Transistors

Abstract

The demand for high frequency wireless data transfer is increasing very rapidly. Some of the present and projected requirements are [24]: Mobile telephones (900 MHz and 1.8 GHz), Communication through low orbit satellite systems (20, 30 and 60 GHz), Automotive industry (collision avoidance radar work at ∼ 77 GHz), High resolution radar for airport surveillance (planned to work at 94 GHz) and space observation systems (140 GHz). GaAs based MESFETs (Metal-Semiconductor FETs) and HEMTs or MODFETs (High Electron Mobility Transistors or Modulation Doped Field Effect Transistors) are the key components for high frequency data transfer systems. High frequency, low power consumption and excellent noise performance are important features of GaAs based FETs. With the advent of strained layer epitaxy, HEMTs containing strained pseudomorphic InGaAs active layers were developed. These transistors, designated as pseudomorphic or PM-HEMTs, are fabricated on GaAs substrates. InGaAs alloys with high In concentration have considerably higher mobility and larger intervalley separation in the conduction band than GaAs. For several years InGaAs strained layer devices dominated the HEMT technology [12]. The main limitation of this technology is that a large fraction of indium can not be built in the layers. If indium concentration is large critical thickness becomes very small. For best performance 30 to 40% mole fraction of In As is required in InGaAs layers. An alternative technology based on InP substrates uses lattice matched In_0.53Ga_0.47As layers. An InGaAs layer with 53% indium is lattice matched to InP. The HEMTs based on InP substrates show higher operating frequencies because of the increased mobility and conduction band discontinuity. Fig. 8.1 shows the evolution of the values of cut-off frequency ft} of both GaAs-substrate and InP-substrate devices. The lowest frequency data point in Fig. 8.1 is for an AlGaAs/GaAs HEMT. The top two points are for the transistors with more than 53% In in the active layers.