1. Direct Current (DC) Characteristics

Figs. 3(a) and 3(b) show the output curves (I${}_{\rm D}$-V${}_{\rm D}$) before and after proton irradiation at a drain voltage (V${}_{\rm D}$) of 10 V. As seen in Fig. 3(b), the current characteristics deteriorated significantly after irradiation with 5 MeV protons at a fluence of $5 \times 10^{13}$ cm${}^{-2}$. The static resistance (R${}_{\rm on}$) value was measured at a gate voltage (V${}_{\rm G}$) of 1 V and drain voltage (V${}_{\rm D}$) of 0.1V. The R${}_{\rm on}$value increased from 62.5 $\Omega\cdot$mm before irradiation to 98.135 $\Omega\cdot$mm after irradiation, at V${}_{\rm G} = 1$ V, respresenting a 57% increase. This indicates substantial damage to the devices from proton irradiation. Additionally, we evaluated the buffer leakage current characteristics by measuring the current. Fig.~4 shows the buffer current before and after proton irradiation. Post-irradiation data, represented by the red open symbols (A and A$'$), show higher current values at oltages below 10 V. As the voltage value increases, the current initially rises (marked as A$'$) and then decreases again indicating changes in the buffer current characteristics due to proton irradiation. Because of thisinflection, it cannot be definitively stated the buffer current decreased after proton irradiation. The observed current characteristics are due to measurements taken through a buffer pattern, which captures both buffer and surface leakage currents. As a result, at voltages below 10 V (marked as A), the post-irradiation current is larger than the pre-irradiation current As the voltage increases, electrons become trapped in th buffer layer's trap, causing the current to decrease.

Figs. 5(a) and 5(b) show the transfer curves (I${}_{\rm D}$-V${}_{\rm GS}$) before and after proton irradiation at a drain voltages (V${}_{\rm D}$) of 0, 0.1, and 7 V. The on-current (I${}_{\rm on}$) values of the before and after proton irradiation are $3.51 \times 10^{-1}$ and $2.57 \times 10^{-1}$ A/mm at V${}_{\rm D} = 7$ V, respectively. The corresponding off-current (I${}_{\rm off}$) values for the AlGaN/GaN HEMT are $1.35 \times 10 ^{-6}$ and $2.56 \times 10^{-6}$ A/mm, respectively, at V${}_{\rm D} = 7$ V. Additionally, the I${}_{\rm on} /$I${}_{\rm off}$ ratios are $2.6 \times 10^{5}$ and $1.0 \times 10^{5}$, respectively. The threshold voltage (V${}_{\rm t}$) was calculated using the g${}_{\rm m,max}$ method, with the V${}_{\rm t}$ values $-3.24$ V before irradiation and $-3.27$ V after irradiation. This indicates that the I${}_{\rm on}$ decreased by 26.78% and the surface leakage current increased, leading to an 89.63% increase in I${}_{\rm off}$.

However, there was no significant change in V${}_{t}$ suggesting minimal damage in the 2DEG channel region that determines V${}_{\rm t}$. For more accurate analysis, the R${}_{\rm on}$ value as a function of D${}_{\rm GD}$ was obtained. Fig. 6 shows the R${}_{\rm on}$ values as a function of D${}_{\rm GD}$ before and after proton irradiation. Closed black symbols represent R${}_{\rm on}$ values before proton irradiation, while open red symbols indicate R${}_{\rm on}$ values after irradiation. For a D${}_{\rm GD}$ of 5 $\mu$m, the R${}_{\rm on}$ value increases from 6.58 $\Omega\cdot$mm before irradiation to 7.66 $\Omega\cdot$mm after irradiation. Similarly, for D${}_{\rm GD}$ values of 10, 20, and 30 $\mu$m, the R${}_{\rm on}$ values change from 7.58, 11.83, and 15.46 $\Omega\cdot$mm before irradiation to 9.14, 13.04, and 16.73 $\Omega\cdot$mm after proton irradiation. The key point in this figure is that the slope of the line representing the R${}_{\rm on}$ values does not significantly change after proton irradiation. This slope corresponds to the sheet resistance (R${}_{sh}$), which is the resistance of the 2DEG channel region formed between the AlGaN and the GaN layers. The slope values before and after irradiation. This slope corresponds to the sheet resistance (R${}_{sh}$), which is the resistance of the 2DEG channel region formed between the AlGaN and the GaN layers. The slope values before and after rradiation are $\sim$0.3552 and 0.3628, respectively, indicating that the degradation of device characteristics is not due to the 2DEG channel layer. Therefore, the changes in device characteristics must be attributed to other factors, specifically device resistance Fig. 7 illustrates the AlGaN/GaN HEMT, where the R${}_{\rm on}$value is the sum of R${}_{\rm S}$, R${}_{\rm Ch}$, R${}_{\rm GD}$, and R${}_{\rm D}$, as shown in Equation (1).

Here, R${}_{\rm S}$ and R${}_{\rm D}$ are the source and drain resistances, which correspond to metal contacts, and R${}_{\rm Sh}$ represents the sheet resistance or 2DEG resistance. Since the slope value representing R${}_{\rm Sh}$ remains nearly unchanged, the increase in R${}_{\rm on}$ value after proton irradiation must be due to the chanages in the source and drain resistances, i.e., the metal resistance. Additionally, it was observed that the metal regions suffered more damage compared to the 2DEG regions when subjected to the same proton irradiation across the device. Figs. 8(a) and 8(b) schematically illustrate proton irradiation throughout the AlGaN/GaN HEMT and the interaction when a proton collides with metal. The higher density ad specific properties of the metal contacts result in greater damage compared to the 2DEG regions when exposed to proton irradiation. When a hydrogen proton irradiates with the device, it collides with the very small metal atoms. This collision produces secondary particles, such as protons or neutrons leading to significant damage around the metal areas.

Fig. 4. Buffer current characteristics of AlGaN/GaN HEMT before and after proton irradiation.

Fig. 6. The R${}_{\rm on}$ value according to D${}_{\rm GD}$ before and after proton irradiation.

Fig. 7. Resistance characteristics of the AlGaN/GaN HEMT.

2. TCAD Simulation-based Analysis of Proton Irradiation Impact

We used the Atlas TCAD semiconductor device simulator from SILVACO for simulation modeling. The modeled HEMT structure consists of a gate, source, and drain are formed on an epitaxy layer with a GaN buffer layer, a channel layer, and an AlGaN layer are grown on a sapphire substrate. The AlGaN layer is 20-nm thick, while the GaN layer is 180-nm thick, with the Al composition ratio in AlGaN being $\sim$25%. The polarization between he AlGaN/GaN heterostructure is controlled based on the AlGaN layer's thickness and Al composition ratio to determine electron configuration and mobility. The model incorporates a polarization model for implementing 2DEG and a thermal model to account for heat dissipation during device operation. Based on this model, we performed transfer curve fitting before proton irradiation.We also accounted for contact resistance, which significantly affects device characteristics, particularly after irradiation. The SRIM simulation results show vacancies occurring when protons irradiate the AlGaN/GaN HEMT ^[19].

At a standard energy of 5 MeV protons are injected to a depth of 120 $\mu$m. Since the protons passe through the AlGaN/GaN heterojunction structure to the sapphire substrate, they affect the entire device in terms of depth, producing many Ga and N vacancies. Figs. 9(a) and 9(b) show the AlGaN/GaN HEMT simulation fitting before and after proton irradiation. The measurement values are represented by lines, while symbols indicate the simulation fitting values. The simulation incorporates polarization for 2DEG implementation and the self-heating phenomenon during device operation, The initial R${}_{\rm C}$ value was based on $1.3 \times 10^{-5}$ as referenced in ^[21]. As shown in Fig. 9(b), when R${}_{\rm C}$ increased to $1.9 \times 10^{-5}$, the simulated current most closely matched the measured value, indicating that R${}_{\rm C}$ increased by $\sim$1.46 times after irradiation compared to that before irradiation.

3. Proton-induced degradation of current collapse characteristics

Fig. 10 shows the current collapse characteristics of the AlGaN HEMT before and after the proton irradiation at V${}_{\rm G} = 1$ V. The gate and drain biases (V${}_{\rm G_B}$, and V${}_{\rm D_B,\ }$, respectively) were $(0$ V, $0$ V$)$, $(-5$ V, $0$ V$)$, $(0$ V, $10$ V$)$, and $(-5$ V, $10$ V$)$. The pulse period and width (P${}_{\rm period}$ and P${}_{\rm width}$) were 5 ms and 100 $\mu$s, respectively. The pink symbols indicate the base voltage section without applied stress voltage. The blue and red symbols represent stress voltage applied to the gate and drain, respectively. This separation of base voltage helps in analyzing the degradation of device characteristics after proton irradiation. The R${}_{\rm on}$ value for gate lag increased from 7.37 $\Omega\cdot$mm before irradiation to 8.59 $\Omega\cdot$mm after irradiation, an increase of 16.58%. For drain lag, R${}_{\rm on}$ increased from 8.16 $\Omega\cdot$mm before irradiation to 10.88 $\Omega\cdot$mm after irradiation, an increase of 33.33%. Table 2 presents the corresponding I${}_{\rm D}$ current values at V${}_{\rm D} = 10$ V. Under the $($V${}_{\rm G_B}$, V${}_{\rm D_B}) = (0$ V, $0$ V$)$ condition, the I${}_{\rm D}$ value decreased by 5.57% after proton radiation, for the $(-5$ V, $0$ V$)$ condition, the decrease was 3.81%. Under $(0$ V, $10$ V$)$ and $(-5$ V, $10$ V$)$ conditions, the decreases were 10.29% and 10.41%, respectively. he overall current characteristic deteriorated after proton irradiation, with the drain lag showing a greater difference than the gate lag. This suggests that more traps formed in the GaN buffer layer than at the AlGaN layer interface. Consequently, as voltage is applied, electron trapping increases, leading to a decrease in the I${}_{\rm D}$ value.

Fig. 10. Pulse output curves before and after proton irradiation.