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12-12-08 - Avalanche Photodiode am 3 me Wilmer ioniziag...

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Unformatted text preview: Avalanche Photodiode am? 3 me Wilmer; ioniziag miiifim am?!“ nemesis} swé fiffi gag stag l. impact ionization creates additional e-h pairs 2. strong B field (>107 V/m) needed to overcome competition with non-ionizing collisions 3. large potential difference (>100 V) needed to obtain sufficient gain 4. solid—state analog of photomultiplier tube Structure of APD l. charges generated in “intrinsic” region are efficiently swept out by B field and injected into high-field region 2. this structure preferred because electrons more efficient that holes in ionization 3. current multiplied by factor M APD responsiVity Reverse Bias in APD 1. linear regime: signal proportional to incident light power 2. Geiger regime: signal pulse independent of number of photons absorbed (used to count photons) 3. temperature control needed to stabilize gain. Signal-to-Noise Ratio electrical signal power SNR= . . __¢.lectnca1 99.136 power. t 3:" ¢ , ,, . :2 Electrical signal P .. h. 3 i_« ~§fi3(?aM i mg w 233; m ( m f“? > RE Pym: E EMERL m Eafii + @531; shot noise electrical power PM m w m 4;, ST 8 thermal noise electrical power SNR 3% PP? _ . W ; ._.i& ...... PM + 233mm ., _. cacti Mama; a» 4kg? .5 Where i3. 5 {Rmfhzsjrgmg Limiting cases for SNR SNR m 1. Large signal i; }}Eg} ~ » rim; 3 V3- ENE-13:: EAR? 2 “if?" V? 3 kng3 2&338 R 1; 2&3 -shot noise from signal current dominates 2. Small signal, large RL i; :3: 123:; i333 1‘3 Vi: -thermal noise from RL dominates °often encountered in practice °tradeoff of SNR and time response F igure-of—merit for detector noise Noise equivalent power (NEP): Incident optical power which gives SNR=,1, . when limited by dark current shot noise, 1% {N EPWWE 1/ in!) QE?AE often optical power given in units of dBm: epiieei pewer iii}; tiBw1—- M It) ieg111 (1 111,111.) Since current density J is the fundamental quantity, write as 111 WE?“ m b“ ‘ fiflfifl “ Figure-of—merit is i? ,1- W fieiw in“... — . .N 1 " :5; L! as: A n “13 :1 {3: mg: > D* smaller for longer 7L detectors because dark current larger for smaller bandgap > D* increases at low temperature because dark current decreases > D* varies with 9» because the absorption coefficient (and nabs) varies with A. , Detector Circuits High impendance amplifier F5?” > Best possible SNR when VB=O (no dark current) > Same bandwidth limitations of previous diode circuit Transimpedance amplifier é > Effective RL=0 (virtual ground, opamp input) > No output saturation until V reaches limit of opamp (usually ~ 10 V) out > Improved bandwidth: CF can be much smaller than Cdiode semitransparent Au °no pn junction fast time response (no e-h recombination °thin depletion region (limits efficiency for IR) 'best for UV-Visible, Where e—h pairs efficiently created in V depletion layer Effect of metal-semiconductor contact potential Question: why can’t we extract power from the diode’s internal potential V0? Answer: In thermal equilibrium, the sum of all voltage drops around a closed loop is zero (required by conservation of energy) The voltage drops across the metal-semiconductor contacts add up to cancel the pn junction potential V0 when no external voltage is applied: ...
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