THE PROTON FLUX INFLUENCE ON ELECTRICAL CHARACTERISTICS OF A DUAL-CHANNEL HEMT BASED ON GAAS

The results of the simulation the influence of the proton flux on the electrical characteristics of the device structure of dual-channel high electron mobility field effect transistor based on GaAs are presented. The dependences of the drain current ID and cut-off voltage on the fluence value and proton energy, as well as on the ambient temperature are shown.


Introduction
The Element base of modern objects of space and nuclear technology is exposed to ionizing radiations, the primary of which are gamma rays (γ), neutron (n), electron (e), and proton (p) radiations. Alfa-particles (α), fission fragments Fp, and other particles from a nuclear reactor or nuclear explosion zone can also influence the degradation of performance characteristics. However, their influence is not so significant (for example, neutrinos, mesons, etc.) [1].
When a particle flux affects a microelectronic device structure, two main mechanisms are possible: ionization and damages caused as a result of elastic scattering of primary particles and fragments formed in nuclear reactions (inelastic scattering) of incident protons or neutrons on target's nucleuses. Ionization is not considered in this work. Neutrons, protons, alpha particles, heavy ions, and photons with very high energies cause displacement effects: the arrangement of atoms in the crystal lattice changes and the number of recombination centers (defects) increases, decreasing the concentration of free charge carriers and deteriorating the performance of device structures.
The parameters of the bulk material most sensitive to the effects of displacements are the lifetime and diffusion length of minority charge carriers, the mobility and concentration of charge carriers [2].
The magnitude of the manifestation of displacement effects depends on the type of particle radiation, the total dose, radiation flux and energy, ambient temperature, operating voltage, as well as the state of the device at the moment of irradiation. These problems complicate testing, increase the complexity of using theoretical calculations to predict radiation effects, increase the time of designing device structures and require a significant number of test samples. In modern computeraided design systems in microelectronics like Silvaco [3] and Synopsys [4], the modules for accounting for displacement effects are implemented, which require adjustment and calibration.
Thus, the paper represents the results of evaluating the processes of degradation of the electrical characteristics of device structures dual-channel high electron mobility field effect transistor (HEMT) based on GaAs under the influence of the proton flux by means of a computer simulation.
Dual-channel GaAs-HEMT device structure and experimental technique A typical device structure of a dual-channel GaAs-HEMT is shown in Fig. 1. The cut-off voltage and drain current for the resulting structure at ambient temperature T = 303 K are equal to VTH = -0.9 V and ID = 0.48 mA (at drain voltage VD = 1 V and gate VG = 0 V), respectively.  To predict the degradation of device parameters caused by penetrating radiation, it is often sufficient to consider only the first stage of the defect formation process. The formation of bulk defects in a semiconductor device structure is proportional to a non-ionizing energy loss (NIEL)the total kinetic energy transferred to the lattice atoms. In modeling and calculations, in addition to the NIEL parameter [MeV•cm 2 /g], the term kinetic energy released in matter Kinetic Energy Released in Material (KERMA) is also used. The relationship between KERMA and NIEL can be written as where F -integral radiation flux (fluence), cm -2 ; m -mass of irradiated material, g.
Parameter NIEL can be used to the extrapolation of a device parameter degradation measured for a particle with the given energy to other energies ("NIEL scalingˮ). The Radiation Fluence Model is used in the microelectronics TCAD software packages to describe the impact of particle flux on material characteristics, which makes it possible to predict the rate of defect generation. According to the model the total density of defect states depends on the radiation flux, NIEL, damage coefficient, and the density of material.
The non-ionizing energy losses for GaAs are determined using the SR-NIEL [5] project for displacement threshold energy values Ed1 = 9,5 eV [6], Ed2 = 10 eV [7], Ed3 = 21 eV [8], and Ed4 = 25 eV [9]. For the obtained values, mean values were determined, which are described by approximating dependence (Fig. 3). The obtained results fully agree with the data presented in the paper [10]. The total NIEL for GaAs is calculated by summing the contributions of each element weighted by its atomic fraction [11].
The simulation of the effect of a proton flux on the performance of a device structure of a dual-channel GaAs-HEMT is carried out. The results of the influence of the flux of protons with an energy of E = 2 keV at the temperature of T = 303 K are shown in Fig. 4. Fig. 5 shows the graphs of the dependence of the drain current ID (at VG = 0 V and VD = 1 V) and the cut-off voltage on the fluence of protons F with energy E = 2 keV. The parameters are expressed in relative units (the values without penetrating radiation are taken as 100 %).
It is shown that the effect of the proton fluence on the values of the drain current and cut-off voltage does not coincide in magnitude, however, starting from values of 10 12 cm -2 , a significant deterioration in the operational parameters of the transistor is observed. So, at F = 10 12 cm -2 , the drain current and cut-off voltage are 0.357 mA and -0.757 V, respectively, and at F = 5·10 12 cm -2 -ID = 0.243 mA (decrease by 32.4 %) and VTH = -0.698 V (7.8 %). The dependence of the drain current and cutoff voltage deviation obeys the linear law (approximation reliability R2 = 0.993 for ID and R2 = 0.996 for VTH). Also noteworthy is the different effect of the proton flux on the drain current at different drain voltages (Fig. 6). At proton fluence F < 5×10 12 cm -2 , its effect on the drain current is higher, the lower the drain voltage is. This is especially noticeable when the voltage at the drain is VD < 0.3 V. At values of the proton fluence F ≥ 5×10 12 cm -2 , there is a shift in the voltage at the drain, at which the effect of radiation is the greatest. It is recommended to limit the use of low drain voltages at the circuit level to increase the radiation resistance of devices using the considered instrument structures. Based on the simulation results, it was found that the proton energy has the greatest effect on the electrical characteristics of a gallium arsenide field-effect transistor in the range from hundreds of eV to 100 keV, which correlates with the data presented in Fig. 3., with the exception of the energy range from 4 to 10 keV (i. e., when E is 6 keV higher than expected), at which there is a strong degradation of the electrical properties of the device structure up to a failure. At energies E greater than 1 MeV, the deviation of the parameters from the values without irradiation does not exceed 20 % for the drain current and 0.1 % for the cutoff voltage.

Conclusion
The model of the dependence of NIEL on the proton energy with different values of the threshold energy of defect formation for GaAs and AlGaAs, that are described in the literature and comply with the latest theoretical and experimental data, has been developed.
The analysis of the simulation results of the effect of the proton flux on the device structure of a dual-channel GaAs-HEMT allows us to reach the following conclusions: the proton fluence has a different effect on the drain current and cutoff voltage (the ratio of the degradation effect that increments ΔID / ΔVTH is not constant and varies from 4,07 to 6,19 for the cases under consideration); the energy of protons has the greatest influence in the region from hundreds of eV to 100 keV (up to the failure of the transistor). To increase the radiation resistance of devices using the considered device structures, it is recommended to provide protection from the proton flux in a narrow energy band (from 4 to 10 keV), and at the circuit level to limit the use of low voltage values at the drain.