Active solid-state devices (e.g. – transistors – solid-state diode – Tunneling pn junction device
Reexamination Certificate
2002-09-12
2004-07-20
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Tunneling pn junction device
C257S013000, C257S025000, C257S079000, C257S103000, C257S183000, C257S918000, C438S022000, C438S024000, C438S025000, C438S028000, C438S029000, C438S046000, C438S047000
Reexamination Certificate
active
06765238
ABSTRACT:
BACKGROUND OF THE INVENTION
Semiconductor lasers generate light that can be used in optical communication systems, compact disc (CD) players, digital videodisc (DVD) players, scanners and other systems. Semiconductor lasers for optical communications include quantum-cascade lasers, vertical-cavity surface-emitting lasers (VCSELs), edge-emitting lasers, in-plane emitting lasers and the like.
Until recently, relatively expensive Fabry-Perot (FP) and distributed-feedback (DFB) lasers have been used to generate light at the wavelengths presently used in the telecommunication industry for transmission via fiber-optic links. Although VCSEL technology has proven to be a viable lower-cost, lower-power alternative well suited for short-haul network applications, the industry has had difficulty to produce reliable, cost-effective VCSELs for use at the longer wavelengths used in medium- and long-haul fiber-optic communications links.
A VCSEL is composed of an active region sandwiched between vertically-stacked mirrors, commonly known as distributed Bragg reflectors (DBRs) or Bragg mirrors. The active region typically includes quantum wells that generate the light. The quantum wells are composed of thin layers of semiconductor materials that differ in band-gap energy. To achieve the necessary reflectivity, the number of semiconductor or dielectric layers constituting each of the DBRs can be quite large. The VCSEL emits the light generated in the active region through one of the mirrors, which has a reflectivity less than that of the other of the mirrors. Light is output from a VCSEL from a relatively small area on the surface of the semiconductor, directly above or below the active region.
The potential for VCSELs to generate light with relatively long wavelengths has not been realized due, in part, to the difficulty of epitaxially growing DBRs that have suitable optical, electrical, and thermal properties on an indium phosphide (InP) substrate. Two of the more significant problems are high optical losses and high joule heating in the Bragg mirror fabricated using p-type semiconductor materials.
The industry has explored incorporating a tunnel junction into a VCSEL to address these problems. Incorporating a tunnel junction allows both DBRs to be fabricated using n-type semiconductor materials. A DBR fabricated using n-type semiconductor materials has significantly lower optical losses and higher electrical conductivity than a DBR fabricated using p-type semiconductor material. Reduced optical losses lead to a lower threshold current and a correspondingly higher differential gain. Higher differential gain is an important parameter for achieving high-bandwidth modulation. High-bandwidth modulation is desirable for optical fiber-based communication systems.
FIG. 1
shows a side view of an example of a prior-art semiconductor device
100
incorporating a tunnel junction structure
102
. The tunnel junction structure is composed of an n-type tunnel junction layer
104
, a p-type tunnel junction layer
106
and a tunnel junction
110
between the tunnel junction layers. The n-type tunnel junction layer is a layer of an n-type semiconductor material. The p-type tunnel junction layer is a layer of a p-type semiconductor material. Applying a reverse bias across tunnel junction
110
will cause a tunneling current to flow across the tunnel junction. A reverse bias is applied by setting n-type tunnel junction layer
104
to a more positive voltage than p-type tunnel junction layer
106
. It is desirable to minimize the voltage drop across the tunnel junction to reduce the overall voltage drop across the VCSEL. To minimize the voltage drop across the tunnel junction, conventional approaches have focused on maximizing the doping concentrations in the materials of the tunnel junction layers.
Also shown in
FIG. 1
are n-type layer
102
on which n-type tunnel junction layer
104
is grown and p-type layer
108
grown on p-type tunnel junction layer
106
. N-type layer
102
may constitute the substrate of semiconductor device
100
. Alternatively, n-type layer
102
may be grown on or over the substrate.
Many different pairs of semiconductor materials that can be used as the materials of n-type tunnel junction layer
104
and of p-type tunnel junction layer
106
are known in the art. In the semiconductor device
100
illustrated in
FIG. 1
, the semiconductor material of n-type tunnel junction layer
104
is n-type indium phosphide (InP) and the semiconductor material of p-type tunnel junction layer
106
is indium gallium aluminum arsenide (InGaAlAs). The material of layer
102
is also n-type InP that has a lower dopant concentration than the material of n-type tunnel junction layer
104
. The material of layer
108
is also p-type InGaAlAs that has a lower dopant concentration than the material of p-type tunnel junction layer
106
.
Tunnel junctions having a low voltage drop are formed of materials that establish a large built-in electrostatic field across the tunnel junction. A large electrostatic field requires a large potential difference across a short distance, and is typically generated by using very high doping concentrations in the tunnel junction layers that minimize the width of the depletion region at the tunnel junction.
FIGS. 2A and 2B
each include an energy diagram
200
and an electrical circuit model
202
that show some of the characteristics of tunnel junction structure
102
.
FIG. 2A
shows the characteristics of the tunnel junction structure at equilibrium.
FIG. 2B
shows the characteristics of the tunnel junction structure under reverse bias. Each energy diagram shows the conduction band energy E
Cn
and the valence band energy E
Vn
of the semiconductor material of n-type tunnel junction layer
104
. Each energy diagram also shows the conduction band energy E
Cp
and the valence band energy E
Vp
of the semiconductor material of p-type tunnel junction layer
106
. N-type tunnel junction layer
104
and p-type tunnel junction layer
106
collectively form tunnel junction
110
.
The energy diagram of
FIG. 2A
shows the depletion region
204
that exists at tunnel junction
110
at equilibrium. At equilibrium, the Fermi level E
Fn
of the material of n-type tunnel junction layer is equal to the Fermi level E
Fp
of the material of p-type tunnel junction layer
106
. The conduction bands of the materials of the tunnel junction layers differ in energy, which establishes the built-in potential barrier
206
at the tunnel junction that prevents conduction through the tunnel junction at low forward bias. The electrostatic field strength E at the tunnel junction depends on the height of the built-in potential barrier and depends inversely on the width W of the depletion region
204
at the tunnel junction.
A forward bias applied across tunnel junction
110
decreases the height of the built-in potential barrier at the tunnel junction. Sufficient forward bias causes current to flow across the tunnel junction in the forward direction. A forward bias is established by setting p-type tunnel junction layer
106
to a more positive voltage than n-type tunnel junction layer
104
. The width of depletion region
204
decreases under forward bias (not shown).
A reverse bias applied across tunnel junction
110
adds to the height of the built-in potential barrier and increases the width of depletion region
204
to W′. The reverse bias separates the Fermi levels E
Fn
and E
Fp
on opposite sides of the tunnel junction. In the example shown, the Fermi level E
Fp
of the material of p-type tunnel junction layer
106
has increased relative to its equilibrium level, whereas the Fermi level E
Fn
of the material of n-type tunnel junction layer
104
remains substantially unchanged. In a conventional p-n junction, only a small leakage current flows across the junction under reverse bias. However, in tunnel junction
110
, the reverse bias causes current to flow occurs due to electrons tunneling through the potential barrier.
The reverse bias elevates the valence band energy E
Vp
of the ma
Chang Yin-Lan
Leary Michael H.
Tan Michael R. T.
Tandon Ashish
Agilent Technologie,s Inc.
Huynh Andy
Nelms David
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