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.

I. Introduction

Few reports exist for wide-bandgap III-nitride-based p-channel field effect transistors (pFETs) [1], [2]. The current performance of III–V pFETs vastly lags behind that of n-channel High Electron Mobility Transistors (HEMTs) Finding methods to boost the performance of pFETs will greatly extend the applications of nitride-based electronics by enabling complementary logic devices. Polarization-induced doping [3] overcomes limitations imposed by the large thermal activation energy for holes [4], and has the potential to enable high-density mobile hole channels. Conducting hole layers were reported in GaN/AlGaN heterostructures [5], [6]. Through polarization field ionization, the effect of impurity scattering can be suppressed, and 2-D hole gases with high mobility are expected. Another possible way to boost the hole mobility is to utilize the crossover of heavy and light hole bands either by strain or in high Al composition AlGaN [7]. To maximize the polarization-induced hole concentration and to employ high Al composition AlGaN channels, AlN is the natural choice as the barrier because of its largest band gap and spontaneous polarization charge in III-nitrides. For metal polar p-channel (In, Al)GaN/AlN heterostructures, AlN acts as the back barrier, providing excellent electrical insulation and simultaneously excellent thermal conductivity. It also enables incorporation of epitaxially strained channel layers, which is expected to play an increasing important role in III-nitride devices of the future, just as it does in silicon devices today.