Polarization-Induced GaN-on-Insulator E/D Mode p-Channel Heterostructure FETs

Polarization-induced
$p$
-type doping is realized in molecular beam epitaxy (MBE) grown ultrathin body GaN/AlN heterostructures. The novel heterostructure consisting of a thin strained GaN layer grown on an AlN template exhibits a hole gas density close to the interface polarization charge
$({\sim}5\times 10^{13}/{\rm cm}^{2})$
. Both enhancement- and depletion-mode (E/D) p-channel heterostructure field effect transistor (HFETs) are demonstrated. Driven by the high hole density, a 2.4-
$\mu{\rm m}$
-long D-mode HFET with alloyed ohmic contacts shows improvement of drain current from 150 mA/mm
$({\rm V}_{\rm GS}=12~{\rm V},{\rm V}_{\rm DS}=30~{\rm V})$
at 300 K to 270 mA/mm
$({\rm V}_{\rm GS}=15~{\rm V},{\rm V}_{\rm DS}=30~{\rm V})$
at 77 K. The extrinsic peak transconductance of the D-mode device increases from 11 mS/mm
$({\rm V}_{\rm GS}=6~{\rm V},{\rm V}_{\rm DS}=30~{\rm V})$
at 300 K to 16 mS/mm
$({\rm V}_{\rm GS}=4~{\rm V},{\rm V}_{\rm DS}=30~{\rm V})$
at 77 K. Both the drive current and transconductance are recorded in nitride p-channel FETs. MBE regrown heavily Mg-doped
${\rm p}^{+}$
-GaN is then employed for ohmic contacts of E-mode p-channel HFETs. A 2-
$\mu{\rm m}$
-long E-mode device with a drain current of 4 mA/mm
$({\rm V}_{\rm GS}=10~{\rm V},{\rm V}_{\rm DS}=80~{\rm V})$
and ON/OFF current ratio of
$10^{3}$
is achieved.