# Monolithic Si-Based AlGaN/GaN MIS-HEMTs Comparator and Its High Temperature Characteristics
## 2 Circuit Simulation  
In this section, three different circuit structures as well as the ADS simulation results are discussed. In Figure 1, three comparator circuit schematics are presented for comparison.  
Structure 1: The diodes in the GaN comparator. The diode is used to replace the PMOS current mirror M\({}_{3}\) and M\({}_{4}\) in the traditional comparator layout. The diode configuration can be seen as a resistor to provide current paths for the differential pair M\({}_{1}\) M\({}_{2}\).  
Structure 2: Figure 1b uses the current source circuit connections for M\({}_{3}\) and M\({}_{4}\). In 2020, researchers used this structure to design current sensing amplifiers [18]. This can be used for the first stage of GaN amplifiers [19]. If NMOSFET is used to replace PMOS for M  
Figure 1: Topologies of three comparator circuits with different active loads. (**a**) Using the diode structure at the top. (**b**) Connecting the gate and source nodes together as the active load. (**c**) Employing the bootstrapped structure at the output current path.  
and M\({}_{4}\) in the amplifier, the current source theory in this circuit, for M\({}_{3}\) and M\({}_{4}\), the current source can provide a higher resistance. The circuit structure that is successfully used in the amplifier circuit indicates it has a relatively high gain in small-signal applications. For a comparator circuit with an input range of several volts, the circuit is expected to perform a comparison among different reference voltage levels.  
Structure 3: Figure 1c demonstrates the bootstrapped structure. The input and output signals are in the same path, where the output is connected to the common gate of M\({}_{3}\) and M\({}_{4}\), forming a feedback loop. This structure can accelerate the output transition from V\({}_{\text{OH}}\) to V\({}_{\text{OL}}\)[20, 21]. In this way, the function of the comparator is improved when the input signal exceeds the reference level.  
In Figure 2, the three graphs from top to bottom are the optimized output voltage transfer curves of structures 1, 2 and 3. The six curves in each graph from left to right correspond to the six reference voltages from 3 V to 8 V with a step of 1 V. For structure 1, V\({}_{\text{OH}}\) cannot remain stable where the reference voltage is higher than 5 V since the structure provides a lower output resistance. The simulation results verify that a low resistance can result in low gain at the output stage, which causes the distortion of voltage transition from high to low. For structure 2, the comparison range is larger while the V\({}_{\text{OH}}\) starts to drop below 10 V at a reference of 5 V; this may cause a problem in some applications. Meanwhile, the shift of the voltage swing at different reference voltage levels is still not solved, as in structure a, which is attributed to the unstable current source with fluctuating output by the n-channel device. The output resistance is also not as high as the p-channel current source structure. Moreover, in order to ensure that M\({}_{3}\) and M\({}_{4}\) are turned on, the upper limit of V\({}_{\text{out}}\) is equal to V\({}_{\text{DD}}\) minus the threshold voltage of the active load, the output swing will be smaller, and the V\({}_{\text{OL}}\) level rises, leading to the reduction of voltage swing with the increase of reference voltage for structures 1 and 2.