Research Article
Vol. 6, No. 6 / June 2019 / Optica
772
Silicon–germanium avalanche photodiodes
with direct control of electric field in charge
multiplication region
XIAOGE ZENG, ZHIHONG HUANG, BINHAO WANG,* DI LIANG, MARCO FIORENTINO,
RAYMOND G. BEAUSOLEIL
AND
Hewlett Packard Labs, Hewlett Packard Enterprise, Palo Alto, California 94304, USA
*Corresponding author: binhao.wang@hpe.com
Received 14 March 2019; revised 4 May 2019; accepted 7 May 2019 (Doc. ID 362416); published 31 May 2019
A CMOS-compatible avalanche photodiode (APD) with high speed and high sensitivity is a critical component
of a low-cost, high-data-rate, and energy-efficient optical communication link. A novel waveguide-coupled silicon–
germanium APD detector with three electric terminals was demonstrated with breakdown voltage of −6 V, bandwidth
of 18.9 GHz, DC photocurrent gain of 15, open-eye diagram at a data rate of 35 Gb/s, and sensitivity of −11.4 dBm at
a data rate of 25 Gb/s. This three-terminal APD allows high-yield fabrication in the standard CMOS process and
provides robust high-sensitivity operation under small voltage supply. © 2019 Optical Society of America under the terms
of the OSA Open Access Publishing Agreement
https://doi.org/10.1364/OPTICA.6.000772
1. INTRODUCTION
Recent information technology advances, such as Big Data, cloud
computation, cloud storage, and Internet of Things, have been
driving exponential growth of data communication in highperformance computers, data centers, and long-haul telecommunication. Silicon photonics is poised to provide a fast on-chip and
off-chip optical link for data communication that has low cost and
high energy efficiency [1]. As an indispensable building block of
an optical link, photodetectors that have high sensitivity, large
bandwidth, and CMOS compatibility play a critical role in reducing power consumption of the link by significantly lowering the
requirement on laser power [2–4].
Photodetectors with internal gain are advantageous by boosting the signal-to-noise ratio and suppressing the adverse effect of
thermal noise in transimpedance amplifiers (TIAs), thus improving detection sensitivity. Linear-mode avalanche photodiodes
(APDs) are most commonly used to improve sensitivity, but their
operations usually require large bias voltages. For example, monolithic silicon–germanium APDs were demonstrated with large
gain-bandwidth products [5]; however, these devices were driven
at bias voltages higher than those available in current computer
power rails [6]. Although germanium APDs can operate at voltages below 10 V, they often suffer from small gain-bandwidth
products and large noise due to the large carrier impact ionization
ratio in germanium [7–9]. Silicon has low charge multiplication
noise due to its small carrier impact ionization ratio; however, it
cannot absorb near-IR light that is used for telecom and datacom
data communication. To take advantage of both large absorption
of near-IR light in germanium and low charge multiplication
2334-2536/19/060772-06 Journal © 2019 Optical Society of America
noise in silicon, a silicon–germanium APD with separate absorption and charge multiplication (SACM) regions has been demonstrated [5]. By employing a P-doped charge layer in silicon to
reduce the electric field in germanium, a low-breakdown-voltage
(≈10 V) silicon–germanium APD has been demonstrated with
large bandwidth and sensitivity [3]. In addition, large gainbandwidth product has been achieved in silicon–germanium
waveguide APDs with a lateral charge multiplication region in
silicon [10], making this device viable in the standard CMOS
fabrication process where there is no epitaxial silicon layer.
However, these conventional silicon–germanium SACM APDs
use one bias voltage to control electric fields in both the light absorption and charge multiplication regions, and thus require complex control of doping in the charge layer to concentrate electric
field for charge multiplication. Silicon–germanium APDs with
ultra-low drive voltages have been proposed in a configuration
of three electric terminals [11,12]; however, no experimental
work has been demonstrated so far.
This paper presents an experimental demonstration of a novel
waveguide-coupled three-terminal silicon–germanium APD detector with a DC photocurrent gain of 15 near low breakdown
voltage of only −6 V. This APD had a 3 dB bandwidth of
18.9 GHz, a clearly open electric eye diagram at a data rate
of 35 Gb/s, sensitivity of −11.4 dBm at bit error rate (BER)
of 2.4 × 10−4 and a data rate of 25 Gb/s for error-free operation
under KP4 forward error correction (FEC) limit, and a sensitivity
of −7.8 dBm at BER of 10−12 and a data rate of 25 Gb/s without
FEC, all of which were measured without a TIA. This novel APD
can be fabricated in the standard CMOS process with high yield,
and has robust operation under low bias voltage, which reduces
Research Article
power consumption and makes it compatible with system
architectures with limited voltage supply.
2. DEVICE STRUCTURE AND WORKING
PRINCIPLE
The structure of this novel three-terminal silicon–germanium
waveguide APD is shown in Fig. 1(a). Light is coupled from an
optical fiber via a grating coupler to a silicon waveguide and is
then evanescently coupled to germanium block above silicon
for absorption. The device was fabricated on a silicon-on-insulator
(SOI) chip with 220 nm silicon and 3 μm buried oxide. The top
silicon layer was first implanted with arsenic and boron to form
N-doped and P-doped interdigitated “fingers,” with finger widths
from 300 nm to 700 nm and spacing between adjacent fingers
from 200 nm to 600 nm. A layer of 400 nm epitaxial germanium
was grown on a layer of 120 nm epitaxial silicon to accommodate
other devices on the same wafer [3], which is not crucial for the
operation of the three-terminal APD. A boron implantation of
medium doping density was first performed throughout germanium. Then another shallow boron implantation of high doping
density was utilized to form electric contact on germanium,
together with two other electric contacts formed on the P-doped
and N-doped silicon fingers, resulting in three electric terminals
in total.
With three electric terminals, two voltage drops can be independently controlled in the three-terminal APD [13,14] [see
Fig. 1(b)]. Such a three-terminal device contains two PIN diode
junctions (i.e., one is between P-Ge and N-Si, and the other is
between P-Si and N-Si), both of which are reversely biased.
Specifically, one voltage drop, V 1 , controls the electric field in
the germanium layer, where light is absorbed and photocarriers
are generated. This electric field separates photon-generated electron–hole pairs and drives electrons toward charge multiplication
regions in silicon below. The other voltage drop, V 2 , directly controls the electric field in intrinsic silicon regions sandwiched
between doped silicon fingers, where charge multiplication occurs
in planar direction once the electric field there is strong enough.
Fig. 1. (a) Structural diagram of a three-terminal silicon–germanium
waveguide avalanche photodiode. (b) Simplified schematic of a threeterminal silicon–germanium APD with three electric terminals providing
two independent voltage drops across two separate regions for light
absorption and charge amplification. (c) Simulated electric field at the
central vertical cross section of the three-terminal APD, with 6 V reverse
bias voltages across both the light absorption and charge multiplication
regions.
Vol. 6, No. 6 / June 2019 / Optica
773
By shrinking the spacing between the P-doped and N-doped silicon regions, a small voltage drop across them is enough to generate avalanche breakdown in the middle intrinsic silicon region.
Figure 1(c) shows simulated two-dimensional electric field in a
vertical cross-section plane at the center of the silicon–germanium
waveguide of the three-terminal APD. This simulation was performed by Lumerical Device [15], where virtual electric terminals
were added at the center of doped silicon fingers in order to emulate the large three-dimensional structure with a simplified twodimensional cross section without losing generality and therefore
reduce simulation time. With a reverse bias of 6 V on both PIN
diode junctions, electric field as high as 6.5 × 105 V∕cm is generated in the intrinsic silicon regions between doped interdigitated
fingers, which is above the impact ionization field threshold for
avalanche multiplication in silicon (≈3 × 105 V∕cm [16]). At the
same time, since the germanium layer is slightly P-doped, voltage
drop in germanium is enough to fully deplete the absorption region without causing charge multiplication, thus further concentrating voltage drop in silicon and suppressing undesired
avalanche breakdown in germanium [17].
Three-terminal silicon–germanium SACM APDs are superior
to their two-terminal counterparts in several ways. Most importantly, with dedicated voltage drop directly applied on the charge
multiplication region, the avalanche breakdown voltage of a threeterminal APD is easily controllable. The breakdown voltage can
be reduced by design, and the low-voltage operation of threeterminal APDs not only reduces power consumption for light detection, but also fits well into computing and storage architectures
that have limited voltage supply [6]. In addition, the reverse bias
voltage applied on a conventional two-terminal SACM APD is
shared by the light absorption and charge multiplication regions
in series, and a charge layer of P-type doping in silicon is required
to help concentrate voltage drop in the silicon multiplication
region. This charge layer requires precise control of the doping
profile, and usually an epitaxial silicon layer is needed to form
this charge layer and intrinsic silicon multiplication region [3,5],
thus adversely increasing fabrication complexity and cost. In addition, the dark current due to dislocation traps at the silicon–
germanium boundary and doping in germanium is reduced
due to lower electric field at the Si–Ge interface. Furthermore,
charge layer doping affects the avalanche breakdown voltage of
a SACM APD, and the multiplication gain of an APD is very
sensitive to bias voltage near breakdown. Therefore the presented
novel three-terminal APDs have larger fabrication yield and are
more robust in operation. Also, APDs with different breakdown
voltages and multiplication gains can be easily fabricated on the
same wafer for different applications. Last, while doping in Ge
could help reduce electric field in Ge in a conventional SACM
Si–Ge APD, the three-terminal design enables easy control of
voltage drop in Ge and depletion of the Ge layer before the onset
of charge multiplication in the Si layer, in order to amplify all
photo-carriers at high speed.
An alternative device structure that allows using three electric
terminals to provide two separate voltage drops on the light absorption and charge multiplication regions, can be realized by directly adding a third electric terminal on the charge layer (P-doped
silicon) of the conventional two-terminal APD [3], and using a
vertical layer structure for the charge multiplication region. With
this approach, the thickness of the epitaxial silicon layer can be
accurately controlled in fabrication, and thinner multiplication
Research Article
layer thickness could potentially allow for even lower breakdown
voltage, while the width and gap of the doping region in a lateral
PIN structure are limited by mask resolution and dopant diffusion. However, there are several drawbacks to this approach. First,
in order to form good electric contact at the charge layer, heavy
doping on the path of light is needed, which would introduce
large optical loss. In addition, although the thickness of an epitaxial layer can be well controlled, the ion implantation depth and
width of a vertical doping structure cannot be accurately controlled. These dopants also need to be kept from diffusing into
the intrinsic charge multiplication region below it. Therefore
three-terminal APDs with lateral PIN diode structure for charge
multiplication are preferred.
3. DEVICE MODEL
The electric circuit model of a three-terminal silicon–germanium
APD is much more complex than that of a conventional twoterminal APD [18], because multiple current paths exist between
any two of the three electric terminals. By analogy with the model
for conventional SACM APDs [19,20], Fig. 2(a) shows an equivalent electric model of the active regions of a three-terminal APD
with parasitics excluded. Three electric terminals (A, B, C) contact germanium, P-doped silicon, and N-doped silicon regions,
respectively. A virtual point, “O,” is used to denote the boundary
between light absorption and charge amplification regions for easier interpretation of carrier flow paths. Photocarriers generated in
germanium by absorption of light have multiple paths to get collected. For example, photoelectrons travel across germanium
under the attraction of higher voltage bias at the N-doped silicon
region. Then a portion of these photoelectrons travels directly to
the N-doped silicon region and is collected by Terminal C, while
more photoelectrons are amplified in the silicon multiplication
region before getting collected by Terminal C. In the comprehensive model illustrated in Fig. 2(a), a pair of lumped capacitor and
Fig. 2. (a) Electric model of the three-terminal silicon–germanium
APD, where each pair of the three electric terminals is connected by
a parallel capacitor and resistor, and an inductor exists between the interdigitated P-doped and N-doped silicon regions. (b) Measured S11 parameters of a three-terminal APD under various reverse bias voltages shown
in a Smith chart. (c) Simplified circuit model of a three-terminal SACM
APD. The parasitics are also included. (d) Measured and fitted electrical
output impedances at 6 V reverse bias. For the results in (b)–(d), the two
terminals contacting germanium and P-doped silicon regions are shorted
by wire bonding.
Vol. 6, No. 6 / June 2019 / Optica
774
Table 1. Fitted Electric Circuit Parameters of a ThreeTerminal Avalanche Photodiode under a Reverse Bias of 6 V
Parameters
Values
Parameters
Values
Rd (Ω)
1053
Rs (Ω)
5
Cd (fF)
57.8
Lp (pH)
71.4
Ra (Ω)
438.6
Rp (Ω)
40
Ca (fF)
30
Cp (fF)
15
La (nH)
6.8
resistor in parallel are used to model the current path between the
three electric terminals and the boundary “O.” In addition, a
lumped inductance is employed to model the charge multiplication regions between interdigitated doped silicon. This extra
inductance originates from the space charge effect as a result of
carrier build-up from charge avalanche multiplication and manifests itself in measured photocurrent pulse in Fig. 4(c).
The circuit elements modeled in Fig. 2(a) can be fitted from
scattering parameter (S-parameter) measurement. Without a RF
probe that can contact all three electric terminals simultaneously,
the S-parameter between any two of the three electric terminals
could be characterized consecutively. In order to measure all photocurrent generated in the device, the P-doped silicon and germanium regions were shorted by wire bonding to simplify the
current measurement with one GS probe [see measured photocurrent in Fig. 4(d)]. Figure 2(b) shows the scattering parameter
S11 of the three-terminal APD with P-doped silicon and germanium regions shorted. This device’s S-parameter changes abruptly
when the reverse bias voltage exceeds 5.6 V, which is consistent
with measured photocurrent shown in Fig. 4(a). This turning
point corresponds to full depletion of the doped germanium
region and the onset of carrier multiplication in silicon.
In order to understand the measured S11 parameters of the
three-terminal APD in Fig. 2(b), the electric circuit diagram in
Fig. 2(a) is simplified, as shown in Fig. 2(c), where the P-doped
silicon and germanium are shorted and circuit elements (i.e.,
capacitances and resistances) are grouped together. Specifically,
the three-terminal APD electrical parasitics include those in
the diode junctions and avalanche regions, as well as substrate
and pad parasitics. Here, R d is the resistance in the absorption
region, C d is the capacitance in the absorption region, and C a
is the capacitance in the avalanche region. The inductor La, together with its series resistance R a , is due to space charge effects.
R s is the diode series resistance, C p is the parasitic capacitance, R p
is the parasitic resistance, and Lp is the parasitic inductance, which
is negligible at reverse bias less than 5.6 V when space charge
effects do not occur [21,22]. Values of these parameters were fitted from S11 characterization, and example values at reverse bias
of 6 V are shown in Table 1. Circuit impedances of the threeterminal APD were then calculated based on fitted parameter
values of circuit components, and they matched very well with
measured values, as presented in Fig. 2(d).
4. DEVICE CHARACTERIZATION RESULTS
Figure 3 shows the optical and electrical setup for testing a fabricated chip with three-terminal APDs. A mode-locked laser was
used to generate femtosecond optical pulses for measuring response speed of photodetectors [3]. Optical data bit stream were
generated by modulating continuous-wavelength (CW) laser light
(1550 nm) with a Mach–Zehnder modulator (MZM) driven by
amplified electric data bit stream. The modulated optical signal
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Fig. 3. Experiment setup for characterization of integrated threeterminal silicon–germanium avalanche photodiodes. Some acronyms
are as follows: CW, continuous wave; MZ, Mach–Zehnder; EDFA,
erbium-doped fiber amplifier; BPF, bandpass filter; DUT, device under
test; VNA, vector network analyser; BERT, bit error rate tester; Scope,
sampling scope.
was then amplified with an erbium-doped fiber amplifier (EDFA)
and filtered with a tunable bandpass filter before it was coupled to
the chip via a grating coupler. An electric bias-tee was used to
apply reverse bias voltages and collect generated photocurrent
simultaneously. Photocurrent signals were then measured by a
vector network analyzer, sampling scope, and BER tester (BERT)
for measurement of electric parasitics, eye diagram, and BER, respectively. As shown in Fig. 2(a) and discussed in Section 3, there
are multiple current paths among the three electrodes. However,
due to limitation of the equipment, only one RF probe could be
laid onto the electric pads. The terminals on P-doped silicon and
P-doped germanium were then shorted by wire bonding in order
to collect total current via one RF probe. As a result, equal reverse
bias voltages were applied on P-doped silicon and P-doped germanium while grounding the N-doped silicon. This test configuration still offers desired operating condition, i.e., with large
electric field in the charge multiplication region and small electric
field in germanium, as confirmed in the experiment.
Figure 4 shows optical characterization of a three-terminal silicon–germanium waveguide APD with a germanium block of
Fig. 4. Optical characterization of a three-terminal silicon–germanium
waveguide APD: (a) photocurrent and avalanche gain versus reverse bias
voltage, showing a breakdown voltage of about 6 V; (b) breakdown voltages of three-terminal silicon–germanium APDs increase with the gap
between P-doped and N-doped silicon regions; (c) photocurrent with
incidence of a femtosecond optical pulse under various reverse bias voltages; (d) 3 dB bandwidth calculated from Fourier transform of its impulse
response in (c).
Vol. 6, No. 6 / June 2019 / Optica
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4 μmwidth × 10 μmlength × 400 nm (thickness). The photocurrent increases with reverse bias voltage as expected, and
charge multiplication occurs at a reverse bias voltage near 6 V,
which is much lower than breakdown voltage of 10 V of twodimensional SACM APDs of similar dimensions [3]. At a reverse
bias of 2 V where unit gain is defined, the responsivity of this
APD is estimated to be 0.48 A/W at 1550 nm. The 10 μm germanium is not long enough to absorb all light, and the responsivity was undermined by lossy evanescent coupling from the
bottom silicon waveguide to the epitaxial germanium layer and
light absorption by a large centered metal electrode on top of
the germanium area. It can be improved by using a longer adiabatic tapered silicon and germanium waveguide and optimizing
the size and location of the metal electrode [23]. Although this
measured responsivity at 2 V bias is lower than that of conventional PIN germanium photodetectors [24], the overall responsivity of this APD increases with avalanche gain, which approaches
15 near avalanche breakdown for CW input light. After the
breakdown voltage, the photocurrent has a sub-exponential increase with bias voltage. This is because of a large resistor formed
by lightly doped fingers in silicon. This resistor is in series with
the lateral PIN diode where charge multiplication occurs. With
large photocurrent at avalanche breakdown, a non-negligible voltage drops across this resistor, causing the actual voltage drop in the
charge multiplication region to be smaller than the DC power
supply output voltage. One drawback of the measured device
is large dark current, which gets amplified together with photocurrent. The dark current at a bias voltage of 5.4 V (right before
the breakdown occurs) is about 10 μA. This large dark current
results mainly from the leakage current in the lateral PIN diode
between P-doped and N-doped fingers in silicon (with a dark current density of 43 A∕cm2 at the doped fingers in silicon at 5.4 V).
The straggle width of the boron doping in silicon under low drive
voltage is not small, and the boron atoms further diffuse during
the following epitaxial silicon and germanium growth processes.
In addition, possible misalignment of these two doping masks
reduces the gap between P-doped and N-doped fingers. The leakage current in the lateral PIN diode can be decreased by better
control of the gap between doped fingers and reducing the straggle width of their doping profiles. Last, before the two terminals
on P-doped silicon and germanium were shorted, a separate voltage was applied on germanium via a DC electric probe. It was
observed that the total dark current decreased slightly when germanium was biased at higher voltage than that on P-doped silicon
and dark current due to defects at the silicon–germanium interface decreasing.
The breakdown voltages of three-terminal APDs with different
gaps between the P-doped and N-doped silicon regions are shown
in Fig. 4(b). Values of gaps shown here are those drawn on the
mask, and the actual gaps in fabricated devices are much smaller
due to mask misalignment and carrier diffusion. When the gap is
smaller than 600 nm, the breakdown voltage increases linearly
with the gap, as the same electric field is needed to initiate the
avalanche process. However, when the gap is greater than 600 nm,
the breakdown voltage converges to 12 V. This is because in the
measured three-terminal APD, avalanche multiplication also
occurs in the epitaxial silicon layer as a result of large voltage drop
between germanium and N-doped silicon, and the breakdown
voltage of this avalanche process is independent of the gap between doped silicon fingers. With a gap of 200 nm, an ultra
Research Article
low breakdown voltage of 4 V was measured. However, with such
a short avalanche region, the achievable multiplication gain is limited. We instead studied mainly devices with a nominal gap of
300 nm that had breakdown voltages near 6 V. It should be noted
that the actual multiplication finger gap is narrower than that designed in the mask (300 nm) due to doping mask misalignment
and dopant diffusion. Last, no correlation was found between
breakdown voltage and doped finger width.
The response speed of the three-terminal APD was characterized by sending a femtosecond laser pulse centered at 1550 nm
to it and measuring the generated photocurrent pulse with a
sampling scope. Figure 4(c) shows such an impulse response measured on the same three-terminal APD in Fig. 4(a) under several
reverse bias voltages. The photocurrent pulse amplitudes were
normalized to highlight the differences in pulse widths. When
the reverse bias voltages are below 5.6 V, the falling edges of
the photocurrent pulses have long tails, indicating that P-doped
germanium is not fully depleted, and carrier diffusion transit time
dominates the response speed. When the reverse bias is greater
than 5.6 V, the germanium layer is fully depleted, and the photocurrent pulse has a second peak after it decays from its major peak
corresponding to the optical pulse peak. This resonance effect
indicates that the response speed is limited by capacitive and inductive circuit elements, which is aligned with the electric circuit
model built from electric parasitics measurement [see Figs. 2(a)
and 2(c)]. A bias voltage of 5.6 V also happens to be sufficient
to cause avalanche multiplication between the P-doped and
N-doped silicon fingers, as indicated in measured photocurrent
in Fig. 4(a).
The 3 dB bandwidth of this APD was obtained by taking
the Fourier transform of its photocurrent impulse response.
Figure 4(d) shows that the 3 dB bandwidth of this three-terminal
APD increases with reverse bias voltage, as increased electric field
further depletes the lightly doped germanium (which reduces
junction capacitance in germanium) and reduces photocarrier
transit time until they reach saturation velocity. At the same time,
the charge multiplication gain also increases as a result of impact
ionization in silicon. As the bias voltage exceeds the impact ionization threshold voltage, the bandwidth levels out as the response
speed is limited by the finite gain buildup time in the avalanche
process. At 6.4 V reverse bias, a 3 dB bandwidth of 18.9 GHz was
measured.
Electrical eye diagrams of the three-terminal APD were also
measured. As shown in Fig. 3, the input optical data to the photodiode were generated by modulating a 1550 nm CW laser with a
40 Gb/s MZM, which was driven by a 29 − 1 pseudorandom
binary sequence (PRBS9) data pattern that was generated in
an arbitrary waveform generator (Keysight 64 GSa/s AWG) and
amplified by a high-speed electrical amplifier with 20 dB gain.
The output electrical signal was recorded by a 63 GHz sampling
oscilloscope. Without using a TIA, the input optical signal and
output electric signals were amplified by an EDFA and an electrical amplifier, respectively. Figures 5(a)–5(c) show clearly open
electrical eye diagrams of the three-terminal APD at data rates of
25 Gb/s, 30 Gb/s, and 35 Gb/s with 6 V reverse bias, due to the
large avalanche gain and high response speed of the device. No
saturation of avalanche gain was observed while varying optical
power to measure the eye diagram. The BER of the three-terminal
waveguide APD was measured by Anritsu MP1800A and is
shown in Fig. 5 (d). Received optical modulation amplitude
Vol. 6, No. 6 / June 2019 / Optica
776
Fig. 5. Electric characterization of a three-terminal silicon–germanium
waveguide APD: electrical eye diagrams at data rates of (a) 25 Gb/s,
(b) 30 Gb/s and (c) 35 Gb/s with 6 V reverse bias voltage, and
(d) bit error rate versus data rate and optical modulation amplitude
(OMA) at the input waveguide.
(OMA) of −11.4 dBm allows error-free operation at a data rate
of 25 Gb/s below the KP4 FEC threshold of 2.4 × 10−4 . And an
OMA of −7.8 dBm leads to a BER of 10−12 at a data rate of
25 Gb/s, which outperforms most PIN photodiodes [23]. It
should be noted that photocurrent gain saturation was observed
when generating the eye diagrams with −2 dBm input optical
power at the photodiode, as a result of the small cross-section area
of the charge multiplication region. The photocurrent gain saturation could be alleviated by increasing the charge multiplication
region size at the cost of increased capacitance.
Although the measured speed, dark current, eye diagram, and
BER of the presented three-terminal Si–Ge APD still have space
for improvement compared to those of conventional two-terminal
Si–Ge APDs with similar dimensions [3], three-terminal APDs
have the advantages of operation at lower drive voltage and better
compatibility with the standard CMOS fabrication process, as
explained in Section 2. There are several approaches to improving
their performances. For example, the eye diagram and BER of a
three-terminal APD can be improved by increasing its speed and
gain together with reducing dark current. Specifically, the speed of
a three-terminal APD can be increased by reducing the capacitance of interdigitated doped fingers in silicon. The avalanche
gain of photocurrent can be enhanced by increasing the charge
multiplication region width at the cost of higher bias voltage.
The sensitivity can be improved by reducing the dark current with
better control of the doping profiles in silicon to suppress undesired leakage current in the lateral PIN diode. Note also that these
measurements were performed with a noisy EDFA (with 2% relative intensity noise at its output) and a noisy electric amplifier
(with noise current of 5.6 μA) rather than a TIA (with typical
noise current 2.5 μA) at the measured 3 dB bandwidth of
18.9 GHz. Higher sensitivity is expected by using a low-noise
TIA and choosing the optimum avalanche gain that maximizes
the signal-to-noise ratio [25].
For a given semiconductor material used for charge multiplication, a threshold electric field is required to initiate avalanche
breakdown. To reduce the breakdown voltage, a shorter multiplication region width is needed; however, the accumulated charge
multiplication gain is also reduced. The presented three-terminal
Research Article
APD reduces breakdown voltage for a given gain by directly applying a bias voltage drop onto the charge multiplication region
without sparing a voltage drop in the light absorption region as in
Ref. [3]. Given the context of developing a low-voltage APD for
data communication, a charge multiplication gain less than 10 is
usually preferred, as higher gain would result in larger excess noise
from the avalanche process.
Vol. 6, No. 6 / June 2019 / Optica
6.
7.
8.
5. CONCLUSION
An integrated silicon–germanium waveguide APD with three
electric terminals has been experimentally demonstrated. The
breakdown voltage of such a three-terminal APD is only 6 V,
which is much smaller than that of a conventional two-terminal
SACM APD of similar device geometry. The reduction of breakdown voltage results from concentrated voltage drop in charge
multiplication regions by directly applying bias voltage onto these
regions via two electric terminals. A third electric terminal is used
to provide electric field to pull photocarriers from the light
absorption region toward the charge multiplication region.
Low-voltage operation of an APD makes it compatible with
the power rails of current computer architectures and reduces
energy consumed by the detector. This novel APD can be fabricated with high yield in the standard CMOS process without
adding extra materials, and different performance metrics of this
APD, e.g., breakdown voltages, can be achieved on the same
wafer by design, e.g., varying lateral charge multiplication region
width.
The presented three-terminal APD has a 3 dB bandwidth of
nearly 19 GHz, a clearly open eye diagram at a data rate of
35 Gbs, and detection sensitivity of −11.4 dBm for error-free
operation at a data rate of 25 Gb/s with KP4 FEC, thus suggesting its potential application in low-power high-speed optical
communication.
The method of using three or more electric terminals to control electric field in multiple device regions can be easily extended
to APDs made of other material systems, as well as other optoelectronic devices. In addition, the same device concept can be
extended from waveguide APDs to those detecting light of normal
incidence, thus improving light detection efficiency in free-space
optics applications such as biological sample imaging and lidar.
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