Multi-chip module integration of Hybrid Silicon CMOS and GaN Technologies for RF Transceivers

Abstract

Digital transceiver architectures offer the potential for achieving wireless hardware flexibility to frequency and modulation scheme for future-generation communications systems. Additionally, digital transmitters lend themselves to the use of switch-mode power amplifiers, which can have significantly higher efficiency than their linear counterparts. Two proposed architectures for realizing digital transmitters will be described in this work, both of which employ a hybrid combination of silicon integrated circuits (IC) and a power technology (e.g. GaN). This hybrid architecture takes advantage of the silicon to implement the high-complexity signal processing required for wireless communications, and uses power devices with high power density and low parasitic capacitance to sufficiently amplify the RF signals for transmission. Unfortunately, interfacing the low-power RF switching signals with off-chip high-power devices poses numerous design challenges, including: generation of integrated silicon power drivers with sufficient voltage swing for controlling power devices such as GaN, mitigation of on-chip current transients, wideband assembly interface from the silicon IC to the power device, and full system design verification using multiple process technologies. This work presents two CMOS driver architectures that can be used to interface low-power CMOS processing circuits with off-chip high-power devices. This work also details the performance limitations when assembling and interfacing multiple process technologies that are not co-located on the same IC. The main function of the driver circuitry within the digital transceiver system is to interface the low-power digital modulator to a large, high capacitance, off-chip power device. The driver must provide adequate transient current to charge/discharge the off-chip power devices' input capacitance through parasitic routing. Furthermore, the driver is designed to exhibit rise/fall times of less than 5% of the switching period and low jitter to meet RF signal quality requirements. Since silicon process technologies typically have much lower voltage breakdowns than those required to drive a power devie (e.g. GaN device), special driver architectures must be implemented to ensure the CMOS devices never exceed their breakdown voltages. Two architectures were implemented within this work to simultaneously achieve RF switching speeds and 5V signal swing from a 0.9V silicon CMOS process technology. The two architectures are: 1) a House-of-Cards configuration, and 2) a Cascode topology. These architectures will be detailed and compared with respect to performance in this presentation. Two of the most common techniques to assemble and connect a silicon IC, which includes the driver circuitry, and a (GaN) power device are: 1) direct wire bonding or flip-chip connection from the IC to the GaN, and 2) connection through a board or package interface circuit. Since most high-performance RF power devices such as GaN have negative threshold voltage, the driver (CMOS) IC must either: 1) have a supply and ground that are shifted to negative voltage values, or 2) decouple the IC's output from the GaN device's input in order to properly control the GaN. Off-chip decoupling is more easily implemented, but may limit maximum operating frequencies due to the added interface network and board/module parasitics. This work shall detail the interface models and compare the assembly procedures and potential performance limits when using both of these most common assembly techniques.