Collapse and large signal modelling of GaAs field effect transistors at 77 K

A complete electrical characterization of different types of GaAs field effect transistors at liquid nitrogen temperature is performed. The trapping-detrapping mechanisms on deep levels are particularly adressed and a method is proposed to circumvent the collapse phenomenom which otherwise limits the electrical performances. From these measurements a HEMT non-linear model is extracted and is found efficient for the prediction of the large signal power out versus power in characteristic of a cooled HEMT. A further application could be the optimized design of a cooled low phase noise oscillator. The sharing of the microwave bandwidth for telecommunication applications has enhanced the need for high spectral purity microwave sources. Cryogenically cooled microwave oscillators [I] could be an answer to this problem thanks to the availability of very high quality factors resonators at low temperatures, such as sapphire-superconductor resonators [2] . However the optimized design of such an oscillator requires a non-linear model of the cooled active microwave device which is not available yet. FET's show enhanced electrical performance under cooling and therefore are thought to be the best devices for such an application. However they are subject to trapping-detrapping phenomena on deep levels which may considerably degrade the expected performance. This phenomena must be considered at the time of modelling and this paper deals with this type of modeliing performed on different commercial FET devices at liquid nitrogen temperature. The different devices under test, a conventional GaAs MESFET, two AIGaAs/GaAs HEMT and two AlGaAsiInGaAs pseudomorphic HEMT, are placed into a 50 R microstrip chip carrier. These devices are characterized at 77 K with respect to their DC and pulsed drain current characteristics and microwave small signal S parameters. As shown on Figure 1, in complete darkness, the DC analysis demonstrates that the HEMT device is dramatically affected by the temperature : at drain to source voltages less than about 1 V a typical collapse of I-V characteristics occurs [3,4,5]. This anomalous behaviour disappears under conditions of white light illumination [4]. Many authors have reported that the collapse strongly depends on the bias history at low temperature and that the effect of a drain stress is predominent [5,6]. On Figure 1, the DC Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1994626 C6-166 JOURNAL DE PHYSIQUE IV characteristics in the dark have been obtained after an initial plot at Vgs = OV sweeping the drain voltage from OV to Vdrnax = 2.5V. It can be observed that such a stress induces a severe collapse. 0 0.0 0.5 1.0 1.5 2.0 2.5 DRAIN VOLTAGE (V) 0 0.1 0.2 0.3 0.4 0.5 DRAIN VOLTAGE Vds (V) Figure 2 : HEMT current-voltage curves at Vgs = OV (sdlid line) and Vgs = 0 . 1 ~ (dashed line). Figure 1 : HEMT Drain current curves for Vgs values Bias : 4Vds = 3V, Vgs = OV; -AVds = 2V, of OV, -0.2V and -0.4V at low temperature (77 K) vgs = ov; -+vds = IV, vgs = ov. Stress : five minutes. On Figure 2, the stress conditions are varied, the drain voltage is maintained at a value Vstress (Vstress = lV, 2V or 3V) during a stress time of 5mn. The corresponding drain current characteristics in the ohmic region are shown in Fig. 2 for these three different stress. The device exhibits severely distorted I-V characteristics mainly if the stress drain voltage is 1V. On the contrary for a drain bias stress equal or higher than 3V a total collapse-recovery phenomenon is clearly observed. As extensively described by Hori et a1 [6], the collapse state and the recovery state can be switched to each other simply by changing the value of the stress drain voltage. Figure 3 shows the collapse enhancement versus stress time (Vstress = lV, Vgs = OV). The collapse is evaluated through the drain current amplitude measured at a fixed drain voltage of 40 mV. This drain current dramatically decreases (Fig. 3) as the stress time increases and a full collapse is observed for a stress time in excess of 2 mn. On the contrary a DC stress of Vstress = 3V, (Vgs = OV) induces quite immediately the recovery. We have also observed a similar behaviour in some pseudomorphic HEMTs (PHEMTs). Figure 4 represents the drain current characteristics (Vgs varied from OV to -0.3V, step 0.1V) for a PHEMT device after an initial bias stress (Vds = 1 V, Vgs = OV) applied during a stress time of 5mn. However two important features must be noticed : In PHEMTs the collapse is much less pronounced and only appears if the stress drain and gate voltages are chosen at around 1V and OV, respectively (as an example, see the fist curve vgs = OV). If Vdmax is above the kink voltage value which can be evaluated from the EV characteristics to be about Vkink = 1.5V (Figure 4) no significant further collapse can be observed (curves Vgs = -O.lV, -0.2V, -0.3V). The origin of the collapse is generally attributed to the existence of electron trapping on the DX centers which are deep donor levels located in the n-AlGaAs layer. The recovery phenomenon has been presented as the result of an impact ionization process [6,7]. Following, this hypothesis, when the electric field is sufficiently high, an impact ionization of hot electron occurs in the high-field channel region and generates holes which flow along the electric field lines and are collected at the gate electrode. Consequently, a dramatic enhancement in the gate current is expected. This expectation has been verified on each of the devices under test and an example of gate current variation versus drain to source voltage Vds is illustrated in Figure 5 for pseudomorphic HEMT's. The enhancement in the gate current at around Vds = 1.5V clearly indicates that the most of holes generated by impact ionization are swept towards the gate. Since the hole concentration increases rapidly and the hole capture cross section of DX center being about several orders of magnitude larger than the electron capture cross section of DX center [8], the traps ionization probably results fiom this hole current. 0 03 I 13 2 2.5 DRAIN VOLTAGE Vds (V) 0 50 100 150 200 250 300