Performance of Optical Receiver Using Avalanche Diodes

Abstract

Avalanche photodiodes APO's with a variety of structures have been used as detectors in fiber optic communication systems.

A photodetector is an optoelectronic device that absorbs optical energy and converts it ·to electrical energy. Which usually mani rests as a photocurrent. There are generally three steps involved in the photocletection process: (I) .. absorption of optical energy and generation of carriers, ( 2) transportation of

the photogenerated carriers across the absorption and/or transit region, with or without gain, and (3) carriers collection and generation or a photocurTent, which flows through external circuitry.

The two main types of detectors are PIN photodiocles, and avalanche photodiodes. PIN photodiodes have no internal gain but can have very large bandwidths.

The p-i-n (or PIN) photodiode is a junction diode in which an undoped i­ region is inserted between p+ and n + regions. Since there is no internal optical gain associated with the operation of a PIN diode (unity gain device), the maximum internal quantum ef!lciency 'lois I 00% and the gain­ bandwidth product is equal to the bandwidth. By careful choice of material parameters and device design, very large bandwidths can be attained. Avalanche photodiode have internal gain, where large gain can be obt1inecl in an avalanche photodiocle (APO). The device is essentially a re\ erse­ biasecl p-n junction that is operated at .a voltage close to the breakdmvn voltage. Photogenerated earners m the depletion region travel at their saturation velocities, and if they acquire enough ener-gy !'rom the field during such transit, an ionizing collision with the lattice can occur. The lield necessary to produce an ionizing collision is in the range of JO-t to 105 V/cm, depending on the material. Secondary electron-hole pairs are produced in

VII

Summury

the process, which agam drift in opposite directions, together with the primary carrier, and all or some of them may produce nevv carriers. The process is known as impact ionization, which leads to carrier multiplication and gain. Depending on the semiconductor material and device design, very large avalanche gain (- 200 or more) cai1 be achieved. and the avalanche photodiode therefore exhibits very high sensitivity. The avalanche process is itself asymmetric (i.e., the probability for initiating an avalanche is usually greater for one type of caiTier than for the other). For example, in si I icon, ionizing collisions are 30-50 times more ti·equent \Vith electrons than with holes. This asymmetry is characterized by the ratio /a, where a and 13 are the impact ionization coefficients for electrons and holes, respectively. Avalanche multiplication noise is lowest in devices in which the avalanche process is initiated by the carrier with the higher ionization rate. In an avalanche photodiode, the process of impact ionization occurs in the high­ field depletion region. The actual energy I ost by the p1·i mary carrier is the threshold ionization energy. To study the avalanche process, the concepts of ionization and ioniz::ttion coefficient must be understood. Impact ionization coefticient for electron and holes, a and ' which characterize the avalanche process are fundamental material parameters. The coefficients a1·e the reciprocal or the average distance traveled by electrons and holes in the direction of the electric field to create the electron-hole pairs. a and B can also be dennec.l as

the average number of ionization collisions per unit length (i.e the number of secondaries). If \V is the width of the region in which avalanche multiplication takes place (which COUld be the \Vidth OJ' the depletion region of the photodiode). Then a\V is the total number ol'.ionizing collisions. In many optical system, avalanche photodiodes (A PO's) are preferred over p-i-n detectors because of the gain they prO\ ide. Unfonunately, this g 1in is