【正文】 for 700 nm radiation, ? =103cm1. How thick must the filter be to attenuate 700 nm background noise by a factor of 104? By what factor is the signal (at 900 nm) attenuated by a filter of this thickness? Neglect reflection at the surfaces. 。 (b) reverse biased with voltage Va ? In conventional mesa devices, a thin, optically transparent Schottkybarrier contact is often used (rather than a p+n junction) to enhance shortwavelength response, by eliminating the strong absorption of these higher energy photons that occurs in the p layer. In a waveguide photodiode, a Schottkybarrier contact is not needed for improved shortwavelength response because the photons enter the active volume transversely. However, ease of fabrication often makes the Schottkybarrier photodiode the best choice in integrated applications. For example, almost any metal (except for silver) produces a rectifying Schottkybarrier when evaporated onto GaAs at room temperature. Gold, aluminum or platinum are often used. Photodiode masking is adequate to define the lateral dimensions during evaporation, and no careful control time and temperature is required, as in the case of diffusion of a shallow p+ layer. Avalanche Photodiode ? The gain of a depletion layer photodiode (. the quantum efficiency), of either the pn junction or Schottkybarrier, can be at most equal to unity, under normal conditions of reverse bias. However, if the device is biased precisely at the point of avalanche breakdown, carrier multiplication due to impact ionization can result in substantial gain in terms of increase in the carrier to photon ratio. In fact, avalanche gains as high as 104 are not unmon. Typical currentvoltage characteristics for an avalanche photodiode are shown in . Response curves for an avalanche photodiode ? The upper curve is for darkened conditions, while the lower one shows the effects of illumination. For relatively low reverse bias voltage, the diode exhibits a saturated dark current Id0 and a saturated photocurrent Iph0. However, when biased at the point of avalanche breakdown, carrier multiplication results in increased dark current Id, as well as increase photocurrent Iph. It is possible to define a photomultiplication factor Mph, given by nbaphphph VVIIM)(110 ???() and a multiplication factor M, given by 00 dphdphIIIIM??? () where Vb is the breakdown voltage, and n is an empirically determined exponent depending on the wavelength of light, doping concentration, and, of course, the semiconductor material from which the diode is fabricated. For the case of large photocurrent Iph0 Id0 the multiplication factor is given by nba VIRVM ])[(1 1???() where I is the total current, given by phd III ??() R being the series resistance of the diode (including spacecharge resistance if significant). The derivation of () assumes that IRVb. For the case of Id0 and Id being negligibly small pared to Iph0 and Iph, it can be shown that the maximum attainable multiplication factor is given by RnIVMMphbph0??() ? Avalanche photodiodes are very useful detectors, not only because they are capable of high gain, but also because they can be operated at frequencies in excess of 10 GHz. However, not every pn junction or Schottkybarrier diode can be operated in the avalanche multiplication mode, biased near avalanche breakdown. For example, the field required to produce avalanche breakdown in GaAs is approximately 4x105 V/cm. Hence, for a typical depletion width of 3?m, Vb equals 120V. Most GaAs diodes will breakdown at much lower voltages due to other mechanisms, such as edge breakdown or microplasms generation at localized defects, thus never reaching the avalanche breakdown condition. In order to fabricate an avalanche photodiode, extreme care must be taken, beginning with a dislocationfree substrate wafer of semiconductor material. Techniques for Modifying Spectral Response ? The fundamental problem of wavelength inpatibility, which was encountered previously in regard to the design and fabrication of monolithic laser/waveguides structures, is also very significant with respect to waveguide detectors. An ideal waveguide should have minimal absorption at the wavelength being used. However a detector depends on interband absorption for carrier generation. ? Hence, if a detector is monolithically coupled to a waveguide, some means must be provided for increasing the absorption of the photons transmitted by the waveguide within the detector volume. A number of different techniques have been proven effective in this regard. Hybrid Structures ? One of the most direct approaches to obtaining wavelength patibility is to use a hybrid structure, in which a detector diode, formed in a relatively narrow bandgap material, is coupled to a waveguide fabricated in wider bandgap material. The two materials are chosen so that photons of the desired wavelength are transmitted freely by the waveguide, but are strongly absorbed within the detector material. An example of this type of hybrid waveguide/detector is the glass on silicon structure, as shown in . Hybrid waveguide detector featuring a glass waveguide coupled to a silicon photodiode The diode was formed by boron diffusion to a depth of about 1?m into an ntype, 5 cm silicon substrate. A 1?m thick layer of thermally grown SiO2 was used as a diffusion mask. The glass waveguide was then sputterdeposited and silve