Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2001-02-05
2004-01-27
Chaudhuri, Olik (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S197000, C257S198000, C257S584000, C257S587000, C257S745000
Reexamination Certificate
active
06683332
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor (hereinafter, referred to as HBT) and a manufacturing method therefor.
Together with the functional improvement of digital portable telephones, there has been a demand for even higher performance of transmission-use high-power amplifiers. The HBT is regarded as promising in the field of high-frequency devices. For further higher performance of the HBT, it is necessary to reduce the parasitic device effects, i.e., parasitic resistance and parasitic capacitance. The parasitic resistance can be classified roughly into emitter resistance, base resistance and collector resistance. In particular, the emitter resistance tends to increase together with decreasing emitter dimensions intended for higher performance. This leads to noticeable deterioration of current amplification rate and cutoff frequency of the HBT.
Conventionally, GaAs based HBTs have been provided, where in the case of npn type, the n-type emitter ohmic electrode is provided by using AuGe based metals, W or other high melting point metals, WN or other high melting point nitrides and WSi or other high melting point silicides, the p-type base ohmic electrode is provided by using Pt, Pd, AuZn, AuBe or the like, and the n-type collector ohmic electrode is provided by using AuGe based metals. Examples of such an HBT and an ohmic electrode used therefor are shown in the following figures (1)-(3):
(1)
FIG. 14
is a schematic sectional, structural view of main part of an HBT disclosed in Reference, S. Hongo et al., SSDM, 1994, pp. 613-615. In
FIG. 14
, a semi-insulating substrate and an n-type collector contact layer formed on the semi-insulating substrate and having a high carrier concentration are omitted.
On the collector contact layer, as shown in
FIG. 14
, a low-concentration n-type collector layer
40
, a high-concentration p-type base layer
41
, a low-concentration n-type emitter layers
42
,
43
, and a high-concentration n-type emitter contact layer
44
are formed one by one. Further, on the high-concentration n-type emitter contact layer
44
is formed an emitter ohmic electrode
46
. Also, on base protective layers
45
which are both end portions of the low-concentration n-type emitter layer
42
, are formed base ohmic electrodes
47
,
47
. Alloyed reaction layers
48
,
48
are formed between these base ohmic electrodes
47
,
47
and the high-concentration p-type base layer
41
, and device isolating layers
49
,
49
are formed so as to sandwich the base layer
41
and the emitter layer
42
. The alloyed reaction layers
48
,
48
extend through the base protective layers
45
,
45
to connect the base ohmic electrodes
47
,
47
and the base layer
41
to each other.
The base protective layers
45
,
45
formed so as to cover the base layer except its portion placed just under the emitter layer
43
(hereinafter, referred to as external base) out of the base layer
41
are provided in order to suppress decreases of the current amplification rate. That is, when the surface of the external base out of the high-concentration p-type base layer
41
is not covered by the base protective layers
45
, a multiplicity of interfacial levels present at the surface causes carrier recombination to occur at the surface of the external base out of the base layer
41
, incurring a deterioration of the current amplification rate. However, by the base protective layers
45
covering the surface of the external base out of the base layer
41
, these base protective layers
45
are completely depleted during the device operation, so that the supply of carriers to the base protective layers
45
is cut off. As a result, the recombination of carriers is suppressed at the high-concentration p-type base layer
41
so that the deterioration of the current amplification rate can be suppressed.
However, when the base ohmic electrodes
47
of the high-concentration p-type base layer
41
are formed, the surface of the external base out of the base layer
41
is covered with the base protective layers
45
, keeping the high-concentration p-type base layer
41
and the ohmic electrodes
47
from making contact with each other, with the result that an ohmic contact cannot be obtained. Therefore, Pt alloyed reaction layers
48
,
48
are formed by forming Pt/Ti/Pt/Au one by one and then subjecting these to heat treatment, and further the high-concentration p-type base layer
41
and the base ohmic electrodes
47
,
47
are put into contact with each other by the Pt alloyed reaction layers
48
,
48
, by which an ohmic contact is obtained. In this case, the Pt alloyed reaction layers
48
,
48
are required to extend through the base protective layers
45
and reach the high-concentration p-type base layer
41
. Also, the emitter ohmic electrode
46
is formed of Ti/Pt/Au by a process other than the process for the base ohmic electrodes
47
,
47
. Although not shown, the collector ohmic electrode is a AuGe based electrode.
(2)
FIG. 15
is a schematic sectional view of main part of an HBT disclosed in Reference, E Zanoni et al., IEEE Device Letters, Vol. 13, No. 5, May 1992. In this HBT, as shown in
FIG. 15
, on a semi-insulating substrate
50
are formed an n-type GaAs collector contact layer
51
, an n-type GaAs collector layer
52
, a p-type GaAs base layer
53
, a non-doped GaAs layer
54
, an n-type AlGaAs emitter layers
55
,
56
,
57
, an n-type GaAs emitter contact layer
58
, an n-type InGaAs emitter contact layers
59
,
60
, one by one. Also, a base ohmic electrode
61
for the p-type base layer
53
is formed of AuBe, an emitter ohmic electrode
62
for the n-type emitter layers
55
,
56
,
57
, as well as a collector ohmic electrode
63
for the n-type collector layer
52
are formed of AuGeNi. Accordingly, the emitter ohmic electrode
62
and the collector ohmic electrode
63
can be formed simultaneously. That is, the emitter ohmic electrode
62
and the collector ohmic electrode
63
can be formed by one manufacturing step.
(3)
FIGS. 16A
,
16
B and
16
C are schematic sectional views of an ohmic electrode disclosed in Japanese Patent Laid-Open Publication HEI 8-222526. As shown in
FIG. 16A
, a Cu layer
71
, a Ge layer
72
and Cu layer
73
are stacked one by one on a high-concentration n- or p-type GaAs layer
70
, and an ohmic electrode is obtained by annealing these layers. Also, as shown in
FIG. 16B
, a Pd layer
75
, a Cu layer
76
, a Ge layer
77
and a Cu layer
78
are stacked one by one on a high-concentration n- or p-type layer
74
, and an ohmic electrode is obtained by annealing these layers. Further, as shown in
FIG. 16C
, a Cu layer
80
, a Pd layer
81
, a Ge layer
82
and a Cu layer
83
are stacked one by one on a high-concentration n- or p-type GaAs layer
79
, and an ohmic electrode is obtained by annealing these layers. The ohmic electrodes of
FIGS. 16A
,
16
B and
16
C can be used as ohmic portions for the high-concentration p-type GaAs layers and the high-concentration n-type GaAs layers in the HBTs. Therefore, ohmic portions for the high-concentration p-type GaAs layers and the high-concentration n-type GaAs layers can be formed by one manufacturing step, thereby facilitating the formation process for the ohmic portions.
However, the HBTs as described in (1), (2) and (3) and the ohmic electrodes to be used therefor have the following problems.
With regard to (1), the process for forming the emitter ohmic electrode
46
, the base ohmic electrodes
47
and the collector ohmic electrode includes a resist formation step for forming photoresist of a pattern corresponding to the configuration of the ohmic electrodes, a metal thin film formation step for forming a metal thin film by using vapor deposition or sputtering process, and a so-called lift-off step for leaving the metal thin film only at necessary portions by removing the photoresist. Also, the base ohmic electrodes
47
are formed of Pt/Ti/Pt/Au, the emitter ohmic electrode
46
is formed of Ti/Pt/Au, and the collector ohmic electrode
Shinozaki Toshiyuki
Tsukao Toshiya
Brewster William M.
Chaudhuri Olik
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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