Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2002-04-03
2004-07-27
Chen, Jack (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S582000
Reexamination Certificate
active
06768140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
GaAs based devices are able to provide the power and amplification requirements of various applications, such as wireless communication applications, with improved linearity and power efficiency. Of particular interest are gallium arsenide (“GaAs”) heterojunction bipolar transistors (“HBT”), which exhibit high power density capability, making them suitable as low cost and high power amplifiers in devices used in CDMA, TDMA and GSM wireless communications. However, GaAs HBTs, in particular GaAs HBTs utilizing an indium gallium phosphide (“InGaP”) emitter, can exhibit undesirable thermal instability, which can lead to catastrophic failure of the HBT.
By way of background, thermal instability can cause an HBT to self-destruct when increasing temperatures inside the HBT cause a decrease in the turn-on voltage of the base-emitter junction of the HBT. As the turn-on voltage decreases, the HBT turns on harder and thus consumes more power, which further increases the temperature in the HBT. The increasing temperature further decreases the base-emitter turn-on voltage, resulting in a positive feedback loop. This phenomenon, i.e. the creation of the positive feedback loop in the HBT due to the above-explained mechanism, is more specifically referred to as “thermal runaway.” Furthermore, as the overall temperature increases in the HBT, the internal areas of the HBT get hotter and, thus, carry more current than the periphery of the HBT. Accordingly, the temperature gradient causes the turn-on voltage in the internal base-emitter junction areas of the HBT to be lower than the turn-on voltage in the periphery areas of the base-emitter junction of the HBT. This results in filamentation, which occurs when localized current causes high power dissipation in a small area in an HBT with a correspondingly high increase in the localized temperature within the HBT. The end result of filamentation is self-destruction of the HBT. This phenomenon, i.e. filamentation and self-destruction of the HBT due to the above-explained mechanism, is more specifically referred to as “thermal collapse.”
In one known method to prevent the above positive feedback loop from occurring, an emitter ballast resistor is integrated into a GaAs HBT by adding a lightly doped epitaxial layer above a high band gap emitter. As the current in the GaAs HBT increases, the voltage across the epitaxial emitter ballast resistor increases which tends to decrease the voltage across the base-emitter junction of the HBT, thereby limiting the current flow into the GaAs HBT, which in turn stabilizes the HBT. For example, K. Yamamoto et al., “A 3.2-V Operation Single-Chip Dual-Band AlGaAs/GaAs HBT MMIC Power Amplifier with Active Feedback Circuit Technique,” IEEE Journal of Solid State Circuits, Vol. 35, No. 8, August 2000, pp. 1109-1120, discloses a lightly doped aluminum gallium arsenide (“AlGaAs”) ballast layer situated on top of a high band gap emitter. By way of further example, G. Gao et al., “Emitter Ballasting Resistor Design for, and Current Handling Capability of AlGaAs/GaAs Power Heterojunction Bipolar Transistors,” IEEE Transactions on Electron Devices, Vol. 38, No. 2, February 1991, pp. 185-196, discloses a lightly doped GaAs ballast layer situated on top of a high band gap emitter.
In a conventional epitaxial ballast resistor design, increased emitter resistance results mainly from two mechanisms: (1) the resistive nature of the low-doped ballast layer itself or (2) an increased thermionic emission barrier between the bottom of a low doped ballast layer, such as an emitter cap layer, and the top of a high band gap emitter. When a relatively thick layer is employed as an epitaxial ballast layer, mechanism (1) dominates, while mechanism (2) plays a significant role when the epitaxial ballast layer is relatively thin. In mechanism (1), increased emitter resistance critically depends on the doping level and thickness of the ballast layer while the emitter resistance will be mainly determined by the doping level of ballast layer in mechanism (2).
In both cases, i.e. in mechanisms (1) and (2), ballast resistance is very sensitive to the doping level of the epitaxial ballast layer and, consequently, requires very accurate doping control during the epitaxial growth process to achieve uniform, reproducible HBT characteristics. The above required accuracy in doping control creates manufacturing challenges that are difficult to meet. In addition, when mechanism (2) plays a significant role in determining total emitter resistance, emitter resistance tends to show increased base current dependency and a more negative temperature coefficient, which undesirably affects power amplifier linearity and thermal stability.
Referring now to
FIG. 1
, a conventional exemplary NPN GaAs HBT is illustrated. GaAs HBT
100
comprises emitter contact
120
, base contacts
122
and
124
, and collector contact
126
. Further, GaAs HBT
100
comprises emitter cap
118
, emitter cap
116
, emitter
114
, and base
112
. In GaAs HBT
100
, emitter cap
118
is indium gallium arsenide (“InGaAs”) grown with an N-type dopant such as tellurium at approximately 4×10
19
atoms per cm
3
, for example. Emitter cap
116
can be gallium arsenide doped with silicon at a relatively low doping level of approximately 5×10
18
atoms per cm
3
. Emitter cap
116
may have a thickness of approximately 2000 Angstroms. Emitter
114
can comprise either Al
x
Ga
(1-x)
As or In
x
Ga
(1-x)
P (referred to hereinafter simply as “AlGaAs” and “InGaP”) doped with silicon at a medium concentration of approximately 3×10
17
atoms per cm
3
. Base
112
can be, for example, gallium arsenide doped with carbon at a typical concentration level of approximately 4×10
19
atoms per cm
3
.
Continuing with
FIG. 1
, as shown, GaAs HBT
100
further comprises collector
130
and subcollector
110
. According to conventional fabrication methods, collector
130
comprises gallium arsenide, which is uniformly and lightly doped with silicon at 1×10
16
atoms per cm
3
. Immediately below collector
130
is subcollector
110
, which also comprises gallium arsenide. However, subcollector
110
is doped with silicon at a significantly higher concentration, typically in the range of approximately 5×10
18
atoms per cm
3
. In GaAs HBT
100
, collector layer
130
can be between 0.3 microns and 2 microns thick, and subcollector
110
can be between 0.3 microns and 2 microns thick.
In previously known GaAs HBTs with an emitter ballast resistor, such as GaAs HBT
100
, emitter cap
116
comprises relatively low silicon-doped gallium arsenide to provide an epitaxial emitter ballast resistor. As the current in GaAs HBT
100
increases, the voltage across the epitaxial emitter ballast resistor provided by emitter cap
116
increases which tends to reduce the voltage across the base-emitter junction of the HBT which, as discussed above, stabilizes GaAs HBT
100
. Accordingly, emitter cap
116
can prevent a positive feedback loop from forming and destroying GaAs HBT
100
, as discussed above. However, emitter cap
116
requires very accurate doping control during the epitaxial growth process to achieve uniform, reproducible HBT characteristics. The required accuracy in doping control of emitter cap
116
is very difficult to achieve at a relatively low silicon doping level, e.g. approximately 3×10
17
atoms per cm
3
, which is required to provide sufficient ballast resistance for GaAs HBT
100
. Additionally, emitter ballast resistance provided by emitter cap
116
in conventional GaAs HBT
100
suffers from undesirable instability in resistance value as the base current is varied.
Another method utilizes an external emitter resistor to protect the GaAs HBT from destruction resulting from filamentation caused by
Burton Richard S.
Green Debora L.
Hong Kyushik
Krumm Charles F.
Matine Noureddine
Chen Jack
Farjami & Farjami LLP
Skyworks Solutions Inc.
Smoot Stephen W.
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