Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1998-03-05
2002-08-20
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S018000, C359S248000
Reexamination Certificate
active
06437361
ABSTRACT:
This application is based on Japanese Patent Application No. 9-66793 filed on Mar. 19, 1997, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to a semiconductor device and a method of using the device, and more particularly to a semiconductor device having a quantum well structure and a method of using such a device.
b) Description of the Related Art
A quantum well structure is formed by inserting a semiconductor thin film having a narrow bandgap between two semiconductor layers having a wide bandgap. The semiconductor layer having narrow bandgap is called a quantum well layer, and the semiconductor layer having a wide bandgap is called a barrier layer. A multiple quantum well structure is formed by alternately stacking a semiconductor layer having a narrow band gap and a semiconductor layer having a wide band gap.
The operation principle of a light modulator using a multiple quantum well structure will be described hereinunder.
FIG. 11A
shows a band energy distribution in a quantum well as a function of position in the thickness direction. A quantum well layer L
2
is sandwiched between barrier layers L
1
and L
3
. In
FIG. 11A
, a kinked line Ec is a conduction band edge, a kinked line Ev is a valence band edge, a broken line el
0
shows a ground level of electron, and a broken line hh
0
shows a ground level of heavy hole.
In the state without applying a bias electric field to such a quantum well structure, a wave function ef
o
of electron at the ground level el
o
of the conduction band and a wave function hf
0
of heavy hole at the ground level hh
0
of the valence band are both symmetrical with the center of the well layer L
2
. The peak position of the electron wave function ef
0
is equal to that of the heavy hole wave function hf
0
. If light having an energy corresponding to a difference Eg or higher between the electron ground level el
0
and the heavy hole ground level hh
0
is incident upon the quantum well structure, electron-hole pairs are generated and light is absorbed.
In
FIG. 12A
, a curve a
0
shows a wavelength dependency of an absorption coefficient in the state shown in FIG.
11
A.
FIG. 11B
shows the band structure when the quantum well structure shown in
FIG. 11A
is applied with a bias electric field along the right direction as viewed in FIG.
11
A. As the electric field is applied, the band edges are slanted. The band edges of the quantum well layer L
2
are slanted upper right so that electrons distribute to the left and holes distribute to the right. As the band edges are slanted, the difference Eg between the electron ground level el
0
and the heavy hole ground level hh
0
becomes small so that the absorption wavelength moves to the longer wavelength side (red shifts) corresponding to a lower energy side.
The wave functions of electron and hole shown in
FIG. 11A
have the same peak position if an electric field is not applied. As an electric field is applied, the wave functions shown in
FIG. 11B
shift in opposite directions, and as the intensity of the electric field increases, the overlap portion of the wave functions reduces. Reduction in the overlap portion of the wave functions means a lowered absorption coefficient &agr;. Therefore, as the electric field is applied to the quantum well structure, the curve a
0
shown in
FIG. 12A
moves to the longer wavelength side, and the height of the curve a
0
lowers.
A curve a
1
, shown in
FIG. 12A
shows the wavelength dependency of the absorption coefficient &agr; in the state shown in FIG.
11
B. As shown, the absorption coefficient &agr; rises on the longer wavelength side than a wavelength &lgr;
0
, and light having the wavelength &lgr;
0
is absorbed. The intensity of light having the wavelength &lgr;
0
can therefore be modulated by controlling an electric field applied to the quantum well structure.
A charp parameter is defined by &Dgr;n/&Dgr;k, where &Dgr;n is a change in the real part of a complex refractive index relative to a change in the electric field applied to a light modulator, and &Dgr;k is a change in the imaginary part thereof. The change &Dgr;k in the imaginary part of a complex refractive index is related to a change rate &Dgr;&agr; of an absorption coefficient relative to a light intensity, by a relationship of &Dgr;k=&lgr;&Dgr;&agr;/4&pgr;. A wavelength change &Dgr;&lgr; of a light pulse generated by modulating an applied electric field is given by &Dgr;&lgr;/&lgr;
2
=−(&Dgr;n/&Dgr;k×dS/dt)/(4&pgr;
0
S), where S is a light intensity which changes with time, c
0
is a light velocity in vacuum. If the charp parameter is not zero, the wavelength changes with modulation of the light intensity.
A conventional electric field—absorption type light modulator has a large positive value of the charp parameter in a transparent state with a weak applied electric field and a small absorption coefficient, and takes a negative value in a non-transparent state with a strong applied electric field and a large absorption coefficient. With a conventional light modulator, the charp parameter is positive in most of an applied electric field which provides a light intensity higher than a certain level. In this case, as a light pulse rises increasing its light intensity, the wavelength of modulated light once shifts to the shorter wavelength side and then recovers its initial wavelength, whereas as a light pulse falls decreasing its light intensity, the wavelength of modulated light once shifts to the longer wavelength side and then recovers its initial wavelength. Namely, in the region having a high intensity of the light pulse, the wavelength of modulated light moves from the shorter wavelength side to the longer wavelength side during the period between the pulse leading and trailing edges.
A quartz single mode optical fiber prevailing in the field of optical fiber communications has so-called dispersion characteristics that a propagation velocity (group velocity) of a light pulse changes with the wavelength of propagating light. Although the wavelength of about 1.55 &mgr;m minimizes a propagation loss, this wavelength band has so-called abnormal dispersion characteristics that the longer the wavelength becomes, the lower the group velocity becomes. In using this optical fiber in the 1.55 &mgr;m wavelength band, since a conventional light modulator has a wavelength change, the propagation velocity at the pulse trailing edge moving to the longer wavelength side becomes lower than that at the pulse leading edge moving to the shorter wavelength side, and the pulse width is broadened during optical fiber transmission. Therefore, the higher the modulation speed becomes and the narrower the pulse width becomes, discrimination between two adjacent light pulses becomes more difficult and transmission errors are likely to occur.
If an optical fiber having such characteristics is used near at the wavelength of 1.55 &mgr;m, a charp parameter is generally desired to be as small as possible. If a negative charp parameter is realized, the light pulse width can be narrowed during optical fiber transmission as opposed to the conventional example. Therefore, transmission errors can be suppressed even if a faster signal is transmitted over a long distance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor device capable of generating a light pulse suitable for long distance transmission.
According to one aspect of the present invention, there is provided a semiconductor device comprising: a quantum well lamination structure having at least one quantum well layer and at least two barrier layers alternately laminated, the quantum well layer forming a quantum well for electron and hole and the barrier layer forming a potential barrier relative to electron and hole, wherein a thickness of the quantum well layer and a height of the potential barrier of a valence band at the interface between the quantum well layer and the barrier layer are s
Armstrong Westerman & Hattori, LLP.
Fujitsu Limited
Jackson, Jr. Jerome
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