Heterojunction bipolar transistor including collector/base...

Details Heterojunction bipolar transistor including collector/base... Heterojunction bipolar transistor including collector/base...

2000-05-19

2002-06-04

Nelms, David (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Heterojunction device

Having graded composition

C257S197000, C257S198000, C257S190000

Reexamination Certificate

active

06399969

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor, and in particular, to a bipolar transistor that has a collector/base heterojunction.
As a conventional heterojunction bipolar transistor, there has conventionally been known a double heterojunction bipolar transistor (DHBT) such that a graded layer whose bandgap linearly changes in the direction of thickness thereof is interposed between a collector and a base in order to remove band discontinuity at a heterojunction located between the collector and the base (the junction being referred to as a collector/base heterojunction hereinafter) (Q. M. Zhang et al., “Analysis of the Emitter-Down Configuration of Double-Heterojunction Bipolar Transistor”, IEEE Transactions On The Electron Devices, vol. 39 No. 10, pp. 2220-2229, 1992). As another double heterojunction bipolar transistor, there is known one such that a setback layer that has a high concentration and a narrow bandgap is interposed between the collector and the base in order to lower a barrier at the collector/base heterojunction (W-C. Liu et al., “Application of &dgr;-doped wide-gap collector structure for high-breakdown and low-offset voltage transistors”, Applied Physics Letters, vol. 73 No. 10, pp. 1397-1399, 1998).
The graded layer and setback layer reduce to some extent the barrier viewed from mobile charge carriers. However, these layers cannot completely remove the barrier. This becomes significant particularly when a bias voltage is low, i.e., when the transistor is operated under bias conditions near a saturation region.
For example,
FIG. 3A
shows a calculated energy band diagram of an npn type DHBT provided with the aforementioned graded layer when a bias set sufficiently away from the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT.
FIG. 3B
shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al
0.3
Ga
0.7
As emitter layer (having a thickness of 800 Å and an impurity concentration n=5×10
17
cm
−3
)
31
, an Al
x
Ga
1−x
As emitter graded layer (having a thickness of 200 Å, an impurity concentration n=5×10
17
cm
−3
and an Al mole fraction x=0.3→0)
32
, a p
+
-type GaAs base layer (having a thickness of 800 Å and an impurity concentration n=4×10
19
cm
−3
)
33
, an n-type Al
x
Ga
1−x
As graded layer (having a thickness of 500 Å, an impurity concentration n=2×10
16
cm
−3
and an Al mole fraction x=0→0.2)
34
, an n-type Al
0.2
Ga
0.8
As collector layer (having a thickness of 6000 Å and an impurity concentration n=2×10
16
cm
−3
)
35
, an Al
x
Ga
0.8
As graded layer (having a thickness of 500 Å, an impurity concentration n=2×10
16
cm
−3
and an Al mole fraction x=0.2→0)
36
and an n-type GaAs subcollector layer (having a thickness of 1000 Å and an impurity concentration n=5×10
18
cm
−3
)
37
. The arrow “→” between the numbers of the Al mole fraction x means that the value of the Al mole fraction x linearly changes in the direction of thickness of the layer.
FIGS. 3A and 3B
show that a collector/base heterojunction barrier
39
cannot be sufficiently reduced by the graded layer when the bias voltage is low.
FIG. 4A
shows a calculated energy band diagram of an npn type DHBT provided with the aforementioned setback layer when a bias set sufficiently away from a saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT.
FIG. 4B
shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al
0.3
Ga
0.7
As emitter layer (having a thickness of 800 Å and n=5×10
17
cm
−3
)
41
, an Al
x
Ga
1−x
As emitter graded layer (having a thickness of 200 Å, an impurity concentration n=5×10
17
cm
−3
and an Al mole fraction x=0.3→0)
42
, a p
+
-type GaAs base layer (having a thickness of 800 Å and an impurity concentration n=4×10
19
cm
−3
)
43
, an n-type GaAs setback layer (having a thickness of 500 Å, an impurity concentration n=2×10
17
cm
−3
)
44
, an n-type Al
0.2
Ga
0.8
As collector layer (having a thickness of 6000 Å and an impurity concentration n=2×10
16
cm
−3
)
45
, an n-type Al
0.2
Ga
0.8
As collector layer (having a thickness of 500 Å and an impurity concentration n=2×10
18
cm
−3
)
46
and an n-type GaAs subcollector layer (having a thickness of 1000 Å and an impurity concentration n=5×10
18
cm
−3
)
47
.
FIGS. 4A and 4B
show that a collector/base heterojunction barrier
49
cannot be sufficiently reduced by the setback layer when the bias voltage is low.
FIG. 5A
shows a calculated energy band diagram of an npn type DHBT provided with a &dgr;-doped layer in addition to the aforementioned setback layer when a bias set sufficiently away from a saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT.
FIG. 5B
shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al
0.3
Ga
0.7
As emitter layer (having a thickness of 800 Å and n=5×10
17
cm
−3
)
51
, an Al
x
Ga
1−x
As emitter graded layer (having a thickness of 200 Å, n=5×10
17
cm
−3
and an Al mole fraction x=0.3→0)
52
, a p
+
-type GaAs base layer (having a thickness of 800 Å and an impurity concentration n=4×10
19
cm
−3
)
53
, an n-type GaAs setback layer (having a thickness of 500 Å, an impurity concentration n=2×10
16
cm
−3
)
54
, a &dgr;-doped layer (donor 2-dimensional doping density Ns=5×10
11
cm
−2
)
55
, an n-type Al
0.2
Ga
0.8
As collector layer (having a thickness of 6000 Å and an impurity concentration n=2×10
16
cm
−3
)
56
, an n-type Al
0.2
Ga
0.8
As collector layer (having a thickness of 500 Å and an impurity concentration n=2×10
18
cm
−3
)
57
and an n-type GaAs subcollector layer (having a thickness of 1000 Å and an impurity concentration n=5×10
18
cm
−3
)
58
.
FIGS. 5A and 5B
show that a collector/base heterojunction barrier
59
cannot be sufficiently reduced when the bias voltage is low even if the &dgr;-doped layer is provided in addition to the setback layer.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a heterojunction bipolar transistor including a collector/base heterojunction such that mobile charge carriers can pass through a collector region from a base region without encountering any barrier, therefore achieving a high operating efficiency.
In order to achieve the above object, the present invention provides a heterojunction bipolar transistor having an emitter region, a base region, a collector region and a subcollector region, which are sequentially arranged in one direction, wherein the collector region includes a plurality of adjacent sub-regions with respect to a thickness direction in which mobile charge carriers move, an energy bandgap in each of the sub-regions is either constant or linearly changes with a position in the thickness d

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