Photodiode and photodiode module

Details Photodiode and photodiode module Photodiode and photodiode module

2002-05-14

2004-04-20

Thomas, Tom (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S431000, C257S461000, C257S490000

Reexamination Certificate

active

06724063

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photodiode (PD) and a photodiode module in optical communication networks, in particular, to a photodiode which is immune from the delay of response due to the diffusion of carriers generated at the periphery of the light receiving region. Rays emitting from an optical fiber or a laser diode (LD) are usually converged for entering the center of the PD chip. However, a part of the rays go into the PD chip at the periphery and make carriers (pairs of electron and hole) at the periphery. Since no reverse bias is applied to the peripheral region of the PD chip, the carriers trek to a p-region or an n-region by diffusion and make a delay photocurrent. The delay of response caused by the peripheral carriers is fatal to high-speed optical communication.
This application claims the priority of Japanese Patent Application No.10-174227 (174227/98) filed on Jun. 22, 1998 which is incorporated herein by reference.
2. Description of Related Art
FIG. 1
is a schematic sectional view of a prior front surface incident type photodiode. A p-region is formed by diffusing a p-type dopant in an n-type substrate. The p-type dopant is, for example, zinc (Zn), cadmium (Cd) or magnesium (Mg). Since Zn is most favorably employed as a p-dopant, the problem will be explained on a Zn-doped PD. Of course, a similar problem accompanies a PD which has a p-substrate and an n-region produced on the p-substrate by diffusing an n-dopant. Here, a prior PD having an n-substrate and a p-region made on the n-substrate by diffusion is explained. The PD has an n-type substrate
1
and an n-type light receiving layer
2
piled on the substrate
1
. The n-type light receiving layer
2
has a plurality of epitaxial layers. A p-region
3
is formed by diffusing Zn atoms at the center of the light receiving layer
2
. A pn-junction is formed between the n-type light receiving layer
2
and the p-region
3
. A p-electrode
4
is fabricated on a part or on the whole periphery of the p-region
3
. An n-electrode
5
is produced overall on the bottom of the n-type substrate
1
. The peripheral part of the top surface of the light receiving layer
2
is covered with an insulating protecting layer which is not shown in FIG.
1
. The PD is reversely biased; i.e., a positive n-electrode
5
and an negative p-electrode
4
. The reverse bias makes a depletion layer
6
which lacks carriers, i.e., electrons and holes. The depletion layer
6
is the region between the dotted line w—w and the solid line u—u. The outside of the line w—w is still the n-light receiving layer
2
. The inside of the line u—u is the p-type light receiving region
3
. A PD device has such a PD chip stored in a package.
The definitions of the depletion layer, the p-type region and the n-type region are first clarified. The n-type region has electrons as major carriers and holes as minority carriers. An n-type semiconductor is produced by doping with an n-type dopant into a semiconductor. The p-region has holes as major carriers and electrons as minority carriers. A p-type semiconductor is produced by doping a semiconductor with a p-type dopant. When a p-region is partially made in an n-type semiconductor, an n-region, a p-region and a pn-junction are formed at the same time. The words are not always used in their correct meaning. Their correct definitions are required for explaining the exact significance of the present invention. The product “np” of the electron concentration “n” and the hole concentration “p” is constant in a semiconductor which depends only upon temperature. A pn-junction is a continual curved plane at which the number of free electrons is equal to the number of free holes (n=p), where n is an electron concentration and p is a hole concentration. Both electrons and holes are sparse at the pn-junction because of n=p. At the pn-junction, a p-dopant density Na (acceptor density) is equal to an n-dopant density Nd (donor density). The pn-junction has a voltage drop which is nearly equal to the band gap.
When a reverse bias is applied between an anode (minus) and a cathode (plus), most of the bias voltage is applied to the pn-junction. The reverse bias pulls holes toward the p-electrode (anode) and pulls electrons toward the n-electrode (cathode). The reverse bias sweeps up the carriers from the pn-junction. There are some portions which has dopant levels (donors and acceptors) but has little carriers in the vicinity of the pn-junction. The parts lack of carriers in spite of high density donors or acceptors. The depopulated part near the pn-junction is called a “depletion layer”
6
. Thus, the depletion layer exists both above and below the pn-junction or both at the n-side and the p-side on the pn-junction. The upper depletion layer and the lower depletion layer sandwich the pn-junction. The thickness of the depletion layer increases in accordance with the reverse bias. But the upper depletion layer and the lower depletion layer don't have an equal thickness. Asymmetry of the depletion layer results from the difference of the dopant densities Na and Nd. The thicknesses of the n-side depletion layer and the p-side depletion layer are denoted by s and t. The neutrality condition requires sNd=tNa. Gauss theorem gives V=e(s
2
Nd+t
2
Na)/&egr; at the depletion layer, where e is an electron charge, V is the reverse bias and &egr; is a dielectric constant of the semiconductor. The n-side depletion layer thickness s and the p-side depletion layer thickness t are,
s=[&egr;VNa/{eNd
(
Nd+Na
)}]
1/2
  (1)
t=[&egr;VNd/{eNa
(
Nd+Na
)}]
1/2
  (2)
s+t=[&egr;V
(
Nd+Na
)/{
eNdNa}]
1/2
  (3)
An increase of the reverse bias V enhances both the p-side depletion layer t and the n-side depletion layer s in proportion to the square root of V. The depletion layer widens in both directions from the pn-junction. The pn-junction intervenes in the depletion layer. But each thickness is not equal (s≠t). In the case of the PD having an n-type substrate, the light receiving layer on the substrate is also n-type. The electron concentration is low (about 10
15
cm
−3
) in the n-light receiving layer. But the p-region is produced by doping a high density p-dopant of about 10
18
cm
−3
. The acceptor density Na on the p-side is about thousand times as much as the donor density on the n-side near the pn-junction (Na=Nd). The n-side thickness s is much larger than the p-side thickness t. The p-side depletion layer thickness t is negligible small. The depletion layer mainly expands in the n-type region due to the sparse donors. The boundary on the p-side of the depletion layer is nearly equal to the pn-junction itself. In
FIG. 1
, the curved solid line u—u is inherently the upper boundary of the depletion layer. But the curved solid line u—u is substantially the pn-junction. The other curved dotted line w—w is the lower boundary (n-side boundary) of the depletion layer.
In
FIG. 1
, the cross-hatched part enclosed by the solid line u—u is the p-region
3
. The region outside of the dotted line w—w is the n-type part of the light receiving layer
2
. The region sandwiched by the solid line u—u and the dotted line w—w is the depletion layer
6
.
Signal light is carried by an optical fiber or so in optical communication. The propagating signal light goes out from the end in air and disperses into rays. Some incidence rays go into the central part of the PD chip. The depletion layer
6
absorbs the central rays and makes pairs of electron
8
and hole
7
by the band gap transition. The depletion layer
6
has no carriers. The newly-borne carriers (electrons and holes) cannot collide and recombine with an extra counterpart in the depletion layer
6
. The electrons
8
progress to the n-region (downward) by the reverse bias. The holes
9
make their way to the p-region
3
by the reverse bias. When an electron inva

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