Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2000-08-03
2002-04-23
Nguyen, Tuan H. (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S200000, C438S312000
Reexamination Certificate
active
06376867
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to heterojunction bipolar transistors (HBTs), and, more particularly, to HBTs constructed to have reduced thermal resistance to permit rapid dissipation of heat.
BACKGROUND OF THE INVENTION
In HBTs operating at high power, the performance of the device is limited by the amount of power that can be dissipated in the device. Typically, a high power HBT will generate a large amount of heat which must be dissipated quickly to permit proper operation.
Recently, metallic thermal shunts have been developed to permit dissipation of heat from the collector of the HBT device. Typically, in any HBT, and especially in InP based HBTs, most of the power dissipation and heat generation are located in the upper region of the collector layer near the base layer, where there is a large voltage drop and a large current.
FIG. 1
shows an HBT with a metallic thermal shunt designed in accordance with known principles to dissipate this heat.
Referring to
FIG. 1
, an HBT
110
is shown. This HBT is constructed with a substrate
114
formed of InP, a sub-collector
116
formed on the substrate, a collector
118
formed on the sub-collector, a base
120
formed on the collector and an emitter
122
formed on the base. An emitter metal
124
is provided on the emitter
122
(with collector metal being provided on the sub-collector
116
and base metal provided on the base
120
).
As can be seen in
FIG. 1
, the majority of the heat generated during operation of the HBT
110
is located in a region
112
near the base-collector junction. In order to dissipate this heat to the substrate, a metallic thermal shunt
126
is coupled between the emitter metal
124
and the substrate
114
. Thus, the heat generated in the collector
118
will dissipate through the base
120
, the emitter
122
, the emitter metal
124
and the metallic thermal shunt
126
to the substrate
114
. It is noted that this arrangement is also taught in U.S. Pat. No. 5,734,193 to B. Bayraktaroglu et al.
From their studies of the thermal shunt technique described above, the inventors have noted certain disadvantages. The first of these is that the heat dissipation is limited by the fact that the heat must go through the base and emitter layers (and an emitter cap layer if one is used), and these layers generally have relatively low thermal conductivity, especially in InP-based HBTs. For example, the material used in the base of InP-based HBTs is the ternary InGaAs that has a very low thermal conductivity. Furthermore, in InP-based HBTs, the emitter material is typically the ternary semiconductor InAlAs, which also has a very poor thermal conductivity. The thermal shunt approach would provide limited improvement for these type of devices. Also, InGaAs is generally used as a cap layer on top of the emitter to improve the ohmic contact resistance to the emitter, which adds another layer with low thermal conductivity in the path for heat dissipation.
The metallic thermal shunt technique was initially developed for AlGaAs/GaAs—based HBTS. In these HBTs, the emitter layer consists of the ternary semiconductor AlGaAs, which has a poor thermal conductivity (see FIG.
4
). Therefore, the thermal shunt technique also provides limited improvement in this instance.
A second disadvantage of the thermal shunt approach is the fact that it requires an extra processing step to construct the thermal shunt after the device itself is completed. Also, additional space is required for the shunt, thereby increasing the overall size of the device.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an HBT with improved heat dissipation.
It is a further object of the present invention to provide an HBT in which heat is dissipated downward to the substrate directly without the need for a thermal shunt.
It is still a further object of the present invention to provide an HBT with reduced thermal resistance between the heat generated near the collector-base junction and the substrate.
Yet another object of the present invention is to provide a method of fabricating an HBT to improve heat dissipation downward to the substrate.
To achieve these and other objects, an HBT is provided with a collector and a sub-collector which are each comprised of InP and located relative to one another so that heat generated in the collector during operation of the HBT dissipates downward from the collector through the sub-collector into the substrate.
REFERENCES:
patent: 5041882 (1991-08-01), Katoh
patent: 5734193 (1998-03-01), Bayraktaroglu et al.
patent: 5946582 (1999-08-01), Bhat
patent: 6049099 (2000-04-01), Vaccaro et al.
S. Yamahata et al., “Over-220-GHz-fnd—fmaxInP/InGaAs Double-Heterojunction Bipolar Transistors with a New Hexagonal-Shaped Emitter”, IEEE, 0703-2966-X/95, pp. 163-166, 1995.
D. Caffin et al., “Base-Collector Leakage Currents in InP/InGaAs Double Heterojunction Bipolar Transistors”,IEEE Transactions on Electron Devices, vol. 44, No. 6, Jun. 1997, pp. 930-936.
H. Fukano et al., “High Speed InP/GaAs Heterojunction Phototransistors Employing a Nonalloyed Electrode Metal as a Reflector”,IEEE Journal of Quantum Electronics, vol. 30, No. 12, Dec. 1994, pp. 2889-2895.
L. Lunardi et al., “Integrated p-i-n/HBT Photoreceivers for Optical Communication”, IEEE, 0-7803-3393-04, IEDM 96-645, pp. 26.1.1-26.1.4, 1996.
Chin Patrick T.
Gutierrez-Aitken Augusto L.
Oki Aaron K.
Streit Dwight C.
Nguyen Tuan H.
TRW Inc.
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