# Monolithic Si-Based AlGaN/GaN MIS-HEMTs Comparator and Its High Temperature Characteristics
## 4 Results and Discussion  
Figure 4 shows the output and transfer characteristics of D-mode and E-mode devices. The I\({}_{\text{D}}\)-V\({}_{\text{D}}\) output curves on the left can reveal the maximum output current (when V\({}_{\text{D}}\) is 10 V). The I\({}_{\text{D}}\)-V\({}_{\text{G}}\) transfer curve determines the threshold voltage. At 25 \({}^{\circ}\)C, E-mode device demonstrated a threshold voltage of 0.88 V and a maximum output current I\({}_{\text{D}}\) of 334 mA/mm when V\({}_{\text{G}}\) was 10 V. The D-mode threshold voltage was read at about \(-8.91\) V, and the maximum current was 502 mA/mm. The GaN MIS-HEMT had an on/off ratio of 10\({}^{7}\) with a contact resistance of 4.5 \(\Omega\)-mm. Figure 5 shows the top-view micrograph of the fabricated comparator. The connections for DC and AC tests are also presented. The gate width W\({}_{g}\) of M\({}_{1}\) and M\({}_{2}\) is 200 \(\upmu\)m, and the L\({}_{g}\)/L\({}_{ds}\)/L\({}_{gg}\) is 3/13/5 \(\upmu\)m. The V\({}_{\text{DD}}\), V\({}_{Ref}\) and V\({}_{Bins}\) were provided using an Agilent power supply. For DC analysis, the input voltage sweep and output voltage capture were completed by the Keysight B1500A analyzer. For AC analysis, the input signal came from the RIGOL signal generator, while the output signal was captured by an Agilent oscilloscope. Figure 4: Output and transfer characteristics of D-mode and E-mode devices. Figure 3: Cross-section view of recessed gate MIS-HEMT E-mode (**left**) and D-mode (**right**) devices. Figure 6 shows the voltage transfer curve of the comparator, in which the reference voltage increased from 3 V to 8 V with a 1 V step. The temperature ranged from 25 \({}^{\circ}\)C to 250 \({}^{\circ}\)C, under a supply voltage of 10 V and a bias voltage of 5 V. The circuit showed a large comparison range, and a high gain of this design. The curve fits the simulated result from the previous Figure 2. When V\({}_{\text{in}}\) was larger than the reference voltage, V\({}_{\text{out}}\) swept from the logic-high stage to logic-low state. The experiment results showed that V\({}_{\text{out}}\) can reach close to V\({}_{\text{DD}}\) of 10 V as well as 0 V, realizing the rail-to-rail operation of the comparator circuit. For the thermal stability test, the curves slightly varied with the temperature increasing from 25 \({}^{\circ}\)C to 250 \({}^{\circ}\)C, showing the V\({}_{\text{th}}\) shift of the device in a high-temperature environment. This figure proves the thermal stability of the circuit, which illustrates the advantage of GaN material for extreme environment applications. Figure 7a shows the first five pulses of the dynamic waveform with a 100 kHz triangular input from \(-\)10 V to 10 V. The test was carried out under different temperature conditions from 25 \({}^{\circ}\)C to 250 \({}^{\circ}\)C. The V\({}_{\text{DD}}\) was 10 V, while the reference voltage V\({}_{\text{Ref}}\) and bias voltage V\({}_{\text{Bias}}\) were both 5 V in this test. It can be seen that the output waveforms maintained a stable square shape at all temperature conditions. Figure 7b shows the zoomed-in view of one period to investigate the circuit stability further. Under the 100 kHz input frequency, the 2.5 us up-sweep from 0 V to 10 V was enough to charge the traps at the gate oxide of M\({}_{2}\). At the down-sweep phase from 10 V to 0 V, the detrapping of gate oxide occurred. The negative phase from 0 V to \(-\)10 V of the triangular wave was used to reset the devices. Figure 7c demonstrates high-temperature dynamic impact on circuit V\({}_{\text{OH}}\), V\({}_{\text{OL}}\) and duty cycle. The V\({}_{\text{OH}}\) suffered a 0.5 V reduction and the V\({}_{\text{OL}}\) showed little variation, while the duty cycle suffered a distortion from the high-temperature environment. This can be  
Figure 5: The top-view micrograph of the fabricated comparator and connections for both DC and AC tests.