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.
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/GaAsxe2x80x94based 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.
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.