Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2002-08-27
2004-04-20
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S199000, C257S438000, C257S458000, C257S481000, C257S656000
Reexamination Certificate
active
06724018
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a pin-photodiode and an avalanche photodiode for sensing near-ultraviolet to blue rays. A photodetector is a sensor which receives light and generates a photocurrent output which is in proportion to the power of input light. A photodiode and a phototransistor having pn-junctions are photodetectors as solid state devices. A photoconductive device without a pn-junction senses the light power by photoconductive effects. Besides the semiconductor devices, there are a phototube or a photomultiplier photodetectors which make use of vacuum tubes.
The present invention relates to a near-ultraviolet-blue photodiode having a pn-junction.
This application claims the priority of Japanese Patent Applications No. 2001-270031 filed on Sep. 6, 2001 and No. 2001-324341 filed on Oct. 23, 2001, which are incorporated herein by reference.
Several kinds of photodiodes have been made and practically used for detecting light of various wavelength ranges. A purpose of the present invention is to provide a photodiode for sensing near-ultraviolet to blue rays of a wavelength between 460 nm and 300 nm. A more concrete purpose is to provide a photodiode for detecting light of a wavelength of 400 nm of HD-DVDs (high definition digital video disc) which will be manufactured and will be sold on the market in the near future.
This invention relates also to a blue-ultraviolet avalanche photodiode (APD) for sensing blue, violet and ultraviolet rays of a wavelength range between about 460 nm and 300 nm. An avalanche photodiode is a high-sensitive photodetector of avalanche-amplifying photocarriers by applying a high reverse bias slightly below a breakdown voltage upon a pn-junction. There has been no blue-ultraviolet avalanche photodiode till now. This invention gives such a blue-ultraviolet avalanche photodiode for the first time.
2. Description of Related Art
A CD (compact disc) and an MD (medium disc) have been widely employed as a recording media of music, movies or data. A CD player has a GaAs-type laser diode which emits near-infrared rays of a 780 nm long wavelength. Sales of the DVD players are going up at a rate of 300% a year. The present DVD players use another GaAs-type laser diode which emits red rays of 650 nm for reading-out data from DVD discs. Video-recorders which make use of rewritable DVD discs are on the market from the end of 1999. The shorter the wavelength of the light source of reading-out data from a DVD disc is, the drastically the data amount which can be stored in the DVD disc increases. Future progress of DVDs ardently desires still shorter wavelength light sources than the 650 nm GaAs-type lasers.
If a blue-violet laser diode of GaN (gallium nitride)-type is put on the market, the short wavelength light of 400 nm will be available for the light source of reading out data of DVDs.
In accordance with the appearance of 400 nm-emitting GaN-LDs on the market, commencement of sales of HD-DVDs which can record HD-TV for two hours is scheduled. Table 1 shows information capacity, recording hours, initially-manufactured years, colors of reading-out light source lasers, emission wavelengths of the lasers, materials of the lasers, kinds of photodetectors for the CD (compact disc), DVD (digital video disc), and HD-DVD (high definition digital video disc).
TABLE 1
Information capacity, recording hours, initially-manufactured years,
colors of reading-out light source lasers, emission wavelengths and
materials of lasers, kinds of photodetectors for CD, DVD, and HD-DVD
CD
DVD
HD-DVD
Information capacity
640 MB
4.7 GB
22.5 GB
Recording hours
Present TV
Present TV
HD-TV
15 minutes
2 hours
2 hours
Initially-
From 1982
From 1997
From 2002
manufactured year
Color of reading-out
Near-infrared
Red
Blue-violet
light source laser
Emission wavelength
780 nm
650 nm
405 nm
Material of laser
GaAs
GaAs
GaN
Kind of
Si-PD
Si-PD
?
photodetector
In the materials on the table, the listed “GaAs” means not the light emitting layer (active layer) but the material of the substrate. Another listed “GaN” is neither the material of the active layer nor the substrate but the material of typical films (buffer layer, contacting layer, or cladding layer). The “GaN”-type laser diode has a sapphire as a substrate and an InGaN layer as an active layer.
The 400 nm HD-DVDs which will be put on the market in 2002 can enhance the memory density by five times as much as the prior 650 nm-DVD. The HD-DVDs will have a 22.5 GB memory capacity which enables a DVD player to record HD-DVD movies for two hours.
An avalanche photodiode is a semiconductor photodetector having an avalanche amplification function which applies a strong reverse bias to a pn-junction, making a strong electric field in depletion layers and the pn-junction, accelerates photocarriers generated by light at the depletion layers, inducing reciprocal collisions of the accelerating photocarriers to lattice atoms, and generating new carriers by the collisions. The avalanche photodiode is an excellent photodiode having a built-in amplification function in itself.
A sensitivity range of a photodiode, in general, is determined by bandgaps of a material of a light receiving layer. A photodiode has a peak sensitivity at an absorption edge wavelength of the material of the light receiving layer. The photodiode has no sensitivity for the light having a longer wavelength than the absorption edge wavelength. The photodiode has sensitivity for the light having a smaller wavelength than the absorption edge wavelength. But, the sensitivity decreases as the wavelength becomes far shorter than the absorption edge wavelength. The tendency is common to the avalanche photodiode (APD). A Si-APD (visible) and a Ge-APD (infrared) are actually on the market. Various type InP-APDs have been proposed (for example, Japanese Patent Laying Open No. 60-198786(198786/'85) and Japanese Patent Laying Open No. 2-262379(262379/'90). But, the InP-APDs (infrared) are unstable and unreliable yet.
A Si-APD has sensitivity for visible light and near-infrared rays of wavelength between 500 nm and 900 nm. A Ge-APD has sensitivity for near-infrared rays. Thus, the Ge-APD and the Si-APD can cover the wavelength range from visible to near-infrared rays. An InP-APD with an InGaAs sensing layer can sense a wavelength range from 1200 nm to 1650 nm. However, the Ge-APDs and the InP-APDs are not put into actual use unlike the Si-APDs.
The Si-APD is made upon a p-type Si substrate unlike other silicon devices which are made on n-type Si substrates. The Si-APD is produced by piling a thin p-Si layer upon the p-Si substrate, making an n-region by thermally diffusing n-type dopant into the p-layer, and making an n-type Si guardring around the n-region by thermal diffusion for stabilizing electric field distribution at a periphery. The Si-device made on the p-type substrate is quite strange, since other Si devices are all made upon n-type Si substrates. Silicon is an ideal semiconductor in which a p-region can be made as easily as an n-region and holes have a similar mobility and an effective mass to electrons. Other semiconductor materials are asymmetric regarding a p-region and an n-region, and a hole mass and an electron mass, and a hole mobility and an electron mobility and so on.
Ge devices which are, in general, suffering from a loose pn-junction has large leak current and big dark current flowing across the pn-junction. An application of a large reverse bias induces a large increase of the dark current in a Ge-APD. Thus, the Ge-APDs and InP-APDs are not in practical use. The Si-APD is a unique practical avalanche photodiode still now.
The matter of the present invention is not a laser diode as a light source of irradiating DVDs for reading-in data but a pin-photodiode and an avalanche photodiode of sensing the reflected reading out light. Inexpensive GaN-type (InGaN) laser diodes will be manufactured at low cost on mass scale as light sources in near future. The progress of photodiodes for sensing the irradiated data with
Ando Koshi
Nakamura Takao
Louie Wai-Sing
Pham Long
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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