823 A Model for Dark Current and Multiplication in HgCdTe Avalanche Photodiodes Journal of ELECTRONIC MATERIALS, Vol. 29, No. 6, 2000 (Received September 22, 1999; accepted November 24, 1999) Special Issue Paper 823 INTRODUCTION Avalanche photodetectors (APDs) provide increased sensitivity when compared to simple p-i-n photo- diodes due to the enhancement of the detected signal by internal gain. For low excess noise factor and improved high-frequency performances, it is advan- tageous to have carriers with highly unequal ioniza- tion coefficients. 1 Because of the large difference in the ionization rates for electron and holes, silicon avalanche photodiodes represent the ideal detector choice for wavelengths below 1.06 μm. For longer wavelengths, necessary to exploit the low-dispersion and the low transmission loss windows at 1.3 μm and 1.55 μm of silica optical fiber, the research has been focused on ternary and quaternary III-V mixed crys- tals: InGaAsP, InGaAs/InP, AlGaAsSb. 2,3 With few A Model for Dark Current and Multiplication in HgCdTe Avalanche Photodiodes S. VELICU, 1,2 R. ASHOKAN, 1 and S. SIVANANTHAN 1 1.—University of Illinois at Chicago, Microphysics Laboratory, Department of Physics, Chicago, IL 60607. 2.—e-mail: silviu@uic.edu In this paper we report the calculated results of the dark current and multiplication factor in MBE grown HgCdTe avalanche photodiodes with separate absorption and multiplication (SAM-APD). The device architec- ture used for this analysis comprises the following layers: p + contact, p junction, n – multiplication, n charge sheet, n – absorber, and n + contact. Various leakage current mechanisms are considered and the generation- recombination term is found to be the dominant one for this device structure. However, experimental reverse bias I-V characteristics re- ported earlier by T. de Lyon et al. shows a large deviation from ideality, which can not be explained in terms of bulk leakage current mechanism. To explain the large difference between experimental and theoretical data we consider that the dominant generation-recombination current is multiplied through impact ionization process. To validate this assump- tion, multiplication is calculated as a function of reverse bias. Electric field profile is obtained and the multiplication is computed using the ionization coefficients and avalanche gain equations. Breakdown voltage is found to be 85 V for room temperature operation in agreement with available data in the literature. The theoretical I-V curves considering multiplication are compared with the experimental ones and a close agreement is found which validate this model. Key words: Avalanche photodiode, HgCdTe, MBE, dark current, multiplication exceptions, these materials have comparable ioniza- tion coefficients for electrons and holes and therefore are unsuitable for applications which require high bandwidth and low noise. HgCdTe is an attractive material for meeting these specifications because of the resonant enhancement of hole ionization coeffi- cient for CdTe fractions between 0.6 and 0.7 when the spin-orbit splitting energy in the valence band (∆) is equal to the fundamental energy gap (E g ). 4 Here, hole ionization rate is enhanced because of a very small momentum transfer in the impact–ionization colli- sion. This leads to a large value for the matrix element of the Coulomb interaction and to a large probability for the associated hole impact ionization process. Also, HgCdTe offers the advantages of tunability over a broad spectral range by choosing the alloy composi- tion, high quantum efficiency and capability for op- eration at room temperature in the cadmium-rich composition range.