Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With particular dopant concentration or concentration profile
Reexamination Certificate
2001-07-02
2004-03-16
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With particular dopant concentration or concentration profile
C257S079000, C257S094000, C257S096000, C257S097000
Reexamination Certificate
active
06707074
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor light-emitting device in a triode configuration such as a light-emitting diode device or a semiconductor laser device and to an apparatus for driving the same.
Light-emitting diode devices have been used widely as low-cost and high-reliability light-emitting devices in remote control equipment and optical fiber communication.
However, conventional light-emitting diode devices have the problems of low response speed and low upper-limit modulation frequency in performing high-speed communication, i.e., high-speed modulation.
Factors that limit the operating speed of a semiconductor light-emitting device represented by a light-emitting diode device include the speed at which carriers injected in the active layer are recombined. The carriers injected in the active region of the light-emitting device do not disappear immediately after current injection is halted but disappear gradually in accordance with a time constant determined by the recombination speed.
Since the light-emitting state continues while the carriers remain in the active region, the carriers remaining in the active region prevent high-speed response of the light-emitting device during modulation. Since the light-emitting diode device utilizes spontaneous light emission and the amount of light emitted therefrom is nearly proportional to the quantity of carriers in the active region, the remaining carriers exert particularly great influence on the response speed of the light-emitting diode device. In a light-emitting diode device composed of a Group III-V compound semiconductor containing, e.g., aluminium gallium arsenide (AlGaAs) as a main component, the time constant determined by the carrier recombination speed is normally several nanoseconds (ns) so that it is difficult to perform high-speed modulation at a modulation frequency exceeding 1 GHz.
As prior art technology for eliminating the limit placed by the carrier recombination speed on the modulation speed, a light-emitting device using a triode configuration similar to that of a transistor device is disclosed in Japanese Unexamined Patent Publication No. SHO 60-167390.
FIG. 17
shows a cross-sectional structure of the triode light-emitting device disclosed in the publication.
As shown in
FIG. 17
, the semiconductor light-emitting device disclosed in the publication comprises a p-type collector layer
902
, an n-type base layer
903
, and a p-type emitter layer
905
formed successively on a p-type semiconductor substrate
901
, similarly to a bipolar transistor.
An active layer
904
is provided between the base layer
903
and the emitter layer
905
. The active layer
904
is surrounded by an n-type buried layer
907
formed in the peripheral region thereof.
An emitter electrode
909
is formed on the emitter layer
905
with a p-type contact layer
906
interposed therebetween. A base electrode
910
is formed on the buried layer
907
with an n-type contact layer
908
interposed therebetween so as to surround the emitter electrode
909
. A collector electrode
911
is formed on the surface of the semiconductor substrate
901
opposite to the collector layer
902
.
A description will be given herein below to the operation of the conventional semiconductor light-emitting device.
FIG. 18
shows the structure of electron energy bands in the conventional semiconductor light-emitting device during a light-emitting period, in which the vertical axis represents the energy of electrons and E
C
, E
V
, and E
F
generally represent energy at the lower end of the conduction band, energy at the upper end of the valence band, and the energy of electrons or holes on a quasi-Fermi level, respectively. The reference numerals associated with the energy levels correspond to the semiconductor layers shown in FIG.
17
.
As an example of driving voltage applied during the light-emitting period, a voltage in a forward direction (forward bias voltage) is applied between the base layer
903
and the emitter layer
905
such that the base layer
903
and the collector layer
902
are set at an equal potential of 0 V.
Since the forward bias voltage is applied between the base layer
903
and the emitter layer
905
, electrons injected from the base layer
903
and holes injected from the emitter layer
905
are accumulated in the active layer
904
and recombined to emit light. Although a depletion layer is formed between the p-type collector layer
902
and the n-type base layer
903
due to the pn junction, at least a part of the base layer
903
is not depleted so that the electrons are supplied from the undepleted portion to the active layer
904
. The base layer
903
functions as a barrier for confining the holes to the active layer.
During a light-extinct period, a voltage in a reverse direction (reverse bias voltage) is applied between the base layer
903
and the collector layer
902
. This depletes substantially the entire region of the base layer
903
, as shown in the energy-band diagram of
FIG. 19
, so that the holes confined to the active layer
904
are extracted to the collector layer
902
. If the holes can be extracted from the active layer
904
with sufficiently high efficiency, the concentration of the holes in the active layer
904
is reduced so that the quantity of carriers recombined for light emission is reduced and light emission is suppressed. Since the hole extracted operation is not dependent on the speed carrier recombination for light emission, light emission can be halted promptly so that high-speed modulation is allowed.
As a result of conducting various studies on the conventional semiconductor light-emitting device in the triode configuration, the present inventors have found the problem that, if low-voltage driving is performed during a light-extinct operation, some of the holes remain in the active layer
904
and emitted light remains even during the extinction period. Briefly, it is difficult to achieve a high extinction ratio, which is the ratio between the amount of light during the light-emitting period and the amount of light during the extinction period.
FIG. 20
shows in enlarged relation a band structure at the upper end of the valence band in the active layer
904
and its vicinity in the conventional semiconductor light-emitting device during the extinction period. As shown in
FIG. 20
, an interface barrier (spike)
920
occurs between the active layer
904
and the base layer
903
during the extinction period due to the offsetting of the valence band caused by the heterojunction therebetween. Even if the absolute value of the potential of the reverse bias voltage applied to the collector layer
902
is increased, the height of the interface barrier
902
(the magnitude of energy) does not change, which forms an obstacle to the extraction of the holes to the collector layer
902
. Although some of the holes move toward the collector by surpassing the interface barrier
902
with the reverse bias voltage, holes with energy lower than the height of the interface barrier
902
remain at the interface between the active layer
904
and the base layer
903
. If a higher reverse bias voltage is applied, some of the holes with lower energy are transported by a tunnel current to the collector layer
902
but the reverse bias voltage with the higher absolute value also increases the amount of heat generated from the device as well as power consumption.
At this time, the holes are supplied from the emitter layer
905
to the active layer
904
so that, if the concentration of the holes is increased at the interface between the active layer
904
and the base layer
903
, the quantity of holes accumulated in the entire active layer
904
is increased. In the conventional semiconductor light-emitting device, therefore, it is difficult to sufficiently reduce the quantity of holes in the active layer
904
with a low reverse bias voltage and a considerable amount of light is emitted from the active layer
904
even during the extinction period.
Thus, it is difficult
Ohnaka Kiyoshi
Yoshii Shigeo
Flynn Nathan J.
Mondt J. P.
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