Radiation Response and Reliability of High Speed AlGaN/GaN HEMTs

Robert Weller for giving me interesting lectures on Monte Carlo simulation and electromagnetic theories, which are extremely helpful in this work. 3-10 Maximum oscillation frequency (fmax) of AlGaN/GaN HEMTs as a function of gate bias before and after proton irradiation, measured at Vd = 5 V. 4-2 Experimental and calculated frequency exponent of noise power spectral density as a function of temperature from 85 K to 450 K (a) before irradiation, (b) after a fluence of 1×1014 cm-2.44.

8 (a) Threshold voltage shifts and transconductance degradation and (b) . temperature-dependent noise measurements on Qorvo devices after high-field stress. 9 Hydrogenated divacancy formation energies in Ga-rich GaN are plotted as a function of Fermi level. 6-4 Experimental and calculated frequency exponent for noise power spectral density as a function of temperature from 85 K to 445 K.

6-5 Temperature-dependent noise measurements from 85 K to 445 K, for GND state radiation (a) after and (b) before half-on stress. 6-6 Temperature-dependent noise measurements from 85 K to 445 K, for radiation in the OFF state (a) after and (b) before half-on stress.

1 ABSTRACT

2 Chapter I

Introduction

In P-based HEMTs, the barrier layer is n-doped and the donors are the sources of the 2DEG electrons. One of the most powerful techniques for growing GaN-based HEMT epi-structures is molecular beam. 10] At lower temperatures (550 ℃ to 0.800 ℃), the PAMBE technique can achieve atomically flat surfaces under Ga-rich growth conditions.

A variety of effects in the characteristics of GaN HEMTs system can occur after exposure to radiation. This thesis will focus on the defects that lead to degradation during proton irradiation and during electrical stress of AlGaN/GaN HEMTs. To obtain a deeper understanding of defect formation during the irradiation, low-frequency noise measurement results are discussed in Chapter IV.

After prolonged hot electron stress, threshold voltage shifts show opposite polarities for Ga-rich and ammonia-rich devices, suggesting different dominant defects. The high field effects of both devices are examined and compared with commercial parts in Chapter V.

Table 1.1. The materials properties of GaN compared to competing materials (after [3])

9 Chapter II

Background

Formation energy of N vacancies as a function of the position of the Fermi level in the band gap of AlGaN. Since 2004, commercial GaN HEMTs have started to appear. on the market, targeting the low frequency, high efficiency end of the market. GaN HEMTs that achieve a high level of reliability and stability with high performance operation. becomes one of the major challenges [49], requiring a better understanding of failure.

Due to their wide band gap and chemical stability, high temperature applications are among the best. distinctive advantage of GaN HEMTs. Thermal stability of other metallizations [53]-[58] also indicate that gate sinking is one of the most important failure mechanisms of GaAs MESFETs and. 16][59], who showed that the electric field in the gate-drain region would increase the voltage in the AlGaN/GaN heterojunction, which is also identified as the 'inverse piezoelectric effect'.

Degradation by hot electrons is one of the most important failure mechanisms of GaN. A schematic cross-section of the GaN HEMT is shown in Figure 2.6(a) and a top view of the device.

Fig. 2-1 Transconductance and saturation current of the HEMT vs. annealing temperature at V ds = 10 V, and V g = 0.5 V

23 Chapter III

Radiation-induced Degradation on GaN HEMTs

MBE Vds = 5 V

The frequency limit for the devices under study is around 10 GHz, as observed from small. Because the non-ionizing energy loss of 1.8-MeV protons is much higher than that of the. The greater RF degradation results from rapid bulk and surface drops below the gate and into the.

The cutoff frequency fT and fmax can also be related to gm, but these parameters are also considerable. Here Cgd and Cgs are the gate-drain and gate-source capacitances, and Ri, Rs, Rg and Rds are the. In this chapter, proton-induced radiation effects in Ga-rich and NH3-rich MBE cultures are discussed.

The difference in RF responses between Ga-rich PAMBE and ammonia MBE devices is due to.

Fig. 3-10 Maximum oscillation frequency (f max ) of AlGaN/GaN HEMTs as a function of gate bias before and after proton irradiation, measured at V d = 5 V

39 Chapter IV

Proton-induced defects Identification via Low Frequency 1/f Noise Measurement

4-1 plots the excess drain voltage noise power spectral density SVd (corrected for background noise) for Ga-rich HEMTs at constant Vg –Vpinch-off = 2 V and Vd = 0.15 V as a function of frequency and. Over the entire temperature range, for both Ga-rich and ammonia-rich units, before and after proton irradiation, the experimental γ values ​​fit the Dutta–Horn model well, as shown in Fig. The energy scale on the upper x-axis is derived from the Dutta-Horn model of low-frequency noise.

Fig. 4-1 Excess voltage noise power spectral density S v as a function of frequency for Ga-rich HEMTs, at 100 K, 300 K and 400 K, (a) before and (b) after irradiation

48 Conclusion

49 Chapter V

High Field Stress and Long Term Reliability of AlGaN/GaN HEMTs

These AlGaN/GaN HEMTs were stressed in the half-ON state with Vgs = -2 V and Vds. The polarity of the V-shift suggests that different defects are generated for NH3-rich devices. The temperature dependence of the low-frequency noise of MBE devices of ammonia is shown in Fig.

To compare the degradation of the UCSB devices with industrial devices, high field stress. We attribute the growth of the ~0.2 eV peak for the two device types to lower stress levels. We attribute the observed broad peak in the noise data at ~0.52 eV to hydrogenated divacancies.

This lowers the formation energy of the FeGa-VN-H defect complex. dehydrogenation of Fe impurity complexes leads to a negative threshold voltage shift [119],. consistent with the Ga-rich devices of Figs. divacancy complexes cause a positive threshold-voltage shift in accordance with the NH3-rich. This indicates that densities of FeGa-VN impurity complexes are larger. for the Ga-rich devices, but less so for the NH3-rich devices, than densities of the divacancy defects. The noise data for the Qorvo devices in Fig. 0.7 eV peaks after semi-ON stress.

The threshold voltage shift, shown in the figure for the Qorvo devices, is qualitatively consistent with that of ammonia-rich MBE devices. We thus attribute the 0.7 eV defect level increase in the Qorvo devices to hydrogen removal from the hydrogenated antisite NGa-Hx, x=1~3, defects. The increase in the broad peak at ~0.8 eV during stress for the UCSB devices is caused by two.

This barrier is lowered in the presence of an electric field, due to the high charge states of the migrating ones. Since the charge transition state of the VGA-ON-H defect is ~0.7 eV, the increase of. The greater overall increase in noise in the NH3-rich UCSB devices at very high voltages is.

Fig. 5-1 Small signal transducer gain as a function of frequency, measured at V ds = 5V with the gate voltage biased to achieve the best performance

66 Chapter VI

Effects of Applied Bias and High Field Stress on the Radiation Response of AlGaN/GaN HEMTs

The hot carrier response of the devices stressed in the semi-ON state is shown in Fig. We now discuss the nature of the defects that lead to the greatest effect on charge trapping. -5 to 6-7 strongly suggests that these increases. in the ∼ 0.2 eV trap level does not strongly depend on either the bias applied during proton irradiation or. the presence or absence of high field voltage before, during or after proton irradiation, at least for the. devices and experimental conditions of this study.

In contrast to the ~0.2 eV trap level, which is now relatively well understood, the phys. HSE06 was used for calculations to reproduce the correct value of the band gap, which is underestimated. The question remains: Why would the ~0.7 eV defect, if associated with NGa, increase during the proton.

Coupling of at least a portion of the ~0.2 eV trap level with an oxygen DX center in AlGaN. Density functional theory calculations in this and previous work suggest that the ∼0.2 eV peak is due to

Fig. 6-1 (a) V TH shift and (b) normalized peak transconductance of AlGaN/GaN HEMTs as a function of hot carrier stress time, for biases of V ds = 25 V and V g = -2 V

84 Chapter VII

Summary and Conclusions

In summary, we have used radiation and DC stress to understand the degradation of AlGaN/GaN.

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