The role of spontaneous and piezoelectric polarization in III-V nitride het
erostructures is investigated. Polarization effects and crystal polarity ar
e reviewed in the context of nitride heterostructure materials and device d
esign, and a detailed analysis of their influence in nitride heterostructur
e field-effect transistors is presented. The combined effects of spontaneou
s and piezoelectric polarization are found to account well for carrier conc
entrations observed in AlGaN/GaN transistor structures with low to moderate
Al concentrations, while the data for higher Al concentrations are consist
ent with defect formation in the AlGaN barrier. Theoretical analysis sugges
ts that incorporation of In into the barrier and/or channel layers can subs
tantially increase polarization charge at the heterojunction interface. The
use of polarization effects to engineer Schottky barrier structures with l
arge enhancements in barrier height is also discussed, and electrical chara
cteristics of transistors with conventional and polarization-enhanced Schot
tky barrier gates are presented. The polarization-enhanced barrier is found
to yield a marked reduction in Sate leakage current, but to have little ef
fect on transistor breakdown voltage. (C) 1999 American Vacuum Society. [S0
734-211X(99)05804-7].