Si Electronics: The Next Frontier

Tomás Palacios
Microsystems Technology Laboratories

Electrical engineering is at a crossroads. For the last fifty years, Si electronics has been driving the development of information technology, which has completely transformed our society. Conventional Si electronics, however, is reaching scaling and performance limits which jeopardizes future developments. The Palacios group at MIT is working on overcoming these limits by developing a new paradigm where performance increase is achieved by integrating Si electronics with multifunctional new materials, instead of by traditional scaling.

Researchers, including EECS graduate student Han Wang and physics graduate student Daniel Nezich under the direction of EECS professors Jing Kong and Tomás Palacios, have succeeded in laying the groundwork for a whole new technology of much faster microelectronic devices using graphene. (photo: Donna Coveney, MIT News Office)

Researchers, including EECS graduate student Han Wang and physics graduate student Daniel Nezich (standing right) under the direction of EECS professors Jing Kong and Tomás Palacios (left), have succeeded in laying the groundwork for a whole new technology of much faster microelectronic devices using graphene. (photo: Donna Coveney, MIT News Office)

Specifically, we are working to seamlessly integrate Si transistors and circuits with two new material systems: nitride semiconductors (GaN) and graphene. The integration with nitride semiconductors allows the combination of the high complexity and flexibility of Si electronics with the vast array of new devices enabled by nitride semiconductors. Some examples of these new devices include light emitting diodes and lasers for optical interconnections and sensing; highly efficient power amplifiers for advanced wireless communications; piezoelectric energy harvesting devices; and high voltage transistors for energy conversion with more than three orders of magnitude lower energy loss than the equivalent Si transistors.

Figure 1. Scanning electron micrograph of a Si (001) transistor in close proximity to a GaN transistor (HEMT). This is the first demonstration of the heterogeneous integration of Si MOSFETs and field effect devices fabricated in compound semiconductors.

Figure 1. Scanning electron micrograph of an Si (001) transistor in close proximity to a GaN transistor (HEMT). This is the first demonstration of the heterogeneous integration of Si MOSFETs and field effect devices fabricated in compound semiconductors.

Our group has recently demonstrated the first on-wafer integration of Si (001) MOSFETs with GaN-based transistors (Figure 1 above). The key enabler of this integration has been the development of a novel wafer bonding technology that allows the fabrication of a Si/GaN/Si hybrid wafer. This wafer, stable at up to 1200°C, can be processed following standard Si technology to fabricate the Si electronics. Then, the top Si layer is removed in selected regions to expose the GaN layer and to fabricate the nitride devices. Our group is using this new technology to develop, among other applications, a GaN/Si hybrid solution for power distribution in a microprocessor. In this new approach, the power is distributed within the chip at high voltages and low currents to minimize conductive losses, and then locally down-converted by nitride electronics to the low voltages required by the Si electronics. This new approach, enabled by the heterogeneous integration of GaN and Si electronics, is expected to reduce power consumption in microprocessors by at least 30%.

The Palacios group is also working on the integration of Si electronics with a one-atom-thick carbon-based material with transport properties several orders of magnitude better than silicon. Using some of the unique properties of this material, we have demonstrated some of the world-first graphene circuits, including frequency multipliers, detectors for RF radiation, and radio demodulators. Figure 2, below, shows an optical micrograph of the first graphene frequency multiplier fabricated on a Si substrate. This new device doubles the frequency of any RF input signal and could revolutionize high-speed communications, sub-mm wave imaging and sensors.

Figure 2, shows an optical micrograph of the first graphene frequency multiplier fabricated on an Si substrate. This new device doubles the frequency of any RF input signal and could revolutionize high-speed communications, sub-mm wave imaging and sensors.

Figure 2 shows an optical micrograph of the first graphene frequency multiplier fabricated on an Si substrate. This new device doubles the frequency of any RF input signal and could revolutionize high-speed communications, sub-mm wave imaging and sensors.

In summary, we believe that the heterogeneous integration of Si CMOS circuits with new advanced materials and devices is going to be key for the development of electronics in the future. The combination, in the same chip, of Si, GaN and graphene devices gives circuit designers unprecedented flexibility to fabricate new electronic systems that will transform society in the 21st century.

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