0.35 μm CMOS avalanche photodiode withhigh responsivity and

responsivity–bandwidth productWolfgang Gaberl, Bernhard Steindl, Kerstin Schneider-Hornstein,* Reinhard Enne, and Horst Zimmermann

Institute of Electrodynamics, Microwave and Circuit Engineering, Vienna University of Technology,Gusshausstrasse 25/354, 1040 Vienna, Austria

*Corresponding author: Kerstin.Schneider‑Hornstein@tuwien.ac.at

Received October 15, 2013; revised December 11, 2013; accepted December 20, 2013;posted December 23, 2013 (Doc. ID 199573); published January 27, 2014

A highly sensitive avalanche photodiode (APD) in 0.35 μm CMOS technology is presented. Due to a thick intrinsicabsorption layer, a high responsivity at a low bias voltage, where the avalanche gain is 1, is combined with anexcellent avalanche gain at high voltages to achieve a maximum overall responsivity of the APD of more than10 kA∕W. This responsivity exceeds that of other submicrometer CMOS APDs by a factor of more than 700. As afigure of merit the responsivity–bandwidth product is defined, and the achieved value of 23.46 A∕W · GHz is 2.4times higher than the values found in the literature. © 2014 Optical Society of AmericaOCIS codes: (230.0250) Optoelectronics; (250.1345) Avalanche photodiodes (APDs); (230.5160) Photodetectors;

(250.5300) Photonic integrated circuits; (250.3140) Integrated optoelectronic circuits.http://dx.doi.org/10.1364/OL.39.000586

Low-cost optical interconnects for gigabit data transmis-sion in electronic systems and short-distance communi-cation are realized nowadays in the wavelength range ofsilicon components (850 nm). In the future optical in-home networks via plastic optical fiber (POF) will alsobe an interesting application, due to very easy installationand handling. The interesting wavelengths here are red(670 nm) and green (520 nm). Vertical-cavity surface-emitting lasers are combined with (often integrated) Siphotoreceivers to realize cheap optical data transmissionsystems. The use of integrated p-i-n photodiodes isalready standard [1], but to increase the sensitivity ofthe system, and consequently the responsivity of thephotodiode, the next step is the integration of avalanchephotodiodes (APDs) into modern technologies. The abil-ity of modern submicrometer CMOS technologies toendure high biasing voltages opens up new possibilitiesfor the design of APDs.Several APDs integrated in standard CMOS processes

have been reported recently. In [2], two different types ofAPDs were researched: an electron-injection-type APDand a hole-injection-type APD that show maximum re-sponsivities of 3.52 and 4.3 A∕W at 850 nm light, respec-tively. The APDs of [3,4] showed maximum responsivitiesbelow 2 A∕W but high bandwidths of up to 8.7 GHz [4].In [5,6] maximum responsivities of 10 and 15.4 A∕W, re-spectively, were reported. Often the avalanche gain (M)is reported as the main goal (see, e.g., [5]), but the overallresponsivity of the APD, consisting of the responsivity atzero biasing (M � 1) times the avalanche gain, is theactual goal. Reference [7] presented a very-high-speedreceiver with a maximum responsivity of 4.67 A∕Wand a responsivity corresponding to the top-speed condi-tions of about 0.3 A∕W again at 850 nm light. Severalother works also investigated shorter wavelengths, forexample, for use with POF. Reference [8] realized an ava-lanche double photodiode in 40 nm CMOS technologyand characterized it at three different wavelengths. For660 nm light, a responsivity of 0.49 A∕W was reached;for 850 nm, 0.84 A∕W; and for 520 nm 2.04 A∕W. Another

APD in 0.18 μm CMOS technology was characterizedfor different wavelengths [9], with maximum responsiv-ities of 2.61, 1.53, and 1.03 A∕W for 405, 650, and 830 nmlight, respectively. In [10], a high-speed APD with a band-width of 7 GHz was reported. It showed, therefore, alower maximum responsivity of 0.65 A∕W for 850 nmlight. Reference [11] also presented an APD in 0.8 μmCMOS technology characterized for 470 nm light witha responsivity of 4.6 A∕W. In this work, we present anAPD with a bandwidth of 1 GHz and very high maximumresponsivity.

Cross section and electric field distribution. Thepresented APD was realized in a 0.35 μm CMOS processwith an intrinsic layer. The active area is 100 μm×100 μm. Figure 1 shows a cross section of the APD,where a detection zone of 12 μm thickness due to the in-trinsic layer with a doping concentration of 2 · 1013 cm−3,

Fig. 1. Cross section of APD.

Fig. 2. Microphotograph of APD.

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as well as an amplification zone consisting of a p-wellarea, can be seen. The thick detection zone allows a highdetection rate of the incoming light, due to the fact thatthe penetration depths of red and near-IR light are ratherlarge, and therefore the responsivity of the diode without

avalanche gain (M � 1) is high. In Fig. 2 a microphoto-graph of the APD is shown.

The simulated electric field distribution of the APD isdepicted in Fig. 3. On the right, the high peak of the elec-tric field is at the interface between the n�� and thep-well area, with 5.4 · 105 V∕cm. Furthermore, the elec-tric field in the detection zone (intrinsic layer) is still highenough to fully deplete this area and therefore ensurescharge-carrier drift.

Photocurrent and dark current. Figure 4 showsthe measured photocurrent for different optical power

Fig. 3. Electric field distribution of the APD at 29 V reversebias (simulated with MEDICI).

Fig. 4. Measured dark current versus reverse bias. Popt,optical input power.

Fig. 5. Measured responsivity and gain at 670 nm light.

Fig. 6. Responsivity versus optical input power at different re-verse bias voltages (Vbias � 25 V, Vbias � 30 V, Vbias � 33.5 V).

Fig. 7. Capacitance versus bias voltage.

Fig. 8. Measured frequency response at 30 V reverse biasvoltage with 670 nm light at 14 μW optical input power.

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values and the dark current over the bias voltage. Thebreakdown voltage is 33.9 V (determined for a reversecurrent of 1 μA), which can be seen in the samefigure.

Responsivity. Responsivity measurements were doneat 670 nm light with different bias voltages and differentoptical input powers. Although the avalanche gain is nota really significant figure, it was also calculated and isdepicted in Fig. 5 together with the responsivity. It canbe clearly seen that the responsivity for low bias voltagesis already high, with 0.35 A∕W at 10 V with M � 1 for670 nm light.

The responsivity rises with increasing bias voltageand reaches a maximum value of 11; 057 A∕W at 33.75 Vat 7 nW optical input power with 670 nm light. This re-sults in a maximum internal gain of the APD of 31,464under the conditions mentioned above. Figure 6 showsthe dependence of the responsivity on the optical powerat constant reverse bias voltages of 25, 30, and 33.5 V. Itcan be clearly seen that the gain and therefore the re-sponsivity is rising with increasing bias voltage anddecreasing optical input power. Due to the fact thatthe avalanche gain at 25 V is much lower than at 33.5 Vreverse bias voltage, the responsivity dependence of theoptical input power is much lower for the lower reversebias voltages.

Capacitance. Figure 7 shows the capacitance againstthe reverse bias voltage. It can be clearly seen that at18.5 V the intrinsic layer is completely depleted, andtherefore the capacitance is constant at 0.15 pF for val-ues above 18.5 V.

Frequency response. The APD shows the highestbandwidth of 1.15 GHz at a reverse biasing of 30 V. Thefrequency response with 670 nm light and an optical in-put power of 14 μW is shown in Fig. 8. The corresponding

Fig. 9. Responsivity versus bandwidth for different opticalinput powers (670 nm light) at 30 V reverse bias voltage (foroptical input power of 14 μW, 2.2 μW, 700 nW, and 220 nW).

Fig. 10. Bandwidth versus bias voltage for 14 μW opticalpower (670 nm light).

Table 1. Comparison with State of the Arta

Refs. Technology

Wave-length(nm)

Area(μm2)

Max.Responsivity

(A/W)

Max.Bandwidth

(GHz)

ResponsivityCorresponding

to Max.Bandwidth (A/W)

Responsivity–BandwidthProduct

(A/W·GHz)

[2] 0.18 μm CMOS (elect.-inj. type) 850 20 × 20 3.52 3.4 0.45b 1.53[2] 0.18 μm CMOS (hole-inj.-type) 850 20 × 20 4.3 3.0 0.26b 0.78[3] 0.18 μm CMOS (EPI) 850 70 × 70 0.7 3.1 0.152 0.47[4] 0.18 μm CMOS 850 50 × 50�oct.) 1.46 8.7 0.045d 0.39[5] 65 nm CMOS 850 30 × 30 10c 3.2 2.94 9.41[6] 0.25 μm CMOS 850 10 × 10 15.4 — — —

[7] 0.13 μm CMOS 850 10 × 10 4.6 6.3 0.3e 1.89[8] 40 nm CMOS 850 30 × 30 0.84 0.7 — —

[8] 40 nm CMOS 660 30 × 30 0.49 1.8 — —

[8] 40 nm CMOS 520 30 × 30 2.04 1.4 — —

[9] 0.18 μm CMOS 405 20 × 20 2.61 0.3 0.071 0.0213[9] 0.18 μm CMOS 650 20 × 20 1.53 1 0.026 0.026[9] 0.18 μm CMOS 830 20 × 20 1.03 1 0.012 0.012[10] 0.18 μm CMOS 850 20 × 20 0.65 7 0.1 0.7[11] 0.8 μm CMOS 470 5 · 105 4.6 — — —

This work 0.35 μm CMOS (EPI) 670 100 × 100 11057 1.15 20.4 23.46aelect.-inj., electron-injection; hole-inj., hole-injection; EPI, epitaxial layer; oct., octagonal.bCalculated from [2].cEstimated from Fig. 3 of [5].dCalculated from [4].eEstimated from Figs. 3 and 4 of [7].

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responsivity is 20.4 A∕W, which results in a responsivity–bandwidth product of 23.46 · 109 A∕W · Hz. With decreas-ing optical input power the bandwidth is also decreasing,but the responsivity is increasing at the same time(see Fig. 9).In Fig. 10 the bandwidth versus the reverse bias

voltage is depicted. It is increasing with increasing volt-age due to higher field strengths and therefore higherdrift velocity in the p-detection zone, but it decreasesagain when the gain in the amplification zone getstoo high and therefore the avalanche buildup time istoo long.Conclusion. Comparing the results of this work to the

state of the art (see Table 1), it can be seen that theachieved maximum responsivity is, by a factor of morethan 700, higher than in the previous published worksin submicrometer CMOS technologies. Even the respon-sivity corresponding to the highest bandwidth is higherthan the maximum responsivity in the APDs of the liter-ature. Due to a high responsivity at low voltages (whereunity avalanche gain is present), and also a high ava-lanche gain, the introduced APD enables these overallhigh responsivities. To take into account the fact thatthe compared APDs from the literature are faster thanthe one in this work, the responsivity–bandwidth productis compared as a figure of merit. In Table 1 the values forthe different APDs are summarized, and it can be seenthat the responsivity–bandwidth product of this workis higher than those of the others by a factor of 2.5.

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