Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
2001-04-19
2004-11-30
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S079000, C257S094000, C257S191000, C257S918000
Reexamination Certificate
active
06825500
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a light-emitting thyristor whose luminous efficiency is improved and a self-scanning light-emitting device using such light-emitting thyristors.
BACKGROUND ART
A surface light-emitting thyristors has been disclosed in the Japanese Patent Publication No. 2-14584, and an end-surface light-emitting thyristor in the Japanese Patent Publication No. 9-85985. The fundamental structure of a surface light-emitting thyristor and that of a end-surface light-emitting are substantially the same, and AlGaAs (Al composition is 0.35, for example) layers are epitaxially grown on a GaAs buffer layer formed on a GaAs substrate, for example.
FIG. 1
is a schematic cross-sectional view depicting a fundamental structure of a light-emitting thyristor. As shown in
FIG. 1
, on a p-type GaAs substrate
10
successively stacked are a p-type GaAs buffer layer
12
, a p-type AlGaAs layer
14
, an n-type AlGaAs layer
16
, a p-type AlGaAs layer
18
, and an n-type AlGaAs layer
30
. On the AlGaAs layer
20
provided is a cathode electrode
22
, and on the AlGaAs layer
18
a gate electrode
24
. An anode electrode
26
is provided on the bottom surface of the GaAs substrate
10
.
In this example, a p-type layer, an n-type layer, a p-type layer, and an n-type layer are stacked in this order on a p-type GaAs substrate via a buffer layer. However, an n-type layer, an p-type layer, an n-type layer, and a p-type layer may be stacked in this order on an n-type GaAs substrate via a buffer layer, in this case the uppermost electrode is an anode one, and the bottommost electrode is a cathode one.
In the above-described publications, the inventors of this application have already disclosed a self-scanning light-emitting device structured by arranging such light-emitting thyristors in an array, a self-scanning function thereof being implemented by providing a suitable interaction between neighbored thyristors in the array. The publications have further disclosed that such self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors in the array.
In the light-emitting thyristor having such structure described-above, Al composition is largely varied, for example from 0 to 0.35, at the interface between the GaAs buffer layer and the AlGaAs layer on the buffer layer. Such rapid variation of Al composition causes the turbulence of lattices or the large variation of energy bands at the interface, while the variation of lattice constants is small. As a result, a lattice-mismatching at the interface become large, thereby causing a dislocation. Also, an energy gap at the interface is increased, so that the deformation of energy bands is made large.
Therefore, for the light-emitting thyristor fabricated by growing the AlGaAs layer on the GaAs substrate interposing the GaAs buffer layer therebetween, there are problems such that a device property is degraded due to the increase of a threshold current and a holding current. This is because lattice deffects due to a lattice-mismatching at the interface between the GaAs buffer layer and the AlGaAs layer are induced, and an unclear impurity level is formed at the interface. There are also problems such that an external. quantum efficiency is decreased, resulting in the reduction of the amount of emitted light. This is because defects which serve as “carrier killers” are generated in the vicinity of the interface.
As shown in
FIG. 2
wherein like elements are indicated by like reference numerals used in
FIG. 1
, an n-type GaAs layer
28
may be provided on an n-type AlGaAs layer
20
in a conventional light-emitting thyristor. In this manner, GaAs is used as the material of the uppermost layer for the facility of making ohmic contact with an electrode and the simplicity of material. Since the wave length of emitted light is about 780 nm, the light is absorbed during passing through the uppermost layer (GaAs layer)
28
so that the amount of light to be emitted is decreased.
In order to reduce the light absorption by the GaAs layer
28
, the thickness of the layer is needed to be thinner. However, if the layer is thinner, additional problems are caused. That is, alloying of electrode material and GaAs by a heat processing is required to from an ohmic electrode, and atoms migrate for a long distance during the heat processing, as a result of which the alloyed area of electrode material is reached to the AlGaAs layer
20
under the GaAs layer
28
. This causes the turbulence of crystalline of AlGaAs, resulting in the scattering of light.
FIG. 3
is a graph showing a light absorption spectrum of an n-type GaAs layer at 297K, wherein ordinate designates an absorption coefficient &agr; and abscissa a photon energy. The amount of absorbed light is represented by the following formula.
1−
e
−&agr;t
(
t
; film thickness)
It is noted from this graph that the absorption coefficient for the light of 780 nm wave length is about 1.5×10
4
. Assuming that the film thickness “t” is 0.02 &mgr;m, it is understood that the amount of emitted light is decreased by 3-4% by calculating the amount of absorbed light based on the above formula. The amount of absorbed light will be further reduced, if the turbulence of atomic arrangement is caused due to the fluctuation of film thickness and the alloying, and the variation of composition.
FIG. 4
shows a light-emitting thyristor in which a GaAs buffer layer
12
is provided on a GaAs substrate
10
, and a GaAs layer
28
is used as a topmost layer. In the figure, like element are indicated by like reference numerals used in
FIGS. 1 and 2
.
In general, a light-emitting thyristor having a pnpn structure is considered to be the combination of a pnp transistor
44
on the substrate side and an npn transistor
46
on the opposite side to the substrate, as shown in FIG.
5
. An anode corresponds to an emitter of the pnp transistor
44
, a cathode an emitter of the pnp transistor
46
, and a gate a base of the pnp transistor
46
, respectively. The holding current of the thyristor is determined by the combination of current amplification factors of respective transistors
44
and
46
. In order to decrease the holding current, it is required to increase current amplifying factors &agr; of respective transistors. A current amplifying factor &agr; is given by the multiplication of an emitter injection efficiency &ggr;, a transport efficiency &bgr;, a collector junction avalanche multiplication factor M, and a specific collector efficiency &agr;*. In order to increase an emitter injection efficiency &ggr;, the impurity concentration of the emitter is designed to be higher that of the base.
The diffusion speed of Zn which is a p-type impurity is very fast, so that Zn is diffused into an n-type semiconductor layer to compensate an n-type impurity. Therefore, if Zn concentration of the anode layer (the GaAs layer
12
and the AlGaAs layer
14
) is higher than Si impurity concentration of the n-type gate layer (the AlGaAs layer
16
), then most of Si in the vicinity of the interface between the anode layer and the gate layer is compensated to decrease a transport efficiency &bgr; of the transistor. Also, non-luminescent center is generated, causing the reduction of the luminous efficiency of the thyristor.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a light-emitting thyristor in which the luminous efficiency thereof is improved, the thyristor being fabricated by growing AlGaAs layers on a GaAs buffer layer formed on a GaAs substrate.
Another object of the present invention is to provide a light-emitting thyristor using GaAs for the material of the uppermost layer, in which the luminous efficiency thereof is improved.
Still another object of the present invention is to provide a light-emitting thyristor including Zn impurity in an n-type gate layer, in which the luminous efficiency thereof is improved.
A further object of the present invention is to provide a self-scanning light-emitting
Crane Sara
RatnerPrestia
LandOfFree
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