Min-Hyeong Kim High-Speed Circuits and Systems Laboratory E.E. Engineering at YONSEI UNIVERITY

[ 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 3  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 InP- based 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.

2. Background 4 Ⅰ. SACM APD (separate absorption, charge and multiplication APD) InAlAs-based APDs (Ref.17) InAlAs-based APDs (Ref.18) Si-based APDs (This work)

2. Background 5 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 = Multiplication coefficient M → Excess Noise factor F(M)

6 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.

7 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.  The breakdown voltage is -25V, and here at the dark current of 10uA.  All these measurements are supported at 1300nm wavelength.

8 Ⅱ. Excess noise factor & Gain-Bandwidth product  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. F(M) 4. Measurement results

9 Ⅲ. Receiver sensitivity & BER A gain of 10 & -20dBm input optical power  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. 4. Measurement results ** Sensitivity in a receiver is normally defined as the minimum input signal S i required to produce a specified signal-to-noise S/N ratio. (So, it is a function of the SNR or BER.)

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. 10