The characteristics of inverted staggered hydrogenated amorphous silicon/silicon nitride (a-Si:H/a-SiNx:H) thin film transistors (TFTs) are reported between 80 K and 420 K. The TFTs are found to have three distinct transport regimes. Between 80 K to approximately 260 K, the transport in the TFT channel is dominated by electrons hopping between localized gap states of a-Si:H and is analyzed using Mott's theory of variable- range hopping. As the tem-perature is increased above ∼260 K the current becomes thermally activated with an activation energy which depends on the gate voltage. The effective field effect mobility, as determined from the TFT characteristics in saturation, is activated in this regime, with an activation energy 0.10 to 0.15 eV. The various activation energies are found to be sensitive to annealing which can be explained by a reduction in deep and shallow states in the a-Si:H active layer. When operated above ∼360 K the TFTs become unstable due to rapid changes in threshold voltage under the applied gate field. The behavior of the threshold voltage is described over the entire temperature range and possible mechanisms are discussed.

Low resistance, alloyed AuGeNi Ohmic contacts have been extensively used in the current manufacturing GaAs devices. However, extension of usage of these devices to Very Large Scale Integration levels requires the contacts with excellent thermal stability, shallow diffusion depth, and smooth contact surface in addition to low contact resistance. In the present paper recent studies for development of “non-gold” Ohmic contacts which improve the poor contact properties of the alloyed Ohmic contacts are rev i ewed.

Recently, thermally stable, low resistance In-based ohmic contacts to n-type GaAs have been developed in our laboratories by depositing a small amount of In with refractory metals in a conventional evaporator, followed by rapid thermal annealing. By correlating the interfacial microstructure to the electrical properties, InxGa1-xAs phases grown epitaxially on the GaAs were found to be essential for reduction of the contact resistance (R c ). This low resistance was believed to be due to separation of the high barrier (φ b ) at the metal/GaAs contact into two low barriers at the metal/InxGa1-xAs and InxGa1-xAs/GaAs interfaces. In this paper the effects of the In concentration (x) in the InxGa1-xAs phases and addition of dopants to the contact metal are presented. High In concentration is desirable to reduce the φ b at the metal/InxGa1-xAs interface. Such contacts were prepared by sputter-depositing InAs with other contact elements, but the low R c values were not obtained. The reason was explained to be due to an increase in the φ b at the InxGa1-xAs/GaAs interface due to the formation of misfit dislocations. However, addition of a small amount of Si to the contact metals reduced significantly the R c value. This contact demonstrated excellent thermal stability: no deterioration was observed at 400°C for more than 100 hrs. In addition, the use of this Ni(Si)InW contact metal allowed us to fabricate the low resistance ohmic contacts by one-step (simultaneous) annealing for “implant-activation” and “ohmic contact formation”, which simplifies significantly GaAs device fabrication process steps. For p-type ohmic contacts, low resistance contacts were fabricated by depositing the same NilnW contact material to p-type GaAs. This contact was also thermally stable during subsequent annealing at 400°C. Within our knowledge this is believed to be the first demonstration of low resistance, thermally stable ohmic contact fabrication using the same materials for both n and p-type GaAs.