What Is Silicon Germanium’s Place at the Semiconductor Table?
Cheaper than gallium arsenide. More flexible band-gap tuning than silicon. What's silicon germanium's place in circuit design?
Silicon germanium (SiGe) is now a popular semiconductor that has skyrocketed in its production since the 90s. But this semiconductor didn't rise to success overnight. In fact, the combination of silicon and germanium was discovered by accident.
The Inadvertent Discovery of SiGe
In the 1970s and 1980s, IBM researcher Dr. Bernard Meyerson accidentally dropped a small piece of silicon he had just cleaned in hydrofluoric acid on the ground. When he rinsed the silicon wafer under water to clean it from any dirt on the ground, he noticed that it was water-repellent.
SiGe chips became a variation to the CMOS transistors used in IBM ICs. Image used courtesy of IBM
Later, Meyerson discovered that when silicon was cleansed in hydrofluoric acid, a protective hydrogen layer would form, negating the need to heat silicon to 1000°C to rid silicon of the contaminating oxide. At 600°C, the hydrogen layer blows off and creates oxide. This discovery allowed scientists at IBM to grow silicon germanium at 550°C.
The Strengths of SiGe
SiGe technology has come a long way since its accidental discovery and commercialization. Since then, circuit designers have discovered that this material is more efficient than silicon—in terms of power consumption and performance—while also increasing the frequency and oscillating capabilities of a device.
IBM shipped its 100,000,000th SiGe chip in 2002. Image used courtesy of IBM
By utilizing SiGe technologies, circuit designers can benefit from a cost-effective solution that is smaller in size than silicon.
Some other benefits of SiGe, according to MACOM, include:
- Low overall component cost
- Power savings and high performance
- Numerous possibilities with integration
- High electronic mobility
- Small size
- High frequency and oscillating capability
Since electron mobility is faster in SiGe than silicon, SiGe is common in consumer electronics, automotive applications, telecommunications, computer technologies, and aerospace devices.
Key Applications for SiGe
Jan Fischer, NXP's product manager for Schottky and SiGe rectifiers, explains that certain industries, like the automotive industry, need a semiconductor capable of working at high temperatures while pushing for efficiency—in terms of miniaturization, performance, or regulatory compliance.
"As we see with other power semiconductor solutions, bulk silicon devices struggle to offer the required performance and thermal stability demanded by some of today’s high-performance electronic systems," Fischer says.
"What is needed is a novel technology for cutting-edge high efficiency, thermal stability, and space-savings."
Many designers consider SiGe a vital material for that "novel technology" to progress innovations in automotive, telecommunications, and solar industries.
As an example of how SiGe is making strides in automotive design, let's assess Nexperia's recently-released SiGe rectifiers.
SiGe rectifiers. Image used courtesy of Nexperia
Nexperia's collection of SiGe rectifiers features 120 V, 150 V, and 200 V reverse voltages. The company describes these SiGe rectifiers as a game-changer for designers since they can withstand 175°C degrees before the thermal runaway kicks in, allowing for a safe operating space.
Designers can take advantage of this thermal capacity—increasing the efficiency of their design without relying on fast-recovery diodes to handle high temperatures.
Because SiGe has a lower saturation voltage than silicon, it can be used in high-current applications. Maxim Integrated illustrates this point in an article on how SiGe technology enhances RF front-end performance.
Maxim compared its SiGe GST-3 bipolar transistor to its silicon GST-2 transistor, displaying the difference in gain vs. noise. In the figure below, you can see that the GST-3 has higher performance and lower noise than the GST-2.
SiGe bipolar transistors demonstrate high gain and low noise. Image used courtesy of Maxim Integrated
A larger, more high-quality signal range can be processed when SiGe is used.
The way SiGe is grown can also contribute to the performance and versatility of SiGe—a reality demonstrated by NASA.
NASA researchers found that by growing SiGe on sapphire substrates using a crystallized lattice growth method, they could use the same wafer to grow a layer of gallium nitride or indium gallium nitride on the opposite side. The end result: a solar-capable LED display.
NASA's patented process for developing crystal-aligned semiconductor wafers vs. existing processes. Image used courtesy of NASA
Solar applications typically have low-energy conversion rates (typically 15% to 20%). But with SiGe solar cells, that number can potentially rise to 30%-40% in energy conversion efficiency. Using single-crystal SiGe can also improve the operational life of a solar panel system from 25 to 30 years to approximately 80 years.
It's worthwhile noting, too, that most materials used for solar applications are more expensive and less abundant than SiGe.
The Trade-Offs of SiGe
While SiGe is useful in applications that require greater electron mobility, it does have trade-offs designers should be aware of. For instance, silicon remains king when it comes to devices that operate at extremely high temperatures. And even though SiGe costs less than gallium arsenide (GaAs), it is more expensive than silicon.
Although silicon and germanium are abundant materials, they do come at a high cost when combined in a lab. Luckily, SiGe's growing popularity has made it more accessible in recent years—especially in components geared for automotive, telecommunications, and solar environments.
What are your thoughts on SiGe as a semiconductor material? Have you encountered applications in which it was especially useful? From your perspective, what are the trade-offs of SiGe? Share your experiences in the comments below.