1. Introduction

The current density and transconductance of a transistor which is monolithically integrated with a GaN LED indicate the current drive ability of the transistor itself. So it is critical to conduct a correct calculation of those two parameters [1] to be fairly compared with other research works. In this comment, we point out some calculation errors and potentially contradictory results in [2] which may be caused by confusing gate width with gate length and suggest that the author can correct them and make their work more accurate and comparable.

2. Current density calculation

According to Fig. 3, the I-V characteristics of the monolithically integrated LED/MOSFET in [2], we can find that at V_DS = 6 V, I_DS = 1050 mA/mm. At the same time, the on resistance of the transistor is extracted to be R_on = 105 Ω. So we can calculate that the drain current at V_DS = 6 V is 6 V/105 Ω = 57.14 mA. Since the MOSFET current density can be given as I_DS/W_g (Here, W_g is the gate width of the transistor [3]), the gate width of the MOSFET is 57.14 mA/ (1050 mA/mm) = 54 µm. Judging from the Fig. 2(a) in [2], the scale bar is 100 µm which is matched with the claimed LED diameter of 300 µm but not matched with the calculated transistor gate width of 54 µm. From the SEM image of the integrated device, we can see that the LED is surrounded by the MOSFET which has a circular-shaped gate (diameter~420 µm). So the gate width should be around 1320 µm. This is not consistent with the I-V characteristics. In this way, the g_m result of 368 mS/mm is also questionable.

Judging from the SEM image in [2], the MOSFET gate length is around 50 µm. So we estimate that the author may confuse the definition of gate width with the one of gate length. As shown in Fig. 1, gate length means the size of gate along the direction from drain to source, while gate width is the size of gate in another dimension. So if the author used gate width to calculate current density and transconductance, the result will be 38.74 mA/mm and 15.05 mS/mm, which becomes much smaller, but more reasonable for MOSFET considering its doping level and carrier mobility.

 

Fig. 1 Schematic to illustrate MOSFET gate length and gate width definition.

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3. Inconsistent I-V characteristics

From the Fig. 5(b) given by [2], we can see that, at V_GS = 1 V, I_DS = 16 mA when V_DD = 7 V and I_DS = 26 mA when V_DD = 15 V. But from Fig. 5(a) in [2], we can see that, at V_GS = 1V, V_DD = 7 V, I_DS = 3 mA and at V_GS = 1 V, V_DD = 15 V, I_DS = 15 mA. So Figs. 5(a) and 5(b) in [2] are inconsistent if they were the same device.

4. Concluding remarks

The author may misunderstand the definition of gate width and gate length, resulting in a wrong calculation of the transistor’s current density and transconductance. It is easy to misunderstand this electronic concept for a photonic researcher when conducting an interdisciplinary research. In addition, what the author was trying to demonstrate is a new concept of monolithic integration of MOSFET with LED, so the calculation errors may be trivial and comprehensible. But correcting those errors can make this work more accurate and less misleading when comparing the transistor’s current drive ability in this work with the one in other monolithic integration works. So we still strongly suggest the author to correct these errors.