Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Having heterojunction
Reexamination Certificate
2001-03-20
2003-04-01
Chaudhuri, Olik (Department: 2823)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Having heterojunction
C438S235000, C257S012000, C257S194000
Reexamination Certificate
active
06541346
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to semiconductor transistors used in the fabrication of very high-speed integrated circuits and microwave power amplifiers. More particularly, the invention pertains to compound semiconductor transistors made from GaAs and InP and related lattice-matched materials epitaxially grown on semi-insulating substrates.
BACKGROUND OF THE INVENTION
Transistors are multielectrode semiconductor devices in which the current flowing between two specified electrodes is controlled or modulated by the voltage applied at a third (control) electrode. Transistors fall into two major classes: the bipolar junction transistor (BJT) and the field-effect transistor (FET). BJTs were derived from the point-contact transistor, which was invented at Bell Telephone Laboratories in 1947 by Bardeen, Brattain, and Shockley. BJTs comprise two p-n junctions placed back-to-back in close proximity to each other, with one of the regions common to both junctions. This forms either a p-n-p or n-p-n transistor comprising three regions—emitter, base and collector. The BJT utilizes the flow of both electrons and holes across the p-n junctions for its electrical behavior. That is, the current flow through the emitter and collector electrodes is controlled by the voltage across the base-emitter p-n junction.
In normal (or forward active) operation of a BJT, the base-emitter p-n junction is forward biased and the base-collector junction is reverse biased. Majority-carrier current flows across the forward-biased emitter-base junction. The emitter is much more heavily doped than the base region, so that most of the total current flow across the base-emitter junction consists of majority carriers from the emitter injected into the base. These injected carriers become minority carriers in the base region, and will tend to recombine. Such recombination is minimized by making the base region very narrow, so that the injected carriers can diffuse across the base to the reverse-biased base-collector junction, where they are swept across the junction into the collector, to appear in the outside circuit as the collector current. The magnitude of this collector current depends on the number of majority carriers injected into the base from the emitter, and thus current is controlled by the base-emitter p-n junction voltage. The output (collector) current is therefore controlled by the input (base-emitter) voltage, and the output circuit of the transistor can be modeled as a voltage-controlled current source (dependent sources), while the input circuit looks like a p-n junction diode.
In principle, the transistor can be operated in reverse active mode by reversing the connections. However, in practice, the transistor is not completely symmetrical. That is, the the emitter is very heavily doped to maximize emitter injection, and the collector is relatively lightly doped so that it can accommodate large voltage swings across its reverse-biased junction. While the electrical characteristics are similar in appearance, the forward characteristics show much greater gain, as expected.
If both junctions are reverse biased, the transistor behaves like an open switch, with only the p-n junction reverse leakage currents flowing. If both junctions are forward biased, there is injection of carriers into the base region from both sides, and a low resistance is presented to current flow in either direction: the transistor behaves like a closed switch, and the base stores the injected charge.
BJTs can be used to provide linear voltage and current amplification: small variations of the base-emitter voltage and hence the base current at the input terminal result in large variations of the output collector current. Since the transistor output has the appearance of a current source, the collector can drive a load resistance and develop an output voltage across this resistance (within the limits of the supply voltage). The transistor can also be used as a switch in digital logic and power switching applications, switching from a high-impedance ‘off’ state in cut-off, to a low-impedance ‘on’ state in saturation. In practice, full saturation conditions of base-collector forward biased are generally avoided, to limit the carrier storage in the base and reduce the switching time. Such BJTs find application in analog and digital circuits and integrated circuits, at all frequencies from audio to radio frequency. For higher frequencies, such as microwave applications, heterojunction bipolar transistors (HBTs) are used.
HBTs are bipolar junction transistor which incorporate a wide band gap emitter, where the emitter-base junction is a heterojunction between semiconductors of different energy band gaps. The following are typical materials for HBTs: aluminum-gallium-arsenide (AlGaAs)(emitter)/gallium-arsenide (GaAs)(base); aluminum-indium-arsenide (AlInAs)/indiumgallium-arsenide (InGaAs); Si/silicon-germanium (SiGe); and indium-gallium-phosphide (InGaP)/GaAs; indium-phosphide (InP)/InGaAs. The wider band gap of the emitter significantly reduces the injection of majority carriers from the base to the emitter, thus maximizing the desired injection of carriers from the emitter to the base. This eliminates the requirement for a heavily doped emitter to achieve the same result, and consequently allows the based doping to be increased. An increase in base doping is desirable from a device viewpoint, as the base resistance can be reduced significantly. This leads to an improvement in the high-frequency performance of the transistor. HBTs are typically used at radio- and microwave frequencies, in integrated circuits (ICs), power applications, optoelectronic ICs, etc.
Compound semiconductor (e.g., GaAs, InP, etc.) HBTs play an important role in present day communications systems. They have been used extensively for power amplifiers in cell phones due to superior efficiency (i.e., longer battery life) and improved linearity (i.e., less distortion and longer operation range) in comparison to standard Si transistor technology. Also, such compound semiconductor HBTs are used in ultra-high speed digital ICs (i.e., operating at 10 Gigabit/sec) for fiber optic communications systems for telecom and Internet backbone transmission. Future generation fiber optic systems are targeted for 40, 80 and 160 Gigabit/sec applications that are well beyond the speeds of silicon (Si) IC technologies. In addition, HBTs may be used in phased-array radar and very high frequency terrestrial and satellite communication systems, and the reliability of HBT devices is not sufficient to incorporate these ICs into such applications, primarily due to the limitations of present HBT process technologies.
HBTs manufactured from compound semiconductor epitaxial layers including AlGaAs/GaAs, InGaP/GaAs, AlInAs/InGaAs, and InP/InGaAs have produced the world's fastest semiconductor transistors with cut-off frequencies of several hundred GigaHertz (GHz) and operating IC speeds above 80 GHz. See, for example, M. Rodwell, “Transferred Substrate InP HBT Technology”, Proceedings from the International Symposium on Indium Phosphide and Related Materials, Williamsburg, Va., 286, IEEE Press (2000). Compound semiconductor HBTs are of great commercial interest for very high-speed optical fiber digital communication systems operating at and above 10 Gigabit/sec, in microwave and millimeter-wave transmitters and receivers, and in high-power X-band (10 GHz) microwave radar systems. Recently, the intrinsic cut-off frequencies of Si bipolar junction transistors (BJTs) and SiGe HBTs have increased to the range of 100 GHz by submicron scaling of the emitter dimensions with reported emitter stripe widths as small as 0.25 &mgr;m. However, Si BJTs and SiGe HBTs suffer from several performance disadvantages in comparison to compound semiconductor HBTs, including, for example, low collector breakdown voltages and the inability to integrate passive components in ICs including resistors, capacitors, and inductors due to the lossy, conductive Si substrate. Therefore,
Chaudhuri Olik
Toledo Fernando
Ward & Olivo
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