Subsections

• 1.2.1 Devices Based on the III-V Material System
• 1.2.2 Devices Based on Silicon

1.2 Devices for RF Applications

Transistors for RF applications have been mostly a market for devices based on the III-V material system rather than for silicon technologies. These compound semiconductors are based on group III elements, for example aluminum (Al), gallium (Ga), or indium (In), and group V elements, for example arsenic (As), phosphorus (P), or antimon (Sb). III-V semiconductors provide better high-frequency performance, because the inherent physical properties such as the electron mobility enable the components to achieve a much higher performance. Consequently, III-V components are particularly useful for applications at higher frequencies or for higher data rates as required for broadband and RF wireless components as well as for satellite communications. However, as discussed in the sections below, silicon technologies have started to be a major competitor for such applications.

Besides of the material system, a distinction between the device type has to be made. Devices for RF applications can be split into two major groups: the heterojunction bipolar transistors (HBTs) and field effect transistors (FETs) such as MESFETs, high electron mobility transistors (HEMTs) and in recent time the RF MOSFET. A heterostructure device consists of two or more adjacent layers of different semiconductor materials. Due to the different material properties of these layers, there is an abrupt transition in the bandgap and carrier transport. The transit times of vertical bipolar devices such as HBTs mainly depend on the thickness of the base layer, which has to be as thin as possible to achieve an $f_\textrm {T}$ above $200\,$GHz [211,113], and on the base-collector space-charge regions. For silicon FETs, the performance depends on the capabilities of the lithography technology [68]. Basically, the most important limiting factor of the response of the transistor is the transit time of the minority carriers across the base region. Due to the performance advantage, the transconductance, high self-gain, low 1/f noise and other benefits bipolar transistors are still the device of choice for many applications, for example in the 40$\,$Gb market [68].

1.2.1 Devices Based on the III-V Material System

The GaAs heterojunction bipolar transistor (HBT) has been among the most popular RF devices and was applied in advanced mobile communication applications. Besides the performance, the advantages are a very low off-state power consumption as well as high current amplification [160].

In recent years a new III-V compound semiconductor technology emerged: devices based on indium phosphide (InP), which are able to replace GaAs as the material of choice for high-performance, high-volume commercial applications [210]. InP based technologies have numerous advantages over the GaAs system for many applications. For example, they offer performance advantages in fiber-optic, millimeter-wave and even wireless applications due to the high gain $\times$ breakdown voltage product and thus yield efficiency. In addition, the ability to produce cost-efficient high-volume InP microelectronics enables several markets for government and commercial applications. InP single heterojunction bipolar transistors demonstrated excellent cut-off frequencies $f_\textrm {T}$ and maximum oscillation frequencies $f_\mathrm{max}$, but due to the narrow bandgap collector only relatively low collector breakdown voltages BV $_{\mathrm{CEO}}$ - conventionally between 0.5$\,$ and 2.0$\,$V - are possible.

In order to increase the breakdown voltage, a second heterojunction and thus a double heterojunction bipolar transistor has been introduced. A number of promising results with cut-off frequencies up to 342$\,$GHz and breakdown voltages of the order of 6$\,$V have been demonstrated [102]. A wide bandgap material, for example typically InP or AlInAs, allows higher BV $_{\mathrm{CEO}}$ - up to 9$\,$V and more - due to the reduced collector fields and thus reduced impact ionization. In addition, the thermal conductivity is higher and the electron saturation velocity V $_\mathrm{sat}$ of InP is about two times higher than that of InGaAs resulting in a short collector transit time at high breakdown voltages [184]. The limitation is the collector current blocking due to the conduction band discontinuity. Thus, the design of the layer structure for the second (base collector) heterojunction is of utmost importance. A quaternary material, that is a positionally step graded InGaAsP, is commonly used to lower the conduction band spike [160].

At the moment, the most prominent high electron mobility transistor is the pseudomorphic AlGaAs/InGaAs HEMT, which still provides competitive performance (tough challenged by the SiGe HBT) for low-noise applications in receiver circuits up to 100$\,$GHz [160,101]. HEMTs based on narrow bandgap materials such as InGaAs and gate lengths below 100$\,$nm show cut-off frequencies beyond $400\,$GHz [160]. Recent results show a cut-off frequency of $547\,$GHz for an InGaAs/InAlAs HEMT [199] and $550\,$GHz for an InP HEMT [198]. Finally it is to note, that devices which incorporate nitrides have become popular in recent time [170]. The so-called III-nitride devices combine different advantages regarding the transport properties, thermal conductivity, and wide bandgaps results in high breakdown fields [160].

1.2.2 Devices Based on Silicon

Heterojunction bipolar transistors (HBT) based on silicon germanium (SiGe) are able to progressively replace devices of the III-V material system, because competitive typical figures of merit are already achieved. For example values for the cut-off frequency of 375$\,$GHz [173] (with associated $\ensuremath{f_\mathrm{max}}= 210\,$GHz), and for the maximum oscillation frequency of 285$\,$GHz [113] are reported. The major benefit of these devices is their compatibility with the standard CMOS process flow, where well-established sophisticated multi-layer metalization layers and interconnects are available [160].

Integrated circuits for optical transmission and wireless communication systems are based on SiGe HBT or BICMOS technologies. Among these applications are $40\,$Gb optical fiber links, $5.8\,$GHz electronic toll collection transceivers for intelligent transport systems, and integrated frequency divider circuits for wireless local area networks [236].

Development of such advanced devices is based on aggressive device scaling. Thereby, the design focuses separately on the emitter, the base, and the collector. The base transit time is reduced by thinning the base film as deposited, reducing thermal cycles in order to minimize the base dopant diffusion, adding carbons to reduce boron diffusion, and increasing the germanium ramp to accelerate electrons. The concentration in the collector is increased in order to reduce the collector-base space-charge layer and to increase the transconductance [69].

Due to the relatively low electron mobility, compared to III-V materials, and the location of the inversion channel near to the interface between silicon and silicon dioxide, silicon MOS transistors have always been regarded as slow devices. In addition, the relatively large gate length contributed to an inferior RF performance. Due to the aggressive downscaling in the last decade, an advanced RF MOSFET does in fact have a smaller gate length than comparable III-V FETs today. Due to the resulting peak cut-off frequencies up to $100\,$GHz with relatively low noise figures, complementary metal oxide semiconductor (CMOS) devices can now be employed also for RF applications.

The main reason for this development can be found in the recent advances in CMOS processing, gate length scaling, and progress in SOI technologies [132]. Despite of several scaling limits regarding short channel and hot carrier effects, gate tunneling and minimum oxide thickness the development has been already continued down below the $100\,$nm feature size. In fact development has sped up, so that the International Technology Roadmap for Semiconductors [67] had to be revised twice recently: the 1999 projection for the 2003 technology node was decreased from $120\,$nm to $100\,$nm, the gate length for devices in high-performance logic circuits from $85\,$nm to $45\,$nm. These developments have also a significant impact on the transistor speed. The continuous scaling has both reduced the chip size and accelerated the transistors.

Furthermore, there has been research in the area of materials compatible with silicon technology. In order to improve the performance of VLSI circuits, silicon germanium is applied due to its material properties. Strained silicon has been started to be used as channel material. This layer, which utilizes an underlying relaxed SiGe layer, allows to enhance both the electron and hole mobilities. In [120], laterally scaled Si-SiGe n-channel FETs are reported with $\ensuremath{f_\textrm{T}}= 79\,$GHz and $\ensuremath{f_\mathrm{max}}= 212\,$GHz for $\ensuremath{L_\mathrm{g}}= 80\,$nm and $\ensuremath{f_\textrm{T}}= 92\,$GHz for $\ensuremath{L_\mathrm{g}}= 70\,$nm. Related results can be found in [121,57].