The Future Of SiGe Devices Beyond HBT Applications
In 1998 Temic (now Atmel) and IBM were the only companies to sell commercial HBT devices and circuits for the communications market. Today almost all BJT and BiCMOS IC suppliers worldwide, and even some competing GaAs companies, are working on SiGe. The fmax values first reported from the labs have since increased by nearly a factor of ten (figure 1), which indicates tremendous progress in device processing. Although production has lagged behind performance, because of a need to develop relaxed and robust structures, the SiGe HBT will continue to move into the semiconductor industry.
In parallel to the developments of HBTs, a second generation of SiGe devices has been investigated: the SiGe HFET. Using Ge to manipulate the device s band alignment and lattice parameters, strained-channel FETs with a Si two-dimensional electron gas (2DEG) or a SiGe two-dimensional hole gas (2DHG) have been developed with carrier mobilities far exceeding those of Si.
Four different types of FET can be distinguished by the layer stack structure shown in figure 2. A surface-channel MOSFET (figure 2a) typically comprises a tensile-strained Si channel on top of a strain-relieved SiGe buffer. The SiGe buffer acts as a virtual substrate and has an adjustable lattice constant. This device can be treated like a conventional CMOS transistor, with only a few restrictions on the thermal budget during processing. With appropriate p- and n-type doping below the channel, the threshold voltage is easily adjustable. In 2001 IBM, as well as ST Microelectronics, Hitachi, Motorola and others, predicted that this device would be the next one on the market after the SiGe HBT.
A buried-channel n-type modulation-doped FET (n-MODFET) (figure 2b) is an upgrade of the surface-channel MOSFET and contains a buried channel that allows modulation doping. Until now, Schottky and junction gates have been predominantly developed on this structure, but variations with gate dielectrics are also feasible. By altering the modulation doping and gate/channel distance, the sub-threshold slope, threshold voltage and breakthrough behavior can be tuned.
Simple buried-channel p-MODFETs (see figure 2c) can be fabricated by depositing a compressively strained SiGe channel directly onto the Si substrate, although the critical layer thickness limits the Ge content to about 30%. Much better performance can be achieved by using Ge-rich or even pure Ge 2DHG channels (figure 2d), but again a relaxed SiGe buffer as a virtual substrate is essential.The need for speedAn important figure of merit for these high-speed devices is their carrier mobility. Surface-channel MOSFETs on relaxed Si1-yGey buffers (with y = 10-30%), exhibit up to an 80% improvement in universal mobility compared with Si n-MOS devices. This can be translated into a performance enhancement of about 30-40% for transconductance, saturation current and ft.
Much higher mobilities can be achieved with the buried-channel n-MODFETs. Unlike gate-oxide surface-channel devices, these devices do not suffer from surface scattering. As a result, electron mobilities of up to 2800 cm2/Vs, which is five times greater than standard Si, have been achieved.
On the downside however, quite high amounts of Ge close to the surface can cause problems in devices that utilize gate oxides. To counter this, investigations into low thermal budget oxides or alternative dielectrics are currently underway.
Taking p-FET devices, improvements in effective-hole mobilities have been even more pronounced (figure 3). Pseudomorphic Si1-xGex channels (with x between 0.15 and 0.36) give mobility improvements of approximately a factor of two, again mainly because of reduced surface scattering.
These simple structures can achieve a p-FET performance close to that of standard Si n-MOS devices, which is an important requirement for future CMOS circuits. High Ge content or pure Ge channels on relaxed buffers benefit from Ge s extremely high hole mobility. For example, room-temperature mobilities of up to 3000 cm2/Vs have been reported for pure Ge 2DHGs, which is a factor of 15 above the mobility in standard Si p-MOS devices.
Over the last decade, researchers have been racing to develop both p-type and n-type MODFETs. Values for fmax have increased by a factor of 10 within the last seven years, and reached 183 GHz in late 2001 (figure 4).
These advances follow reductions in lateral device dimensions - gate lengths have decreased from 1 µm to less than 100 nm - as well as improvements in crystal quality and optimization of the layer stacks. Ge s high hole mobility also means that the performance of p-HFETs is expected to catch up with n-HFET performance in the near future.From prototype to productionTo transfer the high mobilities and oscillation frequencies achieved in prototype devices into real products, a strain-relieved virtual substrate is crucial. Most devices contain a graded buffer that is several microns thick, grown using MBE or UHV-CVD at rates typically below 0.5 nm/s.