p-Channel gallium nitride transistor on Si substrate

Abstract

Gallium Nitride, a wide bandgap (3.4 eV) semiconductor, has outstanding attributes, such as, high breakdown electric field, high electron mobility, which make it suitable for applications requiring high power and high operating frequencies. These intrinsic material properties have been the major driving force to the development of high speed and high power GaN based n-channel transistors (mostly in the form of AlGaN/GaN High Electron Mobility Transistors). However, the full potential of GaN technology cannot be reached without the existence of p-channel GaN transistors. These devices are required for efficient high side switching in the converter circuits and for GaNCMOS technology. Therefore, the aim of this work is to demonstrate a GaN-CMOS compatible p-channel transistor. A stack of MOCVD grown epitaxial layers, is chosen for this work which has both 2-dimensional electron gas (2-DEG) for n-channel transistor and 2-dimensional hole gas (2-DHG) for p-channel transistor. The epitaxial layers chosen for this work are as follows, p++ - GaN (20 nm)/p-GaN (50 nm)/UID-GaN (20 nm)/Alo.25Gao. 75N (20 nm)/UID-GaN (150 nm)/GaNBuffer (3.8 ptm)/Si (1000 [mu]m). From device fabrication point of view, the difficulty of demonstrating a high performing p-channel GaN transistor can be attributed to the high source and drain contact resistances. In this thesis, we successfully improved the contact resistances through the development of optimum fabrication process, and a record contact resistivity of 4.83 x 10-6 [Omega]2 - cm 2 to p type-GaN was demonstrated. Finally, for the first time, a recessed gate p-channel GaN transistor on Si substrate was demonstrated. Direct current measurement of our fabricated devices show excellent off-state characteristics: ION/IOFF 5 , SS= 280 mV/decade and IOFF=- nA/mm. Measured on state characteristics for 2 pm channel length devices are, Ron= 1.7 k[omega]-mm as VGS=12 V, ION=- 3 .5 mA/mm at VGS=10 V and VDs=5 V. From the current-voltage and capacitance-voltage characteristics of 100 pm channel length devices, 2-DHG density and hole C, 2 mobility were found to be 2.4 x 1012 cm 2 and 11cm2/v.s , respectively.