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A new failure mechanism of AlGaAs/InGaAs pseudomorphic high electron mobility transistors has been observed after hot carrier dc accelerated testing. Hot carrier effects have been characterized by means of gate current measurements and electroluminescence spectroscopy. After accelerated testing, a permanent degradation has been found, consisting of the decrease of drain current ID, and of the absolute value of the pinch-off voltage Vp , at low drain-source voltage VDS , resulting in the development of a remarkable ‘‘kink’’ in the output characteristics. Direct current, pulsed, and low-frequency ac measurements demonstrate that the failure mechanism consists of the creation of deep levels under the gate which act as electron traps at low gate-to-drain electric fields. Deep level transient spectroscopy and photoinjection measurements reveal the presence of two levels at 0.77 eV and 1.22 eV. The intensity of the 1.22 eV peak is correlated with the degradation observed in stressed devices

Due to the multiplication of channel electrons i n the low-bandgap InGaAs channel layer, breakdown effects can seriously limit the operating bias of InP-based HEMT's and their circuit applications. Improvements in both on-state and off-state breakdown voltage can be obtained by adopting a combination of a thin layer of InGaAs and InP in a composite channel structure and reducing the thickness of the InGaAs channel layer. In this way the effective bandgap of the channel can be increased due to channel quantization, and breakdown voltage is enhanced. Despite data on the effects of quantization on the off-state breakdown of metal-insulator doped-channel FET (MIDFET's) [I] and on the on-state breakdown of InP-based HEMT's [2] have been presented, no systematic study of multiplication effects in HEMT devices has been reported.In this abstract, we present a detailed study of both on-and off-state breakdown effects in composite channel and conventional InP-based HEMT's. We demonstrate that in both cases breakdown is governed by channel impactionization so that channel quantization not only increases off-state and on-state breakdown voltage, but effectively reduces the gate current due to generated holes in open channel conditions. The temperature coefficient of the impactionization rate has been measured; differently from what happens in GaAs-based FET's, a positive coefficient has been found, i.e. carrier multiplication increases on increasing the temperature, following the corresponding reduction in the energy gap.The devices used in this study are based on a MBE-grown heterostructure on semi-insulating InP. The composite channel structures ccnsists of (bottom to top): a 2500 A undoped A10.48In0.52A~ buffer layer, a 100 8, InP layer doped 2 x lo1* cm' ' , a 50 8, undoped InP layer (spacer and channel), the In0.47Ga0.53As channel. followed by a 15 8, spacer layer and a 50 8, Si-doped A10.48In0.52As donor layer. Finally, a 250 8, undoped A10,60In0,40As layer was grown as the Schottky layer followed by a 70 8, In0.5jGa0.4~As doped cap layer. Three different channel thicknesses (30 A, 50 A and 100 A) have been chosen to study quantization effects. HEMT's with Lg = 0.15 pm and Wg = 50 pm have been measured. Conventional HEMT's having a 300 8, &doped In0.53Ga0.47As channel with L, = 0.5 pm, Wg = 50 pm have been also measured.On-state breakdown effects have been evaluated by measuring the gate current Ig, which is due to holes generated by impact-ionization and collected at the gate electrode. When measured as a function of Vos, Is has a bellshaped behaviour [3]; impact-ionization effects can be detected at Vds as low as 1 .8 V, see Fig. 1.The +/Id ratio is proportional to the impact-ionization rate, which is effectively reduced by channel quantization, see Figs. 2 and 3. Devices with a 30 8, channel have an on-state breakdown voltage of 7 V (at 1 mA/mm); the 100 8, channel ones have typical on-state breakdown voltages of 4 V. Drain-source off-state breakdown voltage VBDS has been measured by adopting the drain-current inje...