Buried channel normally-off AlGaN/GaN MOS-HEMT with a p-n junction in GaN buffer

Abstract

A buried channel normally-off AlGaN/GaN MOS-HEMT with gate recess etching is reported. The buried channel operation is achieved by in situ doping of GaN to introduce a p-n junction in the GaN buffer. The fabricated buried channel MOS-HEMT with 12.5 nm atomic layer deposition (ALD) Al 2 O 3 gate dielectric featured a threshold voltage of 1.3 V with a drain saturation current of 287 mA mm −1 for a device with 3.5 μm long gate length and 11 μm sourcedrain spacing. The field effect mobility improved from 25 cm 2 /Vs for a reference device to 142 cm 2 /Vs for the buried channel device. Due to the presence of the p-n junction depletion region in the GaN buffer, the leakage in the off state decreased by about 4 orders of magnitude (4 nA mm −1 compared to 76 uA mm −1 for the reference device). The buried channel device also gives better breakdown characteristics, with a breakdown voltage of 158 V compared to 98 V for the reference device.

An in situ monitored and controlled etch process to suppress Mg memory effects in MOCVD GaN growth on Si substrate

“…After this etching a 100 nm thick Si:GaN (Si doped GaN) was deposited. An optimised dopant to precursor ratio (Cp 2 Mg/TMG=4.65×10 −5 , SiH 4 /TMG=4.45×10 −6 ) was used to get a dopant concentration of 5×10 18 cm −3 and 3×10 17 cm −3 in the Mg: GaN and the Si:GaN respectively [10]. As shown in our earlier paper at these levels of Mg doping, Ga-polarity is retained [13].…”

“…All the samples in the study were grown on 50 mm (111) Si substrates in a horizontal flow AIXTRON MOCVD reactor. The details of the epitaxial growth have been reported elsewhere [10,13]. Reflectivity oscillations and wafer surface temperature were monitored in situ using integrated control for epitaxy system from k-space associates, inc. TMG, TMA, NH 3 , SiH 4 and Cp 2 Mg were used as the precursors.…”

“…As shown in our earlier paper at these levels of Mg doping, Ga-polarity is retained [13]. The thicknesses and doping concentrations of the n-type and p-type regions were designed to ensure that the depletion layer associated with the junction covered the buffer stack to reduce off-state leakage [10]. Reactor pressure and wafer surface temperature were then changed to 200 mbar and 1050 °C respectively to grow a unintentionally doped (uid) GaN channel layer.…”

“…The dislocation density estimated through TEM imaging on a similar sample is about 1×10 9 cm −2 . Recess etched AlGaN/GaN normally off devices were fabricated on the growth stack as discussed elsewhere [10].…”

“…It has been shown that dislocations in GaN act as high diffusivity pathways [4]. Thus, when such a Mg containing p-type layer is sandwiched between two other layers in the device architecture as is required in the case of trench MOSFETs [5], current apertured vertical electron transistors [6], bipolar transistors [7], tunnel junction light emitting diodes [8,9] and buried channel MOSHEMTs [10], such Mg activity leads to change in doping levels in the p-type layer, counter doping in other layers, introduction of trap states and reduction of electron mobility due to scattering. In all the transistor applications listed above, this is a particular point of concern, as the location of the main conducting channel is close to the Mg doped layer.…”