SiGe Technology for Revolution of Personal Communications

Guofu Niu

Silicon-Germanium (SiGe) technology is the driving force behind the explosion in low-cost, lightweight, personal communications devices like digital wireless handsets, as well as other entertainment and information technologies like digital set-top boxes, Direct Broadcast Satellite (DBS), automobile collision avoidance systems, and personal digital assistants. SiGe extends the life of wireless phone batteries, and allows smaller and more durable communication devices. Products combining the capabilities of cellular phones, global positioning, and Internet access in one package, are being designed using SiGe technology. These multifunction, low-cost, mobile client devices capable of communicating over voice and data networks represent a key element of the future of computing.

The heart of SiGe technology is a SiGe heterojunction bipolar transistor (HBT), which offers advantages over both conventional silicon bipolar and silicon CMOS for implementation of communications circuits. My SiGe research at Auburn includes the design, optimization, and testing of state-of-the-art SiGe heterojunction bipolar transistors and integrated circuits. We work closely with IBM, a world leader of SiGe technology since 1982, and the first company to broadly manufacture SiGe technology. The following gives an overview of this fascinating technology.

1. History of SiGe Technology

The concept of combining silicon (Si) and germanium (Ge) into an alloy for use in transistor engineering is an old one, and was probably envisioned by Shockley in his early transistor game. However, because of difficulties in growing lattice-matched SiGe alloy on Si, this concept is reduced to practical reality only in the last 15 years. SiGe HBT technology was originally developed at IBM for the high-end computing market, that effort, however, failed to CMOS, primarily because of its high power consumption.

In the early 1990s, IBM refocused its SiGe program towards the rapidly developing communications market. Interestingly, for RF communications circuits, SiGe HBT consumes much less power than CMOS to achieve the same level of performance. Since then, significant progress has been made. SiGe technology is being developed and applied around the world, and is in the product roadmap of every major telecommunication company. Applications range from wired and wireless communications circuits, to disk storages, to high speed high bandwidth instrumentation. The use of discrete SiGe HBTs and amplifiers in wireless devices is common place. Integrated SiGe chips can be found in GSM and CDMA wireless handsets and base stations, wireless LAN chipsets, and high-speed 10-40 Gb/s synchronous optical network (SONET) transceivers.

2. SiGe HBT Operation

A SiGe HBT is similar to a conventional Si bipolar transistor except for the base. SiGe, a material with narrower bandgap than Si, is used as the base material. Ge composition is typically graded across the base to create an acclerating electric field for minority carriers moving across the base, typically 30-50 kV/cm, as schematically shown in the Figure below.

A direct result of the Ge grading in the base is higher speed, and thus higher operating frequency. The transistor gain is also increased compared to a Si BJT, which can then be traded for a lower base resistance, and hence lower noise. For the same amount of operating current, SiGe HBT has a higher gain, lower RF noise, and low 1/f noise than an identically constructed Si BJT. The higher raw speed can be traded for lower power consumption as well.

3. SiGe HBT versus CMOS

With well-laid out CMOS devices showing fT and fmax values in excess of 140GHz, an often asked critical competitive question is "Why can't SiGe devices be replaced by CMOS?"

CMOS devices offer the advantages of high fT and fmax as well as superior linearity and lower voltage operation, due to lower threshold voltages (CMOS VT vs bipolar VBE). BJT devices offer the advantages of excellent noise performance and an improved transconductance. The density differences for different circuit applications are also of practical interest. For RF low-noise amplifiers, SiGe HBT circuits occupy one-quarter to one- third the area of CMOS circuits of equivalent functionality. While for dense caches in a microprocessor, CMOS circuits occupy one quarter to one third of the area of BJT circuits for the same functionality.

Noise is perhaps one of the major advantage of SiGe HBT over CMOS for RF design. The 1/f noise due to carrier trapping-detrapping at interface states and thermal noise due to gate and channel resistances are both significantly higher in CMOS than in SiGe HBTs. To reduce noise, very large CMOS devices and large operating current are often required.

Table 1: Comparison of CMOS with conventional and SiGe BJTs (After Harame)
Parameter CMOS Si BJT SiGe BJT
Ft High High High
Fmax High High High
Linearity Best Good Better
Vbe (or VT) tracking Poor Good Good
1/f noise Poor Good Good
Broadband noise Poor Good Good
Early voltage Poor OK Good
transconductance Poor Good Good

4. SiGe BiCMOS Integration

The potential of SiGe technology will never be realized by simply replacing GaAs parts in existing systems. The real strength of SiGe lies in its ability to integrate analog, RF and digital on a single chip using existing CMOS fabs. This is not possible with any other technologies (e.g. GaAs). Furthermore, it makes possible implementation of new architectures such as direct conversion and software radio.

The integration of SiGe HBT with CMOS is much more involved than simply adding a SiGe low-temperature epitaxy (LTE) process. The CMOS performance must be retained (the same as its parent CMOS process) after the addition of SiGe HBT in order to use existing digital ASIC libraries and design methodologies. Similarly, the CMOS processing steps must not significantly alter the doping profiles (and hence performance) of the SiGe HBT. The two primary issues of BiCMOS integration are thermal budget and the trade-off between process modularity and process sharing.

IBM's first-generation (5HP) 0.5um SiGe BiCMOS, used a ``base equal gate'' integration scheme. A common layer stack is used for both the HBT base and the FET polysilicon gates, thus reducing the number of process steps and mask counts. An illustration of this integration scheme is shown below, together with an electronic microscopy picture of the device cross section.

However, problems arise when CMOS advances to 0.24 um (6HP). The CMOS thermal cycle increased significantly to a level well above the SiGe HBT process thermal cycle. The most significant high-temperature steps in CMOS technology are high-temperature source/drain dopant activation for the NFETs and gate sidewall oxidation. To maintain a narrow SiGe base, the exposure of SiGe base to CMOS related high temperature steps must be minimized. A ``base after gate'' integration scheme was then developed, which builds the HBT after major high-temperature CMOS processing steps. Here, the HBT is built after formation of the gate, gate spacer, the LDD implants, and NFETs anneals. This minimizes cycling of the SiGe base. Since HBT processing is at low temperature, thermal effect on the FETs is minimal. The modularity of the ``base after gate'' scheme simplifies BiCMOS integration of SiGe HBT with a newer generation CMOS.

5. Progress and Future of SiGe

At present (Sep 2002), the fastest SiGe HBTs have greater than 210 GHz cutoff frequency (fT) and greater than 285 GHz maximum oscillation frequency. Digital ECL gates built with these HBTs show a gate delay of only 4.3 ps, with just a milliamp of electrical current. This is quite remarkable, as put by Dr. Bernard Meyerson,

Just as aircraft were once believed incapable of breaking an imaginary ‘sound barrier’, silicon-based transistors were once thought incapable of breaking a 200GHz speed barrier (now we are well above 200 GHz with SiGe)!
Undoubtedly, SiGe BiCMOS is the fastest growing semiconductor process ever, and is poised to continue to grow. In the future, faster HBTs will be produced through both scaling and innovative device structure optimization (which we do a lot here at Auburn), enabling higher than ever bandwidth communications. Experience gained and lessons learned from the past will speed the development of next-generation SiGe technologies for higher speed and integration level. Processing modularity and maturity may eventually enable the offering of multiple versions of HBTs optimized for wireless, wired, or storage application respectively, or a single HBT process that provides the best overall performance for all applications. The design and verification tools in current SiGe design kits will be improved to allow first-pass through high frequency mixed-signal design, which is currently feasible only in VLSI digital design.