Radio frequency integrated circuit semiconductor device technology

In recent years, the vigorous development of the wireless communication market, especially the rise of mobile phones and wireless Internet access services, has caused higher demands on wireless communication technologies. Small size, light weight, low power consumption and low cost are the development direction of wireless communication terminals, and radio frequency integrated circuit technology (RFIC) plays a key role in it. The emergence and development of RFIC have put forward new requirements for semiconductor devices, RF circuit analysis methods, and even receiver system structures.

Semiconductor device technology

In the RF field, performance and process requirements are much more complicated than digital integrated circuits themselves. Among them, power consumption, speed, and yield are the most important parameters. At the same time, the RF IC must also consider noise (broadband and narrowband), linearity, gain and power efficiency. In this way, the optimized devices used in RF ICs have been continuously improved and developed. Different RF functional parts will be implemented on different semiconductor device processes. At present, the semiconductor processes used in RFIC mainly include Si, SiGe, GaAs and InP. Their main applications in wireless communications are shown in Figure 1.

● Silicon devices: silicon integrated circuits include silicon bipolar transistors (Si-Bipolar Transistor), silicon-complementary metal-oxygen semiconductors (Si-CMOS), silicon bipolar complementary metal-oxygen semiconductors (Bi-CMOS) or silicon-germanium Quality junction bipolar transistor (SiGe HBT).

At present, the frequency of communication is generally below 2 GHz. In addition to power amplifiers, silicon integrated circuits are dominant in RF / IF modules. Due to the large capacity of silicon technology, single-chip mixed-mode integrated circuits can be composed of RF / IF / fundamental frequencies (single chip mixed mode IC), and can be operated with a single power supply, far more than gallium arsenide devices in price and integration, gallium arsenide and silicon integrated circuits, because of different material characteristics, the design method is also very different The silicon material, because there is no semi-insulating substrate (Semi-insulaTIon substrate), is equivalent to doing circuit design on a high-loss substrate, plus the device itself has a low gain, if you want to achieve the same high-frequency electrical properties as gallium arsenide Silicon RFIC is all about transistor miniaturization (such as sub-micron RF CMOS) or material structure improvement (such as SiGe heterojunction transistors) to increase the device's characteristic frequency fT. It is also necessary to use trench isolaTIon and other processes to improve the isolation and Q value between the circuits. The process is complicated and the number of masks is large. The defect rate and cost are also greatly increased. The high-frequency model is also not easy because of the obvious stray effects. grasp. At present, the silicon process can be qualified for RFICs above 5 GHz, but there are still deficiencies in RF front-ends with low-noise amplifiers, high-power amplifiers, and switches. Therefore, silicon process devices will be positioned in mid-frequency modules or low TIer ) 'S RF module.

It should be particularly pointed out that in the wireless transceiver, the digital signal processing part uses standard Si-CMOS technology, which usually accounts for more than 75% of the chip area. The requirements for indicators such as integration and power consumption make it impossible for him to use other than CMOS Other processes are implemented, so only the realization of CMOS integrated RF front-end can realize the monolithic integrated transceiver and ultimately the monolithic integrated mobile communication products. At present, with the development of CMOS technology, its unity gain cut-off frequency is close to GaAs level. At the same time, some unit circuits and transceivers of RF front-end using CMOS technology have appeared. This also makes it possible to use CMOS technology to achieve single-chip integration of mobile communication products. In addition, compared with other processes, the CMOS process has higher integration, lower cost, and lower power consumption, making it the mainstream direction of RFIC development.

● GaAs devices: The electrical characteristics of GaAs devices at high frequencies, high power, high efficiency, and low noise index far exceed that of silicon devices, and depleted GaAs field effect transistors (MESFETs) or high electron mobility transistors ( HEMT / PHEMT), it can have 80% power increase efficiency (PAE: Power added efficiency) under 3 V voltage operation, which is very suitable for the long-distance and long communication time requirements of high-tier (high TIer) wireless communication. All of them need negative power supply, which will increase the cost of product use. The complicated growth of HEMT devices and the control of the gate width also affect the consistency and ease of production of the process. The enhancement mode E-mode MESFET / HEMT does not require a negative power supply, while maintaining the excellent characteristics of its power amplifier, but its output power will be limited. Heterogeneous bipolar transistor (HBT) is another gallium arsenide device that does not require a negative power supply. Its power density, current drive capability and linearity all exceed FETs, which is suitable for designing high power. , High efficiency, high linearity microwave amplifier, HBT is the best device choice. The HBT device has advantages in phase noise, high gm, high power density, breakdown voltage and linearity. In addition, it can operate from a single power supply, which simplifies the circuit design and the difficulty of sub-system implementation. Development, especially microwave signal source and high linear amplifier circuit.

Circuit CAD technology

For integrated circuit design, design methods and high-level computer-aided design tools are the key to success. For the usual VLSI, there are a series of tools including synthesis, simulation, layout design, verification, test generation, etc. to support the entire design process. But for RFIC, there is not yet a complete set of CAD tools. The main front-end design tool is circuit-level simulation or simulation.

　　 ● Insufficiency of SPICE simulation

The usual circuit simulation uses the simulation technology represented by SPICE, which supports multiple simulations, as shown in Table 1. However, due to the characteristics of RFIC, there are many difficulties in using this type of circuit simulation technology.

First of all, most of the design indicators of RFIC are the indicators when the circuit is in a steady state, such as power gain, intermodulation, and distortion. The time domain simulation with SPICE must go through a transient process to reach the steady state. For longer transient processes Circuit, it takes a lot of calculation.

Secondly, RFIC usually has two or more signals with very different frequencies or varying speeds. A typical situation is a mixer, where the carrier frequency and the signal frequency often differ by several orders of magnitude. Others, such as the PLL capture process, the oscillator start-up process, etc., use SPICE to simulate these situations are very inefficient, because the simulation time depends on the slowest component, and the time step depends on the fastest component.

In addition, there are parasitic elements such as interconnection and packaging in RFIC, which SPICE cannot handle. The exact characteristics of these components are given by electromagnetic field analysis, which is generally suitable for description in the frequency domain and cannot be directly used for analysis in the time domain.

Finally, noise is an important factor that determines the performance of IC systems, such as signal-to-noise ratio and bit error rate. However, SPICE can only perform noise analysis on the situation where the linear amplifier and the noise source is a stable random process, but not on the RFIC system. Linear circuits, such as mixers and oscillators, are modulated by large signals due to noise, the statistical characteristics are no longer stable, and the phase noise characteristics of the mixing noise and the oscillator are different. The noise analysis method of linear circuits in SPICE cannot be used. .