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1、1,雪崩光电二极管（APD）探测器,2,The Multiplication ProcessAvalanche Photodiode DesignsAvalanche Photodiode BandwidthAvalanche Photodiode Noise,Avalanche Photodiode Detectors,3,4,5,6,7,8,9,Measured values of ionisation coefficients e and h for some common semiconductor materials, plotted versus (1/E).,The Multip
2、lication Process,10,The Multiplication Process,We may define ionisation coefficients for electrons and holes, e and h respectively, as the probability that a given carrier will excite an electron-hole pair in unit distance. The coefficients increase so rapidly with increasing electric field strength
3、, that it is often convenient to think in terms of a breakdown field, EB, at which avalanche excitation becomes critical, say becomes of the order 105 106 m-1. Graphs of e and h versus electric field are plotted for a number of semiconductors known to be of interest as detector materials. The curves
4、 refer to room temperature. As the temperature increases, the ionisation coefficients decrease, because the greater number of scattering collisions reduces the high-energy tail of the carrier energy distribution and hence reduces the probability of excitation. In some materials e h, in others he, wh
5、ile in gallium arsenide and indium phosphide the two coefficients are approximately the same. The ratiok = h/eis found to lie in the range 0.01 to 100.,11,12,13,14,2022/12/2,15,16,APD Band width,In this section we avoid a detailed analysis of the consequences of sinusoidal modulation of the incident
6、 light but concentrate instead on the response of an APD to an optical pulse. The full theory, which has much in common with the theory of IMPATT and TRAPATT oscillators is complex, so we limit the discussion to the general physical principles and to estimate the order of magnitude of an bandwidth l
7、imitation. In the n+-p-p+ type of APD illustrated previously the overall response is made up of three parts:A) the electron transit time across the drift region, (ttr)e = w2/se,B) the time required for the avalanche to develop, tA,C) the transit time of the last holes produced in the avalanche back
8、across the drift space, (ttr)h = w2/sh.Parts B) and C) represent delays additional to those experienced in a non-avalanching diode.,17,APD Band width,The avalanche delay time, tA, is a function of the ratio of the ionisation coefficients, k.The distance-time diagrams to follow give a graphic illustr
9、ation of this. When k = 0, the avalanche develops within the normal electron transit time across the avalanche region (wA/se). We assume wA 0, the avalanche develops in multiple passes across the avalanche region and at high levels of multiplication, with 0 k 1,tA MkwA/seThe overall response time, ,
10、 then becomes (w2 + MkwA)/se + (w2 + wA)/vshAnd we should expect the (-3dB) bandwidth to be given approximately byf(-3dB) 0.44/ ,18,APD Distance-Time Diagrams,Avalanche build-up shown on distance-time diagrams: a) k = 0, M=16; b) k = 0.37, M = 24,19,20,21,22,23,24,25,26,APD Noise,The value of the no
11、ise factor, F, and its variation with the multiplication factor, M, are clearly matters which bear on the optimisation of the optical receiver. For purposes of system evaluation the approximation:F Mx Has often been used. The index, x, typically takes on values between 0.2 and 1.0 depending on the m
12、aterial and the type of carrier initiating the avalanche. As we will see, F Mx, may be reasonably valid over a limited range of values of M.A theoretical treatment by McIntyre, yields the following more complex expressions. When the multiplication is initiated by electronsFe = Me 1 - (1-k)(Me-1)2/Me2 When holes initiate the avalanche: Fh = Mh 1 + (1-k)/k .(Mh2-1)2/Mh2 ,27,Comparation of the two theoretical curves： F Mx And Fe = Me 1-(1-k)(Me-1)2/Me2,28,2022/12/2,29,
