Strained indium(0.52)aluminum(0.48)arsenide/indium(x)gallium(1-x)arsenide (x greater than 0.53) high electron mobility transistors (HEMT's) for microwave/millimeter -wave applications.

Abstract

This thesis presents the design, fabrication and detailed characterization of strained $\rm In\sb{0.52}Al\sb{0.48}As/In\sb{x}Ga\sb{1-x}As (x > 0.53)$ HEMT's for microwave/millimeter-wave applications. A set of theoretical criteria have been developed to allow the design of heterostructures for optimum device performance. Transport studies on these heterostructures show enhanced Hall mobilities and velocities when the channel Indium composition (x) increases from 0.53 to 0.65. A higher transconductance (g$\sb{m})$ and current gain cutoff frequency (f$\sb{T}$) is also obtained. 1$\mu$m long gate single-heterojunction HEMT's (SHHEMT's) with x = 0.65 have shown state-of-the-art extrinsic g$\sb{m}$ of 590mS/mm and f$\sb{T}$ of 45GHz. The device performance however degrades for x $\geq$ 0.7 and the output conductance (G$\sb{ds}$) increases considerably. To overcome this, 1$\mu$m double-heterojunction HEMT's (DHHEMT's) were investigated and showed low G$\sb{ds}$ of $\sim$13mS/mm and high maximum oscillation frequency (f$\sb{max}$) of 65GHz. In addition to the theoretical, DC and microwave study of SHHEMT's and DHHEMT's, the devices were also characterized at low frequencies (LF) in order to investigate their 1/f noise, g$\sb{m}$ and output-resistance (R$\sb{ds}$) dispersion properties. LF noise characteristics reveal shallow traps and high noise transition frequencies of $\sim$200-300 MHz. Dispersion in InAlAs/InGaAs HEMT's is attributed to the interface states of the heterointerfaces underneath the gate rather than the surface states of the ungated regions. The $g\sb{m}$ and R$\sb{ds}$ dispersion is small compared to GaAs MESFET's. Submicron (0.9 to 0.2 $\mu$m) HEMT's were also fabricated and characterized. As in the 1$\mu$m gate case, low G$\sb{ds}$ values were found for the DHHEMT's even down to 0.25 $\mu$m; cutoff frequencies of these devices were f$\sb{T}$ = 82GHz and f$\sb{max}$ = 148GHz. Further device performance enhancement in the SHHEMT's was obtained by optimizing the submicron technology. Best results were obtained with 0.2$\mu$m mushroom-gate 65% In SHHEMT's (f$\sb{T}$ = 160GHz and f$\sb{max}$ = 220GHz). Finally, MMIC's using strained InAlAs/InGaAs HEMT's technology are reported for the first time and demonstrate their excellent potential for ultra-high frequency monolithic integrated circuit applications.