【正文】 er wavelength, as shown in . Shift of the absorption edge of GaAs due to the FranzKeldysh effect. (A) Zerobias condition。 (B) Reverse bias applied to produce a field of V/cm Curve A shows the normal unbiased absorption edge for ntype GaAs with a carrier concentration of 3x1016cm3. Curve B is a calculated FranzKeldyshshifted absorption edge for an applied field of V/cm, which corresponds to 50V reverse bias across a resulting depletion width of ?m. At a wavelength of 900nm this shift corresponds to an increase in ? from 25 to 104 cm1 ? hardly a negligible effect! The physical basis for the FranzKeldysh effect can be understood from the simplified energyband bending model diagrammed in . Energy band diagram illustrating the FranzKeldysh effect. The band bending on the nside of a p+n junction ( or a Schottky barrier junction) is shown for conditions of strong reverse bias In this diagram x represents distance from the metallurgical junction plane, in the region far from the junction where there is no electric field, photons must have at least the bandgap energy (EcEv) to produce an electronic transition as in (a). However, within the depletion region where there field is strong, a transition as in (b) can occur when a photon of lessthanbandgap energy lifts an electron partway into the conduction band, followed by tunneling of the electron through the barrier into the conduction band state. The states at the conduction band edge are, in fact, broadened into gap so as to produce a change in effective bandgap ?E, which is given by 3231* )()(23 ??qmE ???() where m* is the effective mass of the carrier, q is the magnitude of the electronic charge, and ? is the electric field strength. The FranzKeldysh effect greatly improves the sensitivity of a detector operating at a wavelength near its absorption edge. ? Perhaps the greatest advantage of electroabsorption detectors is that they can be electrically switched from a low absorption state to a high absorption state by merely increasing the reverse bias voltage. This makes it possible to make emitters and detectors in the same semiconductor material that are wavelength patible. An example of a device making use of this principle is the emitter/detector terminal shown in . This device performs the dual function of light emitter, when forward biased, and light detector, when strongly reverse biased. An integratedoptic emitter/detector terminal employing the FranzKeldysh effect Fabricated in series with a waveguide structure, as shown, it can act as a send/receiver tap on an optical transmission line. Because of the large change in ? produced by the FranzKeldysh effect, operation can be very efficient. For example, consider the case of a p+n junction diode in ntype GaAs with carrier concentration equal to 3x1016cm3, as before. Application of 50V reverse bias changes ? from 25 to 104cm1 at a wavelength of 900nm. Thus, when forward biased, the diode emits 900nm light into the waveguide. When reverse biased with Va=50V, the diode need a length of only 103?m to absorb % of incident 900nm light. When the diode is on standby at zero bias, ? is just 25cm1. Hence, for a typical laser length of 200 ?m, the insertion loss is only 2dB. Such emitter/detector devices may prove to be very useful in systems employing waveguide transmission lines because they greatly simplify coupling problems, as pared to those encountered when using separate emitters and detectors. Factors Limiting Performance of Integrated Detectors ? In the design of an integrated optical detector, there are a number of mechanisms that can limit performance in various ways. Not all of these are important in every application. However, the designer (or user) should be aware of the limitations associated with different device types and geometries. High Frequency Cutoff ? A number of the factors that can limit high frequency response are summarized in Table . Because of the small area of waveguide photodetectors of the type shown in , the RC time constant, which most often limits the response of conventional diodes, can be made small enough to allow frequencies of operation well above 10 GHz, as discussed in . In this case, other potentially limiting effects must be considered. Table Factors limiting high frequency response of a depletion layer photodiode ? RC time constant due to bulk series resistance and junction capacitance ? Carrier diffusion time from regions outside of the depletion layer ? Carrier lifetime and diffusion length ? Capacitance and inductance of the package ? Carrier drift time across the depletion layer ? Carrier trapping in deep levels ? Linearity ? Noise Problems ? 1 We wish to use a photodiode as a detector for a signal of 900nm wavelength. Which would be the best choice of material for the photodiode, a semiconductor of bandgap = eV, bandgap=2 eV, or bandgap=1 eV? Why? (assume all three are direct gap and are equivalent in impurity content, etc.) ? 2 To improve the signaltonoise of the diode in Problem , we wish to use a semiconductor lowpass filter which has the following absorption properties at room temperature: for 900 nm radiation, ?=。 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.