Sub-100�W Low Power Operation of Vibrating Body FETs

Daniel Grogg, Adrian Mihai Ionescu Nanoelectronic Devices Laboratory (Nanolab), Ecole Polytechnique Fédérale de Lausanne (EPFL),

CH-1015 Lausanne, Switzerland, Email: daniel.grogg@epfl.ch, adrian.ionescu@epfl.ch

ABSTRACT

This paper reports the low power operation of Vibrating Body Field Effect Transistors as active resonators for communication applications. For the first time we report active resonators operating at 2MHz and 20MHz with power consumption less than 100�W and Quality factors in the order of 3000. This performance opens new applications of devices for wireless sensor networks.

INTRODUCTION Microelectromechanical resonators could replace quartz crystal

resonators in electronic systems due to their potential for miniaturization and integration [1]. Further miniaturization faces the problem of decreasing signal levels, especially for capacitively coupled devices, as the motional resistance of such devices depends on the electrode area [2]. Among the different strategies proposed to lower the impedance of MEM resonators, piezoresistive [3] or FET [4-6] based current modulation seem best suited. In both methods, the resonator can be built in single crystal silicon without Q-factor reducing elements, such as solid gaps [7] or piezoelectric material [8]. However, they suffer from the drawback of static power consumption in the resonator structure, for instance in [3] a power consumption >1.9mW is reported. In this paper, we demonstrate sub-100�W power consumption for VB-FETs and provide detailed frequency characteristics for low power operation.

DEVICE STRUCTURE AND STATIC CHRACTERISTICS The Vibrating Body Field Effect Transistor (VB-FET) shown in

Fig.1 is fabricated in a 1.25�m thick SOI using the fabrication process previously described [9] to create 180nm lateral air gaps as shown in Fig.1b. A low-doped body region is placed in the center of the beam structure and designed to allow the formation of two independent lateral channels with a length of 1�m and a width of 1.25�m. As the body is n-type doped, the VB-FET behaves similar to a normally-on type transistor, which is represented by a resistor in parallel with two FETs (Fig.1c). Length and width of the beams determine the in-plane flexural vibration frequency and are used as naming scheme in this paper. The four beams characterized are 100�m or 30�m long and 3�m or 4�m wide.

Fig.2 shows the measured power consumption (ID*VD) of the devices calculated from the experimental ID-VD characteristics shown in the inset of Fig.2. At a VD of 50mV, the total power dissipated in the beam is around 1�W from which it is increasing to attain 1mW at a VD > 1.8V. The series resistance (RS) for source and drain, given by the length, the cross section area and the beam doping is the reason for the differences observed in the ID-VD and power-VD plots.

The transconductance (gm) (dID/dVG) of the devices versus VG is shown in Fig.3, which corresponds to a double gate operation.

All structures show a considerable modulation of gm with a maximum around VG=0V. For the 100�m long beams RS seems to be limiting the maximum gm. For negative values of VG this limitation disappears, as Rchan increases and the measured gm of all devices overlap.

Fig.4 reports the transconductance values for different drain voltages at VG=0V (maximum value), which is rising with VD as predicted by the theory of FETs. The slope starts to saturate at lower VD for the beams with a high series resistance RS.

FREQUENCY CHARACTERISTICS The frequency spectra of all four VB-FET resonators are shown

in Fig.5. The resonance frequencies vary between 1.5MHz (a) and 25.7MHz (d). The VG values was kept just below the pull-in voltage for the two 100�m long beams (10V and 16V for a and b respectively) and set to 30V for the two 30�m beams (c and d). The noise in Fig.5 a and b is explained by the very low power handling capability of the two 100�m beams (around -80dBm). As the MOSFET detection is an amplitude dependent detection mechanism, the signal transmission is inversely proportional to the Q-factor and the resonance frequency. Therefore, the measured motional resistances are strongly dependent on the resonance frequency of the device, as with traditional capacitive detection. The improvement over capacitive detection has been detailed elsewhere [6, 10] and is limited by the maximum gain of the integrated FET.

The output impedance (1/gDS) of the VB-FET was measured to be slightly above 2k� for low VD. In the measurement of Fig.5 the device is connected directly to the 50� impedance of the Vector Network Analyzer using a bias-T. This results in a poor impedance matching and additional losses. To improve this we used an external buffer amplifier with unity gain (BUF-634p). Fig.6 gives the frequency spectra of the 100�m long and 3�m wide device without the buffer (a) and with the buffer (b) under equal biasing conditions. In both cases, the strong influence of VD on gm is observed as reduced signal attenuation. The frequency changes slightly with the changing VD due to electrostatic effects. Most important is the improvement of the signal attenuation at resonance by 15dB independent of the Q-factor reduction observed.

CONCLUSION The unique characteristics of the VB-FET are detailed with

respect to possible sub-100�W low power applications. On these first generation devices, a motional resistance of 1.8k� at a frequency of 2MHz is obtained with a power consumption of only 30�W. Further, the importance of impedance matching is shown using a simple voltage buffer.

REFERENCES [1] Wan-Thai Hsu, Proc. of IEEE Freq. Control Symp. 2008, pp. 392-395. [2] C. T. C. Nguyen, IEEE Trans. Ultrasonics, Ferroelectrics and

Frequency Control, Vol. 54, 2007, pp. 251-270. [3] J.T.M. van Beek et al, IEDM Techn. Dig. 2007, pp. 411–414. [4] H. C. Nathanson, W. E. Newell, R. A. Wickstrom, and J. R. Davis, Jr.,

IEEE Trans. Electron Devices, vol. 14, 1967, pp. 117-133. [5] N. Abele et al., IEDM Tech. Dig., 2005, pp. 479-481. [6] D. Grogg et al, Digest of DRC 2008, June 2008, pp. 155-156. [7] L. Yu-Wei et al.,Proc. Joint IEEE Int. Freq. Contr./Precision Time &

Time Interval Symp., 2005, pp. 128-134. [8] G.K. Ho et al., IEEE Journal of Microelectromechanical Systems, Vol.

17, April 2008, pp. 512–520. [9] D. Grogg et al., Proc. of ESSDERC 2008. [10] D. Grogg et al., to appear, IEDM 2008.

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Fig. 1: a) SEM image of a double gate Vibrating Body. The beam is 50�m long amd 4�m wide and the FET-body indicated with a small square is 1�m long. b) A FIB cross-section through a VB-FET beam showing 180nm air-gaps on both sides. C) The n-type doping of the FET-body acts as a resistance in parallel with the two channels formed on the lateral sides of the beam.

Fig. 2: Power consumption versus applied drain voltage calculated from the ID-VD characteristic (inset) of the four beams. Due to the normally-on nature of the current generation VB-FET operation at low VD can reduce the power consumption by orders of magnitude.

Fig. 3: The transconductance as a function of the applied gate voltage in a double gate configuration shows a maximum around VG=0V for all beams. The series resistance to source and drain caused by the beam seems to limit the gm for 100�m long beams.

Fig. 4: Measured transconductance versus VD shows a linear trend for all VB-FETs for low values of VD. The transconductance value of the long beams starts to saturate at around 1V due to the series resistance of drain and source.

(a) (b)

(c) (d) Fig. 5: Transmission scattering parameters measured at a low drain voltage of VD=0.2V for all four VB-FETs. A motional resistance of ~2 k� is obtained for low frequency structures with a power dissipation of 30�W in the beam.

(a)

(b) Fig. 6: The effect of impedance matching was investigated using a buffer amplifier. It is found that (i) a considerable improvement is obtained and (ii) Q-factor loading starts to be a limiting factor.

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