[IEEE IEEE Instrumentation and Measurement Technology Conference - IMTC '94 - Hamamatsu, Japan (10-12 May 1994)] Conference Proceedings. 10th Anniversary. IMTC/94. Advanced Technologies in I & M. 1994 IEEE Instrumentation and Measurement Technolgy Conference (Cat. No.94CH3424-9) - Instantaneous S parameters measurements of MESFETs under burst bias conditions

THAM 2-1 Instanteneous S Parameters Measurements of MESFETs Under Burst Bias Conditions M. Bkgin, F.M. Ghannouchi, L. Selmi* and B. Riccb* Microwave Research Laboratory h o l e Polytechnique de M o n t m P.O. Box 6079, Station A Montrkl, P.Q. Canada, H3C 3A7 *Dept. of Electronics. University of Bologna Vide Risorgimento 2 40136, Bologna, Italy Abstract- An innovative Six-Port Network Analyzer (SPNA) for characterizing GaAs MESFETs under pulsed bias and RF conditions is presented in this paper. This SPNA allows the measurements of instantaneous S parameters of microwave active devices. These S parameters may then be used to extract the electrical model elements (Cgs, Cgd, Gds, Gm, etc.) of the MESFET using standard procedures [l]. This system is particularly suitable for studying trapping and self-heating effects in GaAs devices. INTRODUCTION The ongoing development of mobile and satellite communications technology requires the design of microwave integrated circuits with higher output power, efficiency and linearity. In the same way, there is a growing need for high-speed analog and digital circuits: dynamic RAMS, analog-to-digital converters, RF switches, etc. Advances in GaAs-based fabrication technology along with the higher speed and lower power consumption of GaAs devices make use of this material more and more attractive over silicon. Due to its low thermal conductivity GaAs devices exhibit self- heating effects that limit its performances [2,3]. Furthermore, trap levels located near the surface, at the interface of the active region and the semi-insulating substrate or in the bulk material cause such effects as frequency dispersion and gate and drain transients [3- 51. The design of microwave circuits using CAD techniques requires accurate modeling of the active devices used. Current characterization methods often neglect temperature and trapping effects and do not accurately simulate the actual operating conditions of the transistor under pulsed conditions. This paper proposes a new method for characterizing GaAs MESFETs using a six-port network analyzer. The SPNA is operated in burst mode and is capable of measuring the instantaneous S parameters of the device under test at the maximum speed of 10 Msamplesls. OPERATION OFTHE SIX-PORT NETWORK ANALYZER The six-port network analyzer includes mainly: 1) two six-port junctions, 2) eight linearized diode detectors, 3) a digitizing oscilloscope to record the voltage waveforms at the outputs of the eight diode detectors, 4) a pulse generator and 5) a microwave generator (see figure 1). The linearisation of the diode detectors and the calibration for reflection measurements are described in reference [6,7]. A burst of voltage is applied at the drain of the MESFET while maintaining the gate bias constant. The amplitude of the pulse may be varied from OV to 12V and its width can range from 2us to l.5sec. Voltage waveforms are recorded within the duration of the pulse at the maximum speed of lOMsamples/s. Since only a two- channel oscilloscope was available, a multiplexer was used to record successively the eight output voltage readings from the two six-port junctions. Thus, a minimum of eight bursts must be applied at the drain side of the MESFET in order to convert these voltages in S parameters. Figure 1: Block-diagram of the Six-Port Network Analyzer. 0-7803-1880-3/94/$4.00 01994 IEEE -858- IMTC '94 May 10-12, Hamamatsu EXPWMENTAL RESULTS , ............ j ................................ i ................................ -----!.--a --_ __._* i- 1 3 -L ............................. i ............................... .i .............................. 1 2.5 - I This six-port network analyzer was used to characterize a medium power MESFET (NE900189). The DC I-V curves of the MESFETs are illustrated in figure 2. These curves reveal severe thermal effects resulting in negative output resistance in the saturation region at high drain voltages. Instantaneous S parameters of the device were then measured using the SPNA. The frequency was set to 2.3 GHz and the width of the burst was 1.5 seconde. Fig. 3 and 4 show the results obtained at two bias points, that is Vgs=- l S V , Vds=lO.OV and Vgs=O.OV, Vds=lO.OV respectively. Significant decrease in the magnitude of S21 may be noted especially at Vgs=O.OV where the channel temperature is higher. This effect can best be seen in fig.5 where time is plotted on a logarithmic scale. After lms, the percentage reduction in the magnitude of S21 is similar for both bias points (around 10%) while the overall decrease at the end of the pulse is 16 % for the first bias point (Vgs=-l.SV) and 22% for the second one (Vgs=O.OV). As for the other S parameters, except maybe for S22 (related to the output resistance which is known to be temperature-dependent), only minor changes can be 102 - 9 8 I % 90 seen (less than 5%). Ids(Vds,Vgs) (mA) 200 150 100 50 0 1 - 1 . . . I I I . . ..... f?? .j ............. i ............. i ............. T ..... - : : : . . . . . . . . . : Figure 2: DC I-V curves of the MESFET. 0.85 -90 I.-.-phao(! -95 1 -&g(Sll) 0.81 _ _ ............................. : ............................... i : .............................. ..-a .___.____; + - ..--*.--.--: 0 500 1500 time(ms) Figure3.a: Mag(S11) and Pha(S11) for Vgs=-lSV and Vds=lO.OV. --_-- Pha(S12) 0.08 ........................... ..: ............................... ............................. 0.07 ............................. .............................................................. 0.06 ........................ j ............................. 0.05 ; ............................. : ........ ....................................................... 45 35 0.w F 25 sbo ldoo 1B time(ms) Figure 3.b: Mag(S12) and Pha(S12) for V g s - l S V and Vds=lO.OV. I -Mag(S21) Figure 3.c: Mag(S21) and Pha(S21) for Vgs=-l.SV and Vds=lO.OV. 0.6 ! 1-60 0.65 -C I I ----Phn(S22) . ..... l ........ I- i".." ..... -...r-.- i .....-, A- ...... ..................... -64 -50 -54 0.4 timgms) Figure 3.d: Mag(S22) and Pha(S22) for Vgs=-l.SV and Vds=lO.OV. 0.85 ! I -95 {[-fig(Sl:l) I I--;Pha(Sll) 1 1 - ~ o o 0.81 ........................... + -.. ............................................. ".. ! i 0.77 ) --.- .- -----!-. ----------.- -lo' -1 10 0.73 ........ ...+................................. A:.:.""".:.:"' 0.69 -1 15 -120 i I 0.77 ) .- -----!-. ----------.- -lo' -1 10 0.73 ........ ...+................................. A:.:.""".:.:"' -1 15 0.69 . ...................................................................................... i I 4 -120 I f sbo 0.65 ldoo I time(ms) Figure 4.a: Mag(S 11) and Pha(S11) for Vgs=O.OV and Vds10.OV. 0.08 ! 185 0.07 ............................................................................................ i I jbo Id00 0.03 1 time(ms) Figure 4.b: Mag(S12) and Pha(S12) for Vgs=O.OV and V d s 1O.OV. time(ms) Figure 4.c: Mag(S2l) and Pha(S21) for Vgs=O.OV Vds=lO.OV. and Figure 4.d: Mag(S22) and Pha(S22) for Vgs=O.OV and Vds=lO.OV. 4 3.5 3 2.5 0 time(ms) Fig. 5: Mag(S21) for the two bias points. -860- CONCLUSION An innovative six-port network analyzer operated in burst mode was presented in this paper. This SPNA was used to characterized a MESFET under burst drain bias and RF conditions and thermal effect was observed particularly in the magnitude of S21. Such S parameters give valuable information on the transient and self-heating behaviours of MESFETs. Furthermore, using standard techniques [ 13, a dynamic small-signal electrical model could be extracted from these S parameters. REFERENCES [ l ] DAMBRINE G., CAPPY A., HELIODORE F. and PLAYEZ E., "A New Method for Determining the FET Small-Signal Equivalent Circuit", IEEE Trans. Microwave Theory Tech., [2] SELMI L . and RICCO B., "Thermal Characterisation of GaAs MESFETs by Means of Pulsed Measurements", IEEE International Device Meeting, Dec. 1991. Vol. 36, pp. 1151-1 159, July 1988. [3] CANFIEL C., LAM S.C.F. and ALLSTOT D.J., "Modeling of Frequency and Temperature Effects in GaAs MESFET's", IEEE J. of Solid-state Circ., Vol25, pp. 299-306, February 1990. [4] GOLIO J .M. et al., "Frequency-dependent electrical characteristics of GaAs MESFET's", IEEE Trans. on Elec. Devices, Vol. 37, pp. 1217- 1227, May 1990. [5] LADBROOKE P.H. and BLIGHT S.R., "Low- f i e l d l o w - f r e q u e n c y d i s p e r s i o n of transconductance in GaAs MESFET's with implications for other rate-dependent anomalies", IEEE Trans. on Elec. Devices, Vol. 35, pp. 257- 267, March 1988. [6] DEMERS Y . , B O S I S I O R.G. a n d GHANNOUCHI F.M., "Repetitive and single- shot pulse microwave six-port reflectometer", IEEE Trans. on Instr. Meas., Vol. 39, pp. 195- 200, February 1990. [7] GHANNOUCHI F.M. and BOSISIO R.G., "A wideband millimeter wave six-port reflectometer using four diode detectors calibrated without a power ratio standard, IEEE Trans. Instr. Meas., Vol. 40, pp. 1043-1046, December 1991. - 861