ReviewPerformance trends of Si-based RF transistors
Introduction
About 15 years ago, the field of RF transistors has clearly been dominated by III–V transistors. The only commercially available Si-based RF transistor was the conventional Si bipolar transistor for application in the lower GHz range. Since that time, the situation changed dramatically, and both SiGe HBTs and Si RF MOSFETs became widely accepted RF devices recently. To get an impression on how the frequency performance of Si-based RF transistors evolved and how it competes with that of III–V transistors, Fig. 1, Fig. 2 show the best reported cutoff frequencies, fT, and maximum frequencies of oscillation, fmax, of Si MOSFETs, SiGe HBTs, III–V FETs, and III–V HBTs versus time. The data shown in Fig. 1, Fig. 2 (as well as those in Fig. 3, Fig. 4, Fig. 7, Fig. 8, Fig. 9 to be shown later) have been taken from two comprehensive data collections on the performance of all relevant RF transistor types currently in use or under development [1], [2].
The fT and fmax data vs. time plots clearly indicate that the frequency limits of the Si-based and of the III–V RF transistor types have been enhanced continuously. A closer inspection of the shown data reveals, however, several additional interesting details which are worth mentioning. The record fT and fmax data for III–V FETs stem solely from InP HEMTs, while the record fTs of GaAs MESFETs, AlGaAs/GaAs HEMTs, and GaAs pHEMTs (pseudomorphic HEMTs) are below 200 GHz and the best reported fmaxs of these transistors are below 350 GHz. The record performance of III–V HBTs in terms of fT and fmax has also been obtained almost exclusively with InP transistors. The only exceptions are the fmax records from the 1990s which have been achieved with GaAs HBTs (open triangles in Fig. 2). The current record fT and fmax are:
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fT 562 GHz [3] and fmax 600 GHz [4] for InP HEMTs,
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fT 550 GHz [5] and fmax 687 GHz [6] for InP HBTs,
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fT 380 GHz [7] and fmax 350 GHz [8] for SiGe HBTs,
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fT 330 GHz [9] and fmax 320 GHz [9] for Si MOSFETs.
It should be noted that only III–V HBTs with rather conventional designs have been included in Fig. 1, Fig. 2. Not covered are HBTs fabricated with a sophisticated transferred substrate process. Laboratory InP transferred substrate HBTs showing an fmax around 1 GHz have been reported recently [10]. This is the highest fmax ever reported for a transistor.
We see that presently the frequency limits of Si-based RF transistors are well above 200 GHz. Only a few years ago, such a statement would have been considered as wishful thinking!
In the following sections, we shall take a closer look at some specific aspects of the performance of Si-based RF transistors. In particular, the performance of SiGe HBTs will be reviewed and compared to that of GaAs and InP HBTs. The tradeoff between cutoff frequency and breakdown voltage, BVCEO, of SiGe HBTs and options to optimize the collector design will be discussed. We present the results of a simulation study, where the fT, fmax, and BVCEO have been considered simultaneously. Finally, the frequency limits and noise figures of Si RF MOSFETs will be reviewed and compared to those of GaAs pHEMTs.
Section snippets
SiGe HBTs vs III–V HBTs
In many cases it is important that an HBT shows simultaneously high fT and high fmax, i.e., fT ≈ fmax. To clarify to what extend high-performance SiGe, GaAs, and InP HBTs fulfill this requirement, Fig. 3 shows an fmax versus fT plot. Each data point indicates the fT and fmax obtained with one transistor. The best GaAs HBTs show maximum frequencies of oscillation in excess of 200 GHz, while the cutoff frequency is limited to about 150 GHz. The best reported SiGe HBTs reach both fmax and fT of around
Collector optimization for SiGe HBTs
We have shown that there exists a general tradeoff between speed and breakdown voltage for bipolar transistors and that especially high-speed SiGe HBTs suffer from low breakdown voltages. This leads us to the question whether the collector design can be optimized in order to improve the cutoff frequency without affecting the breakdown voltage.
To investigate this issue, we carried out a simulation study using the commercial device simulator ATLAS [12]. The DC and small-signal characteristics of
Si RF MOSFETs vs GaAs pHEMTs
Traditionally, the Si MOSFET has been considered a slow device not suitable for RF applications. Several reasons contributed to this conviction. First, the electron mobility in Si is by nature lower than in III–V compounds. Second, the current in a MOSFET flows in an inversion channel close to the Si/SiO2 interface where the carriers are subjected to the effects of interface roughness, interface traps, and crystal imperfections. As a result, the mobility is further degraded. Thanks to the
Conclusion
In recent years, the RF performance of Si-based RF transistors has been enhanced considerably. Meanwhile SiGe HBTs with frequency limits (fT and fmax) in excess of 300 GHz and Si RF MOSFETs with frequency limits well above 200 GHz have been realized. The RF noise behavior of Si-based RF transistors has also been improved considerably. Thus, Si-based RF transistors are successfully invading frequency ranges that had been the domain of III–V devices in the past.
Aside from the breakdown voltage,
Acknowledgement
This work has been supported by the Federal State of Thuringia under TMWTA contract No. B0509-03006.
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