High-Speed Photodiodes in Standard CMOS Technology
High-speed Photodiodes in Standard CMOS Technology describes high-speed photodiodes in standard CMOS technology which allow monolithic integration of optical receivers for short-haul communication. For short haul communication the cost aspect is impo
- PDF / 2,689,961 Bytes
- 159 Pages / 453.551 x 680.312 pts Page_size
- 61 Downloads / 218 Views
THE INTERNATIONAL SERIES IN ENGINEERING AND COMPUTER SCIENCE ANALOG CIRCUITS AND SIGNAL PROCESSING Consulting Editor: Mfthammed Ismail. Ohio State Univenitp
DESIGN OF VERY HIGH-FREQUENCY MULTIBATE SWrrCHEO-CAPACITOR CIRCLmS 0. Seng Pan, Mmins, Rui Paulo, Epittiio da Franca, Josi Vol. S67, ISDN: 0-387-26121-4 DYNAMIC CHAKACTERISATiON OF ANAiOGUE-TO-DIGrXAL CONVERTERS Dalfcl, Doinini10 GHz). His approach was to analyze the similarities and differences in optical and electrical systems, which will be briefly investigated in the following subsections.
2.1.1
Electrical and Optical Interconnection - Similarities
At the most basic level, optical and electrical physics are very closely linked. In practice, in both the electrical and optical case, it is the electromagnetic wave that carries a signal through a medium (see figure 2.1). light beam glass
velocity 8
~ 3 x 10 m/s
low-loss coaxial cable low-K dielectric
8
~ 3 x 10 m/s
lossy line
R
8
950 nm. For lower wavelengths on the other hand, λ< 400 nm, excess carriers are generated very close to the photodiode surface. Because typically the surface recombination rate is high then only a small part of the generated carriers contribute to the photocurrent, the usable wavelength sensitivity range of CMOS photodiodes is λ ∈ [400 − 850] nm. For best performance e.g. the highest speed and responsivity, the photodiode should be designed to allow the largest number of photons to be absorbed in
2.5. PHOTODETECTORS - INTRODUCTION
21
depletion regions; in the ideal case photons should not be absorbed until they have penetrated as far as the depletion region, and should be absorbed before penetration beyond it. The relative depth to which photon penetrates is a function of its wavelength (see chapters 3, 4 and 5). Short wavelength light (around blue and violet) are absorbed close to the photodiode surface while those with longer wavelength (infrared) may penetrate 10ths of micrometers deep in the substrate. The values of the absorption coefficient and the corresponding 1/e-absorption depths4 in silicon, are shown in figure 2.9. From this figure we conclude that the difference in absorption coefficient for the two boundaries is very large: α = 7.5 × 102 ÷ 5.5 × 104 cm−1 . As a result, the difference in 1/e-absorption depths for 400 nm and 850 nm light is almost three orders of magnitude.
Figure 2.9: The absorption coefficient α for silicon photodiodes versus input wavelength of the light signal λ.
The light intensity drops exponentially inside silicon: ∂I ∝ αe−αx ∂x
(2.11)
The more light is absorbed in the photodiode, the more excess carriers are generated. We define a parameter G(x) which is the carrier generation rate as 4 The 1/e absoption depth is the depth into the silicon for which the light-intensity is dropped to 1/e of the incidentlight-intensity. This depth is equal to 1/α of the input wavelength
22
CHAPTER 2. SHORT RANGE OPTICAL INTERCONNECTION
a result of the incident light in the unity of time often modelled as: G(x) = Φ0 αe−αx
(2.12)
where Φ0 is the photon flux
Data Loading...
