January 11, 2003

Semiconductor electronics has been advancing and evolving at an enormous pace. This can be attributed mainly to dramatic reduction of the device dimensions and therefore integration of more and more transistors onto a single Silicon (Si) chip, that is Si VLSI (very large scale integration). Thanks to these advances, microprocessors now contain several tens of millions of transistors, Gbit DRAMs are commercially available, and the Si VLSI is multi-billion dollar industry. The trend of ever-increasing integration levels and decreasing device dimensions is expected to continue at least for the next eight years.

Besides Si VLSI, however, there are other emerging fields in microelectronics. Despite the fact that their current market share is much smaller than that of Si VLSI, some of these fields are in the state of dynamic growth. Among them, the radio frequency (RF) electronics with RF transistors as its basic building block is likely the most prominent one. Currently, we witness far-reaching upheavals in civil communication technology that have created mass consumer markets for RF systems. Mobile communications including cellular phones, mobile internet access, and new communication services will have an impact on human society at least as large as personal computers had in the past ten years. These new communication systems transmit, process, and receive great amounts of data in a very short period of time and in the GHz frequency range. RF transistors are the backbone of these modern communication systems. For example, the widespread use of mobile phone during the 90s created the first real mass market for transistors. In 1998, for the first time, more mobile phones than PCs had been produced. Till date, CMOS is the standard device and Si is the only semiconductor used in VLSI. In RF electronics, on the other hand, a wide variety of different semiconductor materials (Si, SiGe, GaAs, InP, and wide band-gap materials) and various transistor types, such as, bipolar junction transistor (BJT), heterojunction bipolar transistor (HBT), metal semiconductor field effect transistor (MESFET), high electron mobility transistor (HEMT) and metal oxide semiconductor field effect transistor (MOSFET) are also available.

Despite the higher cost of material and processing, HBTs have gained popularity in digital and microwave applications primarily because of their superior speed performance. Due to wide band-gap emitter used in HBTs a much higher base doping concentration (1019cm-3) can be used while still maintaining a reasonable current gain. Such a high base doping concentration thus allows the use of very thin base layer (e.g. 1000Ao) without having to be concerned about punch-through in the base. As a result the base resistance and the transit time are reduced and the “Early voltage” is increased, which leads to high switching speed and high cut off frequency >100 GHz.

In addition to cutoff frequency, HBTs have high current handling capability and excellent threshold voltage control; they do not suffer trapping effects, which are causes of the hysteresis in FETs; and they have the wide dynamic range and high output resistance due to large “early voltage”. For HBTs, however the thickness of the base layer, which is the primary region needs to be sufficiently wide to prevent punch-through but can be reproducibly fabricated in the neighborhood of 1000Ao or less by epitaxial growth technology such as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD)

The idea of utilizing the wide band-gap emitter to improve device performance was first proposed by Shokley in 1951 and later developed by Kroemer, who both were recipients of Nobel prize in physics for their respective work on BJT and HBT. But most developments on the AlGaAs/GaAs HBT, which has become the standard HBT technology, did not start until the early 1980, when the technology for MOCVD and MBE became practical. An ultra high speed AlGaAs/GaAs HBT with a frequency of unity current gain of 105GHz and fmax 175GHz has been reported.

Other types of HBTs have also emerged recently and shown promising applications. The two most notable ones are Si/SiGe and InP-based HBTs. The former has the advantage of being fabricated using the existing Si CMOS technology, with only few extra steps added. The latter is attractive due to the superior carrier transport properties of Inp (that is, higher electron velocities, narrow band-gap and lower surface recombination velocity than GaAs).

Greater than 95 per cent of today’s $200+ billion global market uses the semiconductor silicon to realize a host of ICs ranging from 233-MHz microprocessors to 64-Mb dynamic DRAM chips. This profound dominance of Si rests on a number of surprisingly practical advantages Si has over the other numerous semiconductors, including: (1) an extremely high quality dielectric (SiO2) can be trivially grown on Si and used for isolation, passivation or as an active layer (for instance, gate oxide); (2) Si can be grown in very large, virtually defect-free single crystal (200mm in production today, rapidly moving to 300 mm), yielding many low-cost ICs per wafer; (3) Si has excellent thermal properties allowing for the efficient removal of dissipated heat; (4) Si can be controllably doped with both n- and p-type impurities with extremely high dynamic range (1014-1022cm-3); (5) Si has excellent mechanical strength, facilitating ease of handling and fabrication; (6) it is easy to make very low-resistance ohmic contacts to Si, thus minimizing device parasitics; and (7) Si is extremely abundant and easily purified. Thus from an IC manufacturing stand point, Si is a dream come true.

The first SiGe BiCMOS technology was reported in 1992, and first large-scale integrated (LSI) circuit, a 1.2-Gsample/s digital-to-analog converter in 1993. The first SiGe HBTs with frequency response greater than 100 GHz were demonstrated in 1993-1994 and the first SiGe HBT technology entered commercial production on 200-mm wafers in 1994. During last decade a large number of different SiGe HBT technologies have been demonstrated throughout the world using a variety of SiGe epitaxial growth techniques, and during the past three to five years, these technologies have produced a large number of impressive circuit demonstrations for practical digital, RF, and microwave applications.

As mentioned earlier, there is an increasing trend of fabricating the Si/SiGe HBT from CMOS process, thus taking the advantage of using the existing Si technology and high performance of HBT. There are three main differences in such a device compared to epitaxial-growth HBT: (1) the doping in the emitter and base are non-uniform, (2) the spike in the conduction band is negligible, and (3) the alloy content (that is, Ge content) is non-uniform.

The first successfully realized HBTs for microwave applications were based on GaAs. Much work on InP and SiGe HBTs have also been done recently. HBTs have promising ultra high-speed digital-circuit applications such as for frequency dividers, analog-to-digital (digital-to-analog) converters, shift registers and logic families, and as well as low-noise microwave circuit applications, since they posses low base resistance, low base-emitter capacitance, and thus high cutoff frequencies, while maintaining large current drivability. Recently, InP device based on GaAs material with an fT of 170GHz and fmax of 230GHz have been reported. These devices have great potential for use in digital ICs and broadband amplifiers used in optical-fibre transmission systems with bit rate over 10Gb/s.

Many of the III-V compound semiconductors (for instance, GaAs or InP), enjoy higher mobilities and saturation velocities and because of their direct gap nature, make excellent optical devices. In addition the way they are grown, can be compositionally tailored for a specific need application. With these materials there are practical deficiencies associated for making highly integrated low-cost ICs. There is no decent grown oxide for GaAs or InP, for instance and wafers are smaller with much higher defect densities, more prone to breakage, poor heat conductors, etc. This translates into generally lower levels of integration, more difficult fabrication, lower yield, and ultimately higher cost.

The SiGe HBT is one promising candidate for microwave/millimeter-wave wireless communication systems and optical communication systems operating at 10GB/s and over. This is because HBTs can be used in both high-speed digital circuits and high frequency analog circuits. Moreover, the fact that a multifunctional LSI can be obtained by integrating HBTs with CMOS transistors, is a big advantage over compound semiconductor devices. The IBM’s SiGe HBT technology has been qualified in 1998, and in commercial production on 200-mm wafers in an advanced CMOS fabrication facility, and is thus arguably the most “real” SiGe HBT technology world-wide.

An important emerging market for RF and microwave circuit technologies is in space-born satellite systems, key components in the requisite infrastructure to support global communications networks for voice, video, and data transmission. Space is an amazingly hostile environment, due not only to the extreme temperature variations encountered between solar shade and solar illumination, but importantly from radiation standpoint. From a radiation immunity viewpoint, SiGe HBT structure has several intrinsic advantages: (1) the EB spacer is very thin and composed of radiation-hard oxide-nitride composite; (2) the extrinsic base doping under the EB spacer is very high, effectively confining any ionization damage in the region; (3) the active device region is very thin (<200 nm) and, hence, the total volume exposed to practical displacement damage is minimal; and (4) the deep- and shallow-trench isolation minimizes the exposure of oxides that can contribute to junction leakage. Thus, SiGe technology offers promise as a high-speed low-cost alternative for applications requiring some level of radiation tolerance.

The past five years have seen a dramatic increase in the number of practical circuits implemented in SiGe HBT technology. The targeted applications range in frequency from cellular phones at 900MHz to optical data links at 40Gb/s. The present high volume RF market exist at 900MHz and 2.0GHz. Clearly, a 70-GHz fmax transistor is not needed here. As application frequencies continue to rise over time and these higher frequency markets develop and mature, SiGe HBT technology will increasingly favored over other Si technologies because of its performance advantages. SiGe technology appears to clearly be capable of meeting performance specs at system frequencies as high as 20GHz, and probably 40GHz, at least for not requiring high-breakdown voltage.

The operating frequencies of civil RF applications range from a few hundred of MHz up to 100GHz. Currently most systems having real mass markets operate at frequencies below 5GHz. The number of units sold in these markets is in the order of millions per year. At lower bound of this frequency spectrum, a mass market is the pagers operating at frequencies from 200 to 900MHz. Cellular phones and the global system for mobile communications running around 900MHz represent another large market for RF transistors. More advanced mobile communications services, such as digital European cordless telecommunications (DCET), personal communications system (PCS), Japanese personal hand-phone systems (JPHS), and digital communications systems (DCS), operating in the frequency range from 1.8 to 2GHz are also on the rise. A new system called the wireless local area network (WLAN) occupies the frequency band around 2.4GHz. WLAN had been expected to be produced in large volumes some years ago but the acceptance of traditional inexpensive coaxial-based systems by the majority of the users prevented the breakthrough of WLANs. Currently, number of large companies is having another try under the denotation of Bluetooth. Bluetooth is designed to wirelessly link devices like computers (desktops and notebooks), printers, cellular phones, digital cameras, etc. not directly connected to mobile communications is the global positioning system (GPS) operating at 1.8GHz. GPS is another market segment within expected large volumes for RF transistors.

Examples for civil communication system in the higher frequency range are direct-to-satellite communication (20GHz downlink, 30GHz uplink), local multi-point communication services (LMCS) (27.5 to 29.5GHz), millimetre-wave digital radio systems (23 and 38GHz, and possibly 12, 15, 18, 26GHZ). Another field of civil RF applications is automobiles. The envisaged applications include the GPS mentioned above, collision avoidance radar (77GHz in Europe), vehicle identification, traffic management, and others. Certain radar and sensor applications will operate around 94GHz. When InP HEMTs became operable above 100GHz in late 1990, discussions started focusing on use by RF transistors also in frequency bands in the range of 140 to 220GHz. There are low attenuation atmospheric windows at 140-165 and 200-220GHz for these applications.

In general, the civil RF markets in the different ranges mentioned above are expected to grow quickly in the coming years. Most of the market segments are not saturated yet, and it is certain that new applications will create even larger consumer markets. There is, however, no guarantee that a new and useful RF product will automatically find commercial acceptance at the market. Examples for that are the delay in the large volume production of WLAN systems and the current economic turbulence of the direct-to-satellite communication system IRIDIUM.

The writer is a Fulbright fellow and Professor at the department of Electronic & Telecommunication Engineering, Mehran University of Engineering & Technology, Jamshoro

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