The Best Material for the Function: Seamless On-Wafer Integration of GaN and Si Devices

Hyung-Seok Lee, Kevin Ryu, Jinwook Chung, and Tomás Palacios*
Department of Electrical Engineering and Computer Science
Massachusetts Institute of Technology
77 Massachusetts Ave., Bldg. 39-567B, Cambridge MA 02139, USA

Abstract

The outstanding properties of nitride semiconductors are allowing this semiconductor family to revolutionize both electronics and optoelectronics. Their tunable bandgap, from 0.6
eV in InN to 6.2 eV in AlN allows the fabrication of light emitting diodes and lasers with output wavelengths varying from the deep ultraviolet to the green. In addition, the high
electron mobility and charge density possible in AlGaN/GaN and InAlN/GaN heterostructures has enabled the demonstration of power amplifiers with at least one order of magnitude higher output power density than GaAs or Si devices. The frequency performance of these devices is also quickly improving, with current gain and power gain cut-off frequencies approaching 300 GHz. However, in spite of this excellent performance, nitride-based devices cannot compete with Si CMOS electronics in terms of cost, scalability and circuit complexity. The seamless integration of these two semiconductors would give the circuit and system designer unprecedented flexibility to use the best material and devices for each function. Our group has been working for the last few years on developing manufacturable new technologies to enable this seamless integration. In this talk, we will review some of these technologies as well as the many new applications and opportunities that they enable.

The proposed GaN/Si integration is based on a Si(100)/GaN/Si hybrid wafer fabricated by wafer bonding technology as shown in Figure 1. For this integration, a Si-GaNSi
hybrid wafer is formed by bonding a Silicon (100)-on-Insulator wafer to the nitride surface of an AlGaN/GaN-on-Si (111) wafer, and subsequently etching away the top silicon and
oxide layers of the SOI structure. Hybrid wafers with diameters as large as 4” have been recently demonstrated (Figure 1). Table 1 shows some of the most relevant parameters in the fabrication of these hybrid wafers.

The Si/GaN/Si hybrid wafers are thermally stable up to 1,000 °C and Si CMOS electronics can be fabricated on them in a process similar to conventional silicon-on-insulator
technology. Once that the Si devices are finished, the top Si layer is removed from the regions where GaN devices are needed, and GaN transistors, surface acoustic wave devices, detectors or other GaN devices are fabricated following a CMOS-compatible technology [1]. Finally, the devices are interconnected and passivated.

Figure 2 shows the scanning electron micrograph of GaN and Si transistors fabricated using the technology described above. The current-voltage characteristics of some typical
devices are shown in Figure 3. These devices are currently being used to demonstrate hybrid circuits that take advantage of the seamless integration of GaN and Si devices. Some examples of these circuits include DC-DC power converters where the high voltage is controlled by GaN switches and the low voltage stage relies on Si devices (Figure 4), GaN power amplifiers in close proximity with Si linearizing circuits, and high power digital-toanalog converters. In this talk, we will review some of these applications and the implications that heterogeneous integration of devices has for the device, circuit and system designer.

Paper 10a.2.pdf