GaAs PHEMTs

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Microelectron. Reliab., Vol. 36, No. 11/12, pp. 1899-1902, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0026-2714/96 $15.00+.00

) Pergamon

PII: S0026-2714(96)00224-7

THE EFFECT OF HOT ELECTRON STRESS ON THE DC AND MICROWAVE C H A R A C T E R I S T I C S OF A1GaAs/InGaAs/GaAs PHEMTs R. MENOZZI 1, M. B O R G A R I N O 1, P. COVA l, Y. B A E Y E N S 2 and F. F A N T I N I 1 1Dipartimento di Ingegneria dell'lnformazione, Universit£ di Parma, Viale delle Scienze, 43100 Parma, Italy. 2KULeuven/IMEC, Mercierlaan, B-3001 Heverlee, Belgium.

A b s t r a c t : This work reports on hot electron reliability of 0.25 #m A/0.25-

Gao.7sAs/Ino.2Gao.sAs/GaAs PHEMTs from the viewpoint of both DC and RF characteristics. The changes of DC and RF behavior after high drain bias stressing are shown to be strongly correlated. Both Call be attributed to a decrease of the threshold voltage, yielding different effects on gain depending on the bias point and circuitry chosen for device operation: a fixed current bias scheme is shown to minimize the changes induced by tile stress. Copyright © 1996 Elsevier Science Ltd INTRODUCTION Although hot electron and impact ionization effects are of great relevance in millimeter wave FETs, due to the very short gate lengths required to achieve high operating frequencies and to the low InGaAs-channel bandgap energies, limited data has been made available so far on the possible reliability implications of such phenomena. In particular, few papers have been published [1]-[4] on the effect of hot electron stressing on the DC behavior of A1GaAs/InGaAs/GaAs pseudomorphic HEMTs (PHEMTs) which, in terms of gain, noise figure and cutoff frequency, represent the state of the art of commercially exploitable devices for microwave and millimeter wave applications. Furthermore, with the exception of a few hints in [2], no indications can be found on the high frequency effects of hot electrons. From a practical viewl)oint it would be most useful to have indications about the potential degradation of the device RF gain, and its correlation with the changes observed in the easily measurable DC characteristics. This work is aimed at giving a contribution to this issue.

EXPERIMENTAL

RESULTS AND

DISCUSSION

We applied hot electron stress cycles to 0.25 #m Alo.25Gao.rsAs/Ino.2Gao.sAs/GaAs PHEMTs designed and fabricated at IMEC for millimeter-wave low-noise applications. The devices, passivated with SiN, feature maximum gin's of about 600 mS~ram and unity 36/11/12-V

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current gain cutoff frequencies (fT") around 70 G H z . The effect of hot electrons on the device performance was evaluated by completely characterizing the samples at DC and RF before and after 60 rain accelerated stress at VDS = 5.5 V , Vc;s = 0 V .

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Figure 1: a) Output characteristics of one of the devices under test before and after a 60 min stress at VDS = 5.5 V, VGs = 0 V. VGs ranges from --0.4 V to 0.3 V with 0.1 V increments, b) Transconductance measured at Vos = 2 V before and after the stress of Fig. la.

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Vos [V] Figure 2: Gate current measured in the conditions of Fig. la before and after the stress of Fig. la. V~s ranges from -0.4 V to 0.2 V with 0.2 V increments. Fig. l a shows t h a t after the hot electron stress the DC o u t p u t characteristics display a relevant increase of drain current in the whole bias range explored. This increase is entirely due to a threshold voltage (VT) decrease, as clearly suggested by Fig. lb, where the gm curve is seen to be shifted by about 60 - 70 mV in the negative VGs direction, without any change of shape, after the stress. The gate reverse current (which under high drain bias conditions is made up of contributions due to leakage and to collection of holes generated in the channel by impact ionization) remains at low values of few hundred nA after the stress, as shown by Fig. 2. This indicates t h a t the gate contact area has not been severely altered during the hot electron stress; the small increase of the magnitude of the gate current can be attributed to the drain current increase enhancing the amount of holes generated by impact ionization.

Hot electron stress on DC and microwave 16

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f r e q u e n c y [GHz]

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Figure 3: a) Magnitude of $21 measured at Vas = 0 V, VDs = 2 V before and after the stress of Fig. la. After the stress, IS211 has been measured also at Vas = -0.065 V to compensate for the VT decrease and get the same It)s as before the stress, b) Magnitude of ,5~21 measured at Vc_;s = 0.3 V, VDs = 2 V before and after the stress of Fig. la. After the stress, IS21l has been measured also at Vas = -0.235 V to compensate for the VT decrease and get the same IDs as before the stress.

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frequency [GHz] Figure 4: Magnitude of h2x measured at Vcjs = 0.3 V, VD._~-= 2 V before and after tim stress of Fig. la. f T decreases from 72 G H z to 66 G H z after the stress.

To get a more complete picture, we have measured the device S - p a r a m e t e r s at different bias points, in the 1 - 50 G H z range, using a HP-8510 network analyzer and a Cascade coplanar probe s t a t i o n . T h e most i m p o r t a n t effect observed was a change of $21, strictly linked with the variation of gm described above. Figs. 3a and 3b show the m a g n i t u d e of $21 as a function of frequency, at Vc;s = 0 V and VGS -- 0.3 V, respectively ( V D 8 -- 2 V in b o t h cases). At V(js --- 0 V gm is increased by the stress (Fig. lb); as a consequence, IS21i increases as well (Fig. 3a). On the contrary, at VGS -- 0.3 V gm is lower after the stress (Fig. lb), and the device RF gain decreases accordingly (Fig. 3b). Once more we realize t h a t the only change b r o u g h t a b o u t by the stress is a VT decrease: if we m e a s u r e the post-stress S - p a r a m e t e r s a d j u s t i n g VG8 in such a way as to get the same drain current we had before the stress, while kpeping the drain bias fixed at 2 V, i.e. if we c o m p e n s a t e for the VT shift, $21 is seen to coincide in the virgin and stressed devices (Figs. 3a and 3b). It is w o r t h pointing o u t t h a t this has i m p o r t a n t practical implications: due to the

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spread of threshold voltage values, HEMTs are COlnmonly biased by imposing the drain current rather than the gate-source voltage; in this case the observed change of VT is automatically compensated by the bias circuitry, and the RF behavior does not change. Finally, Fig. 4 shows the impact of the stress on the unity current gain cutoff frequency fT at V D S - - 2 V, VGS = 0.3 V: fT decrea.ses from 71 G H z to 66 G H z ms a consequence of the 9m reduction at this bias point. As to the physical mechanism underlying the measured phenomena, the results obtained on these and other P H E M T devices indicate that a build-up of positive charge in the semiconductor region underneath the gate is responsible for the decrease of VT. The positive charge accumulation is likely due to deep traps capturing holes generated by impact ionization and flowing toward the gate. In a study performed on PHEMTs from a different manufacturer but showing the same kind of change of the DC characteristics after the hot electron stress, a clear activation energy could be extracted for the trapping process, corresponding to a DX-like level in the A1GaAs [1]. A similar process might take place here, even though, unlike in [1] (where the recovery was complete), only about 25% of the VT shift is recovered over several weeks of device storage at room temperature. Different trap levels, some of which very deep, may thus be involved in the process, although presently we have no quantitative information on the related energy levels and time constants. Different phenomena capable of producing a change of the gate built-in voltage and of VT, such as "gate sinking", are unlikely to occur due to the very low gate current density and to the fact that the stress temperature is limited to the value caused by device self-heating. SUMMARY To conclude, we have for the first time shown data correlating the DC and RF effects of hot electron stress of millimeter-wave GaAs-based PHEMTs. DC and RF effects show a clear correlation, which allows to skip the costly and time-consuming high frequency characterization in the hot electron reliability evaluation of the PHEMTs under test. The stress effect on device performance, namely a negative shift of the threshold voltage, varies depending on the operating biers point and on the bias circuitry adopted: in particular, results have shown that a current-driven arrangement would minimize the stress effects. References 1. C. Canali, P. Cova, E. De Bortoli, F. Fantini, G. Meneghesso, R. Menozzi, and E. Zanoni, Enhancement and degradation of drain current in pseudomorphic AlGaAs/InGaAs HEMT's induced by hot-electrons, IEEE Int. Reliab. Phys. Proc. 205-211 (1995). 2. Y. A. Tkachenko, C. J. Wei, J. C. M. Hwang, T. D. Harris, R. D. Grober, D. M. Hwang, L. Aucoin and S. Shanfield, Hot-electron induced degradation of pseudomorphic high-electron mobility transistors, IEEE 1995 Microwave and Millimeter-Wave Monolithic Circuits Syrup. Proc. Orlando, Florida, 115-118 (1995). 3. R. Menozzi, P. Cova, C. Canali and F. Fantini, Breakdown walkout in pseudomorphic ttEMT's, IEEE T, uns. Electron Dev. 43, 543-546 (1996). 4. G. Meneghesso, C. Canali, P. Cova, E. De Bortoli and E. Zanoni, Trapped charge modulation: a new cause of instability in A1GaAs/InGaAs pseudomorphic HEMTs, IEEE Electron Dev. Lctt., 17, 232-234 (1996).