Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2001-02-26
2004-09-14
Kang, Donghee (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S436000, C257S437000, C257S440000, C250S214100, C438S048000, C438S072000, C438S636000
Reexamination Certificate
active
06791152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photodetector device and a method for manufacturing the same, and more particularly, to a photodetector device capable of high speed response at low driving voltages, and a method for manufacturing the same.
2. Description of the Related Art
Optical data communications, i.e., optical interconnections, require optical signal sending and receiving portions, i.e., light emitters to produce an optical signal, and photodetectors to detect an optical signal. As the need for high speed transmission increased, optical interconnections, which are applicable for communications through local area networks (LANs), computer-to-computer, computer-to-peripheral device, board-to-board, and chip-to chip, have replaced conventional copper wire. To meet the requirements for high speed transmission, an optical interconnection requires a rapidly responsive photodetector, which is operable with low voltages.
However, a common photodetector formed of a compound semiconductor, which is used for high response speed, cannot satisfy both the need for high response speed and the need for a low driving voltage in optical interconnections for the following reasons.
For p-i-n photodetectors, which are extensively used, electrons and holes are generated in an intrinsic semiconductor material layer by the absorption of light incident through a light-receiving surface. The response characteristics of a p-i-n photodetector is determined by the duration of time required for the thusly generated electrons and holes to reach n-type and p-type semiconductor material layers and by the capacitance, which is a function of the area of the photo-receiving surface and the thickness of the intrinsic semiconductor material layer. In other words, the higher the transport speeds of the electrons and holes and the lower the capacitance, the higher the response speed.
The critical speed of electrons and holes is 1.0×10
5
m/s for a Silicon (Si) layer and 1.4×10
5
m/s for a Gallium Arsenide (GaAs) layer. Accordingly, GaAs is the preferred photodetector material, relative to Si, with respect to response speed. However, the difference in critical speed between GaAs and Si is not substantial. Thus, the response characteristics of a p-i-n photodetector are markedly influenced by the capacitance of the device.
One of the determinative factors for the capacitance of a p-i-n photodetector is the intrinsic characteristics of the material used to form the photodetector. For example, a photodetector formed of Si has a lower capacitance than one formed of GaAs due to the relatively low dielectric constant of 11.9 for Si, relative to the dielectric constant of 13.1 for GaAs.
Another factor which determines the capacitance of a p-i-n photodetector is the thickness of the intrinsic semiconductor material layer. The capacitance of the photodetector is inversely proportional to the thickness of the intrinsic semiconductor material layer. In particular, for a general p-i-n photodetector formed of Si, the intrinsic semiconductor material layer must be as thick as 20-25 &mgr;m, due to the low absorbency of Si, which has an indirect transition band gap, thereby lowering the capacitance of the photodetector. In contrast, for a p-i-n photodetector formed of GaAs, the intrinsic semiconductor material layer can have a thickness of 3-5 &mgr;m, due to the high absorbency of GaAs, which has a direct transition band gap, thereby increasing capacitance.
Thus, a p-i-n photodetector formed of Si, which has a thick intrinsic semiconductor material layer, needs high driving voltages in order to facilitate the migration of holes generated along with electrons by light absorption, which migrate much slower than electrons, thereby increasing the response speed. Accordingly, a p-i-n photodetector formed of Si is inappropriate in application fields which require both a high speed response and a low driving voltage. In contrast, a p-i-n photodetector formed of GaAs has a thin intrinsic semiconductor material layer and thus is operable with a low driving voltage. Accordingly, p-i-n photodetectors formed of GaAs are suitable for application fields which require a low driving voltage. The high capacitance of GaAs photodetectors, however, limits their response speed.
Another factor which determines the capacitance of p-i-n photodetector is the photo-receiving area. Capacitance is proportional to the photo-receiving area, and thus capacitance can be lowered by reducing the photo-receiving area. When a GaAS photodetector is used to receive light in a high-frequency band, the photo-receiving area must be further reduced. However, for this case, a restrictive control of allowable error is required for the alignment with an optical axis, thereby increasing the packing and alignment cost.
FIG. 1
shows an example of a conventional p-i-n photodetector. The photodetector is integrated on a n-type substrate
10
beginning with an n-type semiconductor material layer
21
, upon which an intrinsic, i.e., undoped, semiconductor material layer
23
and a p-type semiconductor material layer
25
are stacked in sequence. On the top of the p-type semiconductor material layer
25
, an annular p-electrode
27
is formed of metal. Also, an n-electrode
29
is formed on the underside of the substrate
10
.
The photodetector has a mesa
20
, which is etched around the outside of the p-electrode
27
to a depth which extends just inside the n-type semiconductor material layer
21
. The back side of the substrate
10
is lapped to have a desired thickness. In order to avoid the occurrence of dark current, the exposed sidewalls of the mesa
20
are covered with an insulating layer (not shown) or polyimide (not shown). For the conventional p-i-n photodetector, the top of the p-type semiconductor material layer
25
inside the p-electrode
27
serves as a photo-receiving surface
25
a.
Another conventional p-i-n photodetector is shown in FIG.
2
. In this case, the p-i-n structure is implemented on an n-type substrate
10
by diffusion. For convenience and clarity, corresponding layers having like structures and functions are denoted by the same reference numerals as in FIG.
1
. Reference numeral
26
represents an insulating layer between the p-electrode
27
and the intrinsic semiconductor material layer
23
.
In such conventional p-i-n photodetectors, as a reverse bias voltage is applied between the p-electrode
27
and the n-electrode
29
, incident light enters the p-type semiconductor material layer
25
through the photo-receiving surface
25
a
and is absorbed in the intrinsic semiconductor material layer
23
to produce electron and hole pairs. The electrons migrate toward the n-electrode
29
, while the holes migrate toward the p-electrode
27
, so that a current is output in proportion to the amount of received light.
When Si is used to manufacture p-i-n photodetectors having the above configurations, a high driving voltage is needed, which limits application to, for example, optical interconnection, which requires a low driving voltage. On the other hand, if the above p-i-n photodetectors are manufactured using GaAs, designing a pre-amplifier IC for amplifying the detection signal of a photodetector becomes complicated due to the photodetector's relatively high capacitance, thereby increasing the manufacturing cost. In addition, when a smaller photo-receiving area is required to receive high-frequency light, the packaging and optical alignment costs increase.
On the other hand, conventional approaches have suggested a resonator type photodetector, as shown in
FIG. 3
, which is operable with low driving voltages even when the photodetector is manufactured using relatively low absorbency Si. Like a resonator, the conventional resonator type photodetector shown in
FIG. 3
includes a first distributed Bragg reflector (DBR)
100
on the p-type semiconductor material layer
25
, and a second DBR
101
between the intrinsic material layer
23
and the n-type semiconductor material
Chang Dong-hoon
Hwang Woong-lin
Kim Jun-young
Kang Donghee
Samsung Electronics Co,. Ltd.
Sughrue & Mion, PLLC
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