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Min-Hyeong Kim
High-Speed Circuits and Systems Laboratory
E.E. Engineering at YONSEI UNIVERITY
2011. 5. 11.
[ Contents ]
1. Abstract
2. Background
- SACM APD structure
- Ionization/multiplication coefficient
3. Device structure
4. Measurement results
I.
Dark current
II.
Excess noise factor & Gain-Bandwidth product
III. Receiver sensitivity & BER
5. Conclusion
2
1. Abstract
Monolithic Ge-Si SACM APD
operating at 1300nm
(separate absorption, charge and
multiplication avalanche photodiodes)
Gain-BW product : 340GHz
K_eff : 0.09
A receiver sensitivity
: -28dBm at 10Gb/s
Si material properties allow for high gain with less excess noise than InPbased APD and a sensitivity improvement of 3dB or more.
With Si, an even higher gain–bandwidth product could be achieved based
on a simple layer structure with relatively large process tolerances.
3
2. Background
Ⅰ. SACM APD (separate absorption, charge and multiplication APD)
InAlAs-based APDs
(Ref.17)
InAlAs-based APDs
(Ref.18)
Si-based APDs
(This work)
4
2. Background
Ⅱ. Ionization/Multiplication coefficient
K : Ratio of the ionization
coefficients of electrons and holes.
A low k value is desirable for
high-performance APDs.
Impact Ionization probability = W
Multiplication coefficient M
1 W (W ) 2 (W )3
1
1 W
( for W 1)
→ Excess Noise factor F(M) kM (2
1
)(1 k )
M
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3. Device structure
SACM APD
Punch through voltage -22V
Breakdown voltage -25V
with Responsivity 5.88A/W
Designs for a floating guard ring (GR) with various distances (1–3 mm)
between the guard ring and the mesa edge were introduced to reduce
the surface electric field strength at the silicon/insulator interface to
prevent premature breakdown along the device perimeter.
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4. Measurement results
Ⅰ. Dark current
When the reverse bias increase, not
only the gain becomes large but also
the dark current increases.
It is because of (1) junction leakage
current (generation and recombination)
and (2) tunneling current.
T e L
The breakdown voltage is -25V, and
here at the dark current of 10uA.
All these measurements are supported
at 1300nm wavelength.
7
4. Measurement results
Ⅱ. Excess noise factor
& Gain-Bandwidth product
1
F(M) kM (2 )(1 k )
M
After measurement of excess noise
factor, the k value is calculated about
to 0.09 by using above equation.
All measured devices had a gain–
bandwidth product over 300 GHz. The
highest gain–bandwidth product
obtained was 340 GHz.
The 3dB BW was measured using
Agilent 8703A Network Analyzer. The
bandwidth is limited by RC and transit
time effect.
As the gain is increased beyond 20,
the bandwidth dropped owing to the
avalanche build-up time effect.
8
4. Measurement results
A gain of 10
& -20dBm input optical power
Ⅲ. Receiver sensitivity & BER
APD+TIA+CDR for BER
measurement
A data rate of 10Gb/s
Using a pseudo-random binary
sequence(PRBS) and extinction
ratio(ER) of 12dB.
In this set, the input optical
power(Receiver sensitivity) was
maximun-28dBm.
** Sensitivity in a receiver is normally defined as the minimum input
signal Si required to produce a specified signal-to-noise S/N ratio.
(So, it is a function of the SNR or BER.)
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5. Conclusion
• To improve more,
(1) Reducing the dark current of the APDs.
: Better control of the germanium profile with respect to the electric field
distribution in the device can reduce the tunneling current.
(2) Reducing the value of k_eff.
: Studies have shown that k_eff can be reduced by optimizing the
multiplication region thickness. By this, we believe that a sensitivity of
approximately -32 dB m could be achieved.
• Demonstrate a monolithically grown, CMOS-compatible Ge-Si
SACM APD device with a gain–bandwidth product of 340 GHz
and a k_eff of 0.09 at 1300nm wavelength.
• The optical receivers built with this Ge-Si APDs demonstrated a
sensitivity of -28 dBm at 10Gb/s.
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