 1 μm 50 % (NW 50 ) 1 μm 2 μm 33 % (NW 33 ) - - 0 % (NW epi ) TAB. 1: DESIGN PARAMETERS AND N-DOPING CONCENTRA- TION OF THE ADJUSTABLE BASE D. Bandwidth Junction capacitances can be found between base and emitter as well as between base and collector (C BE, C BC ). These two capacitances of the PTs are of major importance for the dynamic behaviour of the PTs. The sizes of these capacitances depend strongly on the thickness of their relevant SCRs. Furthermore the thicknesses of the SCRs depend on the doping concentration in the base. These capacitances and the base transit time are the main parameters defining the -3 db bandwidth of PTs. The -3 db bandwidth is indirect proportional to the three mentioned parameters. Equation (1) describes the relation between the -3 db bandwidth the above mentioned parameters [5]. where f 3dB 1 kbt 2 C C qie B BE BC f 3dB -3 db Bandwidth of the phototransistor; forward current gain of the phototransistor; B base transit time; k B Boltzmann constant; T absolute temperature; q elementary charge; I E emitter current of the Phototransistor; C base-emitter capacitance; BE C base-collector capacitance. BC E. Forward Current Gain Furthermore the thickness of the area between the SCRs (this equals the effective base thickness) is of major importance for the current gain of the PT. Thick SCRs lead to small capacitances and to a thin effective base. In a rough estimation, the current gain is indirect proportional to the square of the effective base thickness. This relation is shown in equation (2) [6]. (1) Fig. 2: CROSS SECTION OF THE ADJUSTABLE BASE DESIGN 1 2 B Dn B D W W N 2 D D L N b p p n A (2) where Fig. 3: TOP VIEW OF TWO EMITTER DESIGNS W B b D p D n forward current gain of the phototransistor; effective base thickness; minority carrier lifetime in the base; diffusion coefficient of holes in the base; diffusion coefficient of electrons in the base; L n diffusion length of electrons in the emitter; N D donor density in the base; N donor density in the emitter. A 47
III. RESULTS AND MEASUREMENTS Electrical and optical (at 675 nm and 850 nm) characterizations of the PTs were done by three measurement setups. 1) Gummel measurements: Gummel measurements were done to characterize the electrical current amplification. A depiction of the gain over the collector current I C for four PTs with a small centered emitter and different types of base doping is shown in Fig. 4. PTs with lighter doped base have due to thicker SCRs a thinner effective base width W B and a minor donor density N D than other PTs. According to equation (2) this properties lead to a higher current gain. The PTs depicted in Fig. 3 show current amplifications between 57 and 187 for I C < 13 na. 2) DC light measurements: DC light measurements were done by sweeping the optical light power and varying the collector-emitter voltage from -1 V to -8 V. Fig. 5. depicts the responsivity of two PTs at three different Fig. 4: CURRENT GAIN FOR FOUR PTS WITH DIFFERENTLY DOPED BASE AND SMALL CENTER EMITTER collector-emitter voltages for 675 nm DC light. For this measurement the light power was swept from -55 dbm to -26 dbm. The PT with the full plane emitter and the 50% doped base shows the largest responsivity of 76A/W at V CE = -8 V. In devices with small centered emitter, electrons have to travel longer distances to reach the emitter due to an inhomogeneous electric field. This leads to recombination of holes and electrons and furthermore to a decreased responsivity. The small sized center emitter PT has a higher emitter resistance compared to the full emitter PT. The higher resistance is due to the smaller emitter area. This leads to a stronger decrease of the responsivity according to the optical power due to stronger operating point variations for the PT with the small center emitter. 3) AC light measurements: AC light measurements were done for determining the responsivity and bandwidth at 675 nm as well as 850 nm. The measurements were done at an optical power of -24.6 dbm at 675 nm light and -21.2 dbm at 850 nm light. Due to the difference in the penetration depth of 675 nm and 850 nm light, the used optical power was adjusted to meet the same collector current for each wavelength. The devices were characterized at three different collector-emitter voltages and five different operating points (including floating base): V CE = -2 V, V CE = -5 V, V CE = -10 V, I B = 0 A, I B = 1 μa, I B = 2 μa, I B = 5 μa and I B = 10 μa. The base current I B was adjusted via an on-chip 1 M resistor. Responsivities at different operating points for three PTs at 675 nm (top table) and 850 nm (bottom table) are show in Tab. 2. In Tab. 3 the corresponding bandwidth values are shown. According to equation (1) and (2), the device with the smallest emitter shows the highest bandwidth and the smallest responsivity. In contrast to this device, the device with the full plane emitter has the smallest bandwidth and the highest responsivity. The PT with the full emitter shows also a strong increase of the responsivity with a collectoremitter voltage increase. The small emitter devices have a small but rather constant responsivity. All PTs show a bandwidth increase when increasing the collector-emitter voltage. The PTs achieved a higher responsivity at 675 nm, due to a shorter penetration depth of 675 nm compared to 850 nm light. Maximum achieved values for the responsivity are 98 A/W at 675 nm and 37.2 A/W at 850 nm light. Furthermore, bandwidths up to 8.97 MHz at 675 nm and 13.46 MHz at 850 nm were achieved. IV. CONCLUSIONS Fig. 5: DC RESPONSIVITY FOR TWO PTS (50 % DOPED N- WELL BASE WITH FULL AND CENTERED EMITTER) AT THREE DIFFERENT COLLECTOR-EMITTER VOLTAGES AT 675 NM. This paper reports on fully integrated silicon PTs in a CMOS OPTO ASIC technology. The use of a special starting material is the only difference compared to a standard OPTO ASIC technology. Electrical and optical measurements were done for the PTs characterizations. Electrical current gains up to 187 as well as responsivities up to 98 A/W and bandwidths up to 13.4 MHz were achieved. The PTs in this work achieve about 25 times more responsivity compared to devices presented in [4]. The full silicon integration of the PTs makes them well suited for many optical sensing applications and cheap OEICs. 48
Tab. 2: RESPONSIVITIES IN A/W FOR THREE PHOTOTRANSISTORS AT 675 NM (TOP-TABLE) AND 850 NM (BOTTOM-TABLE) AT AN OPTICAL LIGHT POWER OF -24.6 DBM FOR 675 NM AND -21.2 DBM FOR 850 NM Tab. 3: BANDWIDTHS IN MHZ FOR THREE PHOTOTRANSISTORS AT 675 NM (TOP-TABLE) AND 850 NM (BOTTOM-TABLE) AT AN OPTICAL LIGHT POWER OF -24.6 DBM FOR 675 NM AND -21.2 DBM FOR 850 NM V. ACKNOWLEDGEMENTS VII. VITAE This work received funding from the Austrian Science Fund (FWF) in the project P21373-N22. VI. REFERENCES [1] H. Zimmermann, Integrated Silicon Optoelectronics, 2 nd ed., Springer-Verlag, Berlin, Heidelberg, 2010. [2] S. Cova, M. Ghioni, A. Lacaita, C. Samori and F. Zappa, Avalanche photodiodes and quenching circuits for single-photon detection, Applied Optics, vol. 35, no. 12, pp. 1956-1976, 1996. [3] A. Pauchard, A. Rochas, Z. Randjelovic, P.A. Besse and R.S. Popovic, Ultraviolet Avalanche Photodiode in CMOS Technology, IEEE IEDM, 2000, pp. 709-712. [4] T. Yin, A. M. Pappu and A. B. Apsel, Low-cost, high-efficiency, and high-speed SiGe phototransistors in commercial BiCMOS, IEEE Photonics Technology Letters, vol. 18, no. 1, pp. 55-57, 2006. [5] G. Winstel and C. Weyrich, Optoelektronik II, Springer-Verlag, Berlin, Heidelberg, 1986 [6] P. Gray, P. Hurst, S. Lewis and R. Meyer, Analysis and Design of analog integrated Circuits, Wiley, New York, 2008 Plamen Kostov graduated with distinction from the Vienna University of Technology in 2009 with a MSc. degree in electrical engineering. He joined the Institute of Electrodynamics, Microwave and Circuit Engineering at Vienna University of Technology in 2009 in order to pursuit the Ph.D. degree. His main interests include photodetectors and optoelectronic integrated circuit design. Wolfgang Gaberl started in telecommunication industry with design and realization of custom circuits for small PABX systems and technical consulting. He received the MSc. with distinction from the Vienna University of Technology, Austria. During the last years, he has been with the Vienna University of Technology, Austria. His major fields of interest are analog circuit design in general, photodetectors and integrated photoreceiver design. Dr. Horst Zimmermann, received the diploma in Physics in 1984 from the Univ. of Bayreuth, Germany and the Dr.- Ing. degree in the Frauenhofer Inst. for Integrated Circuits (IIS-B). Since 2000 he is full professor for Electronic Circuit Engineering at Vienna Univ. of Technology, Austria. He is author of the Springer books Integrated Silicon Optoelectronics, Silicon Optoelectronic Integrated Circuits and Highly Sensitive Optical Receivers. He is also author and co-author of more than 300 publications. IEEE Senior Member since 2002. 49