For well over a century RF technology has been understood in sufficient detail for the design of basic communications systems. The application of RF to radar began in the 1930s and accelerated during World War II, driven by the pressing needs of the major war effort.
In both instances–communications and radar–electronic vacuum tubes (or “valves”) formed the active devices. What changed the entire electronics scene forever were the pivotal inventions of the transistor in 1947 and the integrated circuit in 1958. Key developments in microwave integrated circuits during the 1950s (right up to the present era) ensured continuous progress for solid-state RF technology. Twenty-first century RF systems exhibit the following trends: they are increasingly digital, increasingly software-based and almost entirely solid-state. At the systems level software-defined radio (SDR) is now at a high level of maturity and cognitive systems (radar and radio) are under development.
The overall physical dimensions of critical components and devices involved in communications systems and radars embrace an extremely wide range, from massive communications “towers” and phased-array radars all the way down to the nanometer-scale semiconductor devices involved in the electronics. Between these extremes there exist many types of modules and subsystems that perform specific signal-processing functions. Semiconductor devices form the critical technologies for all systems and therefore such devices form the initial considerations.
RF CMOS
Back in the 20th century it was almost unheard of to even remotely consider CMOS for RF applications because the highest speeds possible precluded such considerations. However, as MOS transistor feature sizes continued to shrink (and therefore operating speeds increased) so the possibilities of at least low-power microwave applications began to look feasible. Nowadays RF CMOS is well established for many microwave applications–especially where very densely-packed mixed-signal RFICs are concerned. The basic CMOS logic inverter represents an important example of how two contrasting types of MOS transistor are interconnected to form a fundamental type of circuit configuration, shown in Figure 1.1
Figure 1 CMOS logic Inverter Circuit.
The small circle symbol on the (gate) input to the upper transistor means this PMOS transistor’s gate directly connects to an “N-well” region. In contrast the lower (NMOS) transistor’s gate directly connects to the P-type substrate. When both transistors have minimum feature dimensions down into the sub-micron levels (increasingly nm) these types of circuits can be designed to process low-power microwave and mmWave signals.
RF CMOS and its derivatives now represent a mainstream RF technology that can be adopted for the relatively low-power portions of MMICs/RFICs.
SiGe BiCMOS
In order to increase the operating speed (hence also frequency) it is also possible to add one or occasionally two bipolar transistors to a CMOS circuit stage. Where such a BJT is NPN type then the base is often doped with germanium (Ge)–which serves to shrink the bandgap substantially. This silicon-germanium alloy is always termed “SiGe” and the resulting overall technology is: SiGe BiCMOS.1
Compared to silicon alone, the addition of Ge:
- Leads to a much higher value of the current gain (ß = Ic / IB), and;
- Also leads to a much larger transition frequency (fT).
SiGe BiCMOS technology is extensively and increasingly being adopted for RFICs going into high-volume applications such as automotive radars.
Some 130 nm SiGe BiCMOS transistors operate close to 1 THz. But a serious disadvantage is the very low breakdown voltage of typically around 1.7 V.
Adding BJTs to CMOS requires high-level multi-masking fabrication which is relatively expensive and is certainly not cost-effective for low-to-moderate production runs.
Another technology termed laterally-diffused metal-oxide-silicon (LDMOS) transistors is used for many u.h.f. high power amplifier applications.