Normally-Off AlGaN/GaN/AlGaN Double Heterostructure FETs With a Thick Undoped GaN Gate Layer

Abstract

In this letter, we report on polarization charge engineering enabling normally-off operation for a doubleheterostructure Al 0.26 Ga 0.74 N/GaN/Al 0.07 Ga 0.93 N-based field effect transistors (DHFETs) using a 35 nm thick undoped GaN layer underneath the gate metallization. The combined effect of the negative polarization charge induced by the AlGaN back barrier and the undoped GaN gate layer ensures the total depletion of the channel, and provides a positive threshold voltage. The fabricated DHFET exhibits normally-off operation with a threshold voltage of +1.2 V, a maximum drain current density of 370 mA/mm, and a high ON/OFF current ratio of 10 7 , at a gate bias of +7 V. A transistor with gate-drain distance of 6 µm demonstrates 300 V off-state breakdown voltage. Index Terms-Normally-off, AlGaN/GaN/AlGaN DHFET, AlGaN back-barrier, thick undoped GaN gate layer, polarization charge engineering.

Device characteristics of enhancement mode double heterostructure DH‐HEMT with boron‐doped GaN gate cap layer for full‐bridge inverter circuit

“…The presence of B‐doping in GaN on AlGaN layer causes a large conduction band offset shows improved Schottky barrier height with low surface‐related defects would cause V T change toward positive (E‐mode type) V T = 1.92 V for Al 0.23 Ga 0.77 N/GaN/Al 0.07 Ga 0.93 N DH‐HEMT devices through the application of high quality B‐doped GaN cap layer on top of Al 0.23 Ga 0.77 N barrier with a GaN channel. It is to be noted that the obtained V T values are better than the reported GaN cap gate HEMT structures . Figure shows the variation of V T and ON‐state resistance ( R ON ) for various values of “x” in the Al x Ga 1‐x N buffer for B and Mg‐doped GaN cap DH‐HEMTs.…”

“…For accurate simulation, the physical model parameters for HD transport model are calibrated with the reported measured data of as shown in Figure . The values of model parameters are then extrapolated for HEMT according to .…”

“…Experimental (solid line) and simulated (symbols) transfer characteristics of Mg‐doped and undoped GaN cap HEMT after tuning the simulation model to match the experimental curve with our proposed B‐doped GaN cap gate DH‐HEMT…”

“…Therefore, enhancing V BR,OFF and V T is one of the most significant challenges in the evolution of GaN‐based efficient power switches . While, both aspects still need the consideration by researchers, certainly, obtaining enhancement mode (E‐mode) behavior in GaN‐based devices with sufficiently high V T has always been a struggle . Several approaches, such as high Al content channels or the use of quaternary barriers to allow a zero polarization difference at the barrier/channel interface, cap layer doping under the gate (p‐InGaN, p‐AlGaN), and recessed gate etching techniques have been implemented to achieve the E‐mode AlGaN/GaN HEMTs.…”

“…However, the reported devices and GaN‐based cap gate HEMTs show drawbacks such as low I ON , V T , high gate and drain leakage currents, and low V BR,OFF. Thus, in order to overcome the current drawbacks of GaN‐cap HEMTs, it is crucial to achieve a much higher value of V T than the reported data for safe E‐mode operation of low‐power consumption, circuit simplicity, and fail‐safe requirements. In this respect, boron (B) or magnesium (Mg) doped GaN cap‐layer is used under the gate to achieve high V T , low‐leakage, and high V BR,OFF DH‐HEMT devices.…”