Journal of Optical Communications 18 (1997) 3 99 J. Opt. Commun. 18 (1997) 3, 99-103 è Journal of Optical Communications © by Fachverlag Schiele & Sch n 1997 Frequency Response and Gain of Multiquantum Well (MQW) Avalanche Photodiode Yousef Zebda, Omar Qasaimeh Summary In this paper, we investigate the effect of the carrier trans- port mechanism between the bound and the continuum energy states on the frequency response and the gain of Multiquantum Well (MQW) avalanche photodiode. It is clear that the frequency response is a strong function of the LO phonon scattering rate and the carrier capture rate. The calculated 3 dB bandwidth for the photodiode considered in this analysis is 14 GHz for LO phonon scattering time (T sc ) equal 1 ps, while the 3 dB band- width is dropped to 9.5 GHz if the LO phonon scatte- ring time is increased to 3 ps. An expression of the effec- tive impact ionization rate, and the effective generation rate are derived and it is shown to be a function of the operating frequency, the phonon scattering rate, and the continuum recombination rates. 1 Introduction The impact ionization coefficient ratio (oc/ ) is equal unity for most III-V bulk semiconductors. Enhancement of á/â ratio in avalanche photodiode leads to a better performance in terms of noise, gain, and speed of ope- ration. Lately, several reports 1-5] are directed to verify experimentally the enhancement of the á/â ratio in GaAs/AIGaAs multiquantum wells. Juang et al. [5] mea- sured and observed an enhancement in á/â ratio for GaAs/AIGaAs multiquantum wells with well and bar- rier widths of 50 nm. The electron ionization rate á sig- nificantly increases in avalanche photodiodes made from InGaAs/InAIAs multiquantum well in comparison with bulk InGaAs. The enhancement of á/â ratio in InGaAs/InAIAs MQW avalanche photodiodes increases the gain-bandwidth product. The frequency response of avalanche photodiode made from bulk material was calculated by Emmons [6] and Chang [7]. Kahraman et al. [8] have developed a nume- rical method to solve the coupled transport equation for both electrons and holes in arbitrary structure and super- lattice multiquantum well avalanche photodiodes. In their analysis the ionization rates are localized to the bandgap transition regions. Ja Woong Lee et al.[9-10] analyze the impulse response of extremely shallow quan- tum wells (ESQW) p-i-n photodiode. Their analysis was given in terms of LO phonon scattering rate in the well and the carrier transport coefficient in the continuum sta- tes. In this paper, we present an analysis of the frequency response and the gain of MQW avalanche photodiode in terms of LO phonon scattering rates. In this analysis the impact ionization rates, and the generation rate are being function of distance in the structure. 2 Analysis 2.1 Frequency response analysis In multiquantum wells (MQW), carriers are characteri- zed by the energy state in which they are located, eit- her in the continuum energy state or in the bound ener- gy state. In the MQW avalanche photodiode, impact ionization occurs in a discrete location at the bandgap transition regions [11-12]. This process occurs when the carriers (which are in the continuum energy state) gain sufficient energy that is capable of generating an addi- tional carriers from the bound energy state. Because of this, the mechanism of the carriers movement between the continuum and the bound state become a major fac- tor in limiting the values of the effective impact ioniza- tion rates, and therefore, the frequency response of the MQW avalanche photodiode. In the bound state, carriers are generated mainly by two process either the incident light generation rate or the impact ionization process. The vast majority of the generated electron-hole pairs are rapidly moved to the continuum state by LO pho- non scattering process, while a little of them are recom- bined. In the continuum state, most of the carriers are drifting in the electric field with a saturated velocity. Some of the drifted carriers may be captured in the bound state, while a little of them are recombined. The electrons continuity equation in the continuum state is given as: Address of authors: Electrical Engineering Department Jordan University of Science and Technology Irbid, Jordan Received 13 October 1995 Brought to you by | University of Glasgow Library Authenticated Download Date | 6/27/15 7:14 AM