Short-Channel Effects in Iii-V Compound Semiconductor Field-Effect Transistors

Abstract

The most efficient way of improving the high-frequency performance of a field-effect transistor (FETs) is to reduce the gate length. However, as the gate length decreases, short-channel effects, such as a decrease in the transconductance, an increase in the subthreshold current, and a negative threshold shift, become more prominent degrading the device performance. This thesis describes the fabrication and characterization of short gate-length FETs. The emphasis is on the study of the short-channel effects, and the factors limiting the performance of the high-speed FETs.

Electron beam lithography has been used to pattern the gates for the FETs. A PMMA bilayer process and a trilayer process for T-gates with minimum gate lengths of $\sim$30 nm and $\sim$80 nm, respectively, are developed. Also, a process for fabricating T-gates using a silicon nitride passivation layer is demonstrated.

Gallium arsenide metal-semiconductor field-effect transistors (MESFETs) with gate lengths down to $\sim$30 nm have been fabricated and characterized. Three molecular beam epitaxy-grown (MBE) structures, each with a different buffer layer, are compared. Due to the selective wet etching for the gate recess, the channel thickness is known accurately for each device. Extensive dc characterization is performed to study the short-channel effects. Microwave measurements and delay time analysis have been used to study the high-speed characteristics of the devices. It is shown that at very short gate lengths the device speed is not determined by the gate length alone, but the parasitic delay terms become dominant degrading the device performance.

Lattice-matched, planar-doped InAlAs/InGaAs/InP modulation-doped field-effect transistors (MODFETs) with gate lengths from 1 $\mu$m down to 0.15 $\mu$m have been fabricated on organometallic vapor phase epitaxy-grown (OMVPE) layers. A unity current-gain cutoff frequency of 187 GHz has been measured for a 0.15 $\mu$m gate-length MODFET. Using dc and rf measurements, the effects of scaling the planar doping level, the InAlAs-barrier layer thickness, and the gate length on the device performance have been investigated.