Impact Ionization in AIGaAsSb/InGaAs/AIGaAsSb Metamorphic HEMTs

bandgap of the channel material, 0.47 eV, makes these HEMTs susceptible to impact ionization. In this paper, we present a measurement of the impaction ionization current and show that it is consistent with a physics based model for collection of impact ionization generated holes at the gate electrode. Fig. 1 shows measured drain current voltage characteristics of a O.1Sum x 64 um AIGaAsSbho 8Gao *As HEMT. The maximum drain current at VD of 1.2 V and VG of 0.2 V is 390 mA/mm. A peak dc transconductance value of 564mS/mm at VG=.ISV and VD=0.75 is measured. Impact ionization is initiated at VDG (VD - V,) of O.8V with VD = 0.9 and VG = 0.1V as shown in Fig 2. In Fig. 2 the drain conductance is plotted as a function of VDG with VG as a parameter. The presence of impact ionization is also indicated by the inductive behavior of S22 at low frequency which is shown in Fig. 3. The total gate current is plotted as a function of gate voltage is plotted in Fig. 4. The gate current is composed of the Schottky gate current and the hole current due to impact ionization. In Fig.5 the impact ionization component of the gate current is plotted by subtracting the Schottky component of the gate current from total gate current as a function of drain bias with gate bias as a parameter. In the inset the impact ionization gate current calculated from a physics based model is compared to experimental data, at Vg=OV, to show good agreement. The hole current due to impact ionization is calculated by incorporating both tunneling and thermionic components.