4.2. COMMON PHOTODETECTORS 139
Figure 4.5: (a) Ap–i–nphotodiode together with the electric-field distribution under reverse
bias; (b) design of an InGaAsp–i–nphotodiode.
layer consists of nearly intrinsic material, such a structure is referred to as thep–i–n
photodiode. Figure 4.5(a) shows the device structure together with the electric-field
distribution inside it under reverse-bias operation. Because of its intrinsic nature, the
middlei-layer offers a high resistance, and most of the voltage drop occurs across it.
As a result, a large electric field exists in thei-layer. In essence, the depletion region
extends throughout thei-region, and its widthWcan be controlled by changing the
middle-layer thickness. The main difference from thep–nphotodiode is that the drift
component of the photocurrent dominates over the diffusion component simply be-
cause most of the incident power is absorbed inside thei-region of ap–i–nphotodiode.
Since the depletion widthWcan be tailored inp–i–nphotodiodes, a natural ques-
tion is how largeWshould be. As discussed in Section 4.1, the optimum value ofW
depends on a compromise between speed and sensitivity. The responsivity can be in-
creased by increasingWso that the quantum efficiencyηapproaches 100% [see Eq.
(4.1.5)]. However, the response time also increases, as it takes longer for carriers to
drift across the depletion region. For indirect-bandgap semiconductors such as Si and
Ge, typicallyWmust be in the range 20–50μm to ensure a reasonable quantum effi-
ciency. The bandwidth of such photodiodes is then limited by a relatively long transit
time (τtr>200 ps). By contrast,Wcan be as small as 3–5μm for photodiodes that use
direct-bandgap semiconductors, such as InGaAs. The transit time for such photodiodes
isτtr∼10 ps. Such values ofτtrcorrespond to a detector bandwidth∆f∼10 GHz if
we use Eq. (4.1.9) withτtrτRC.
The performance ofp–i–nphotodiodes can be improved considerably by using a
double-heterostructure design. Similar to the case of semiconductor lasers, the middle
i-type layer is sandwiched between thep-type andn-type layers of a different semicon-
ductor whose bandgap is chosen such that light is absorbed only in the middlei-layer.
Ap–i–nphotodiode commonly used for lightwave applications uses InGaAs for the
middle layer and InP for the surroundingp-type andn-type layers [10]. Figure 4.5(b)