Graduate Thesis Or Dissertation
 

Electrical properties of GaAs FET structures

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  • The electrical properties of ion-implanted GaAs FET channels are investigated by two methods. First, the channel current (I) as a function of voltage (V) is examined at different temperatures and using different voltage ramp rates. The standard FET I-V curve, which can be observed on a commercial curve tracer, is not observed at slow ramp rates. The curve exhibits an abrupt decrease above 220°K and a stepwise increase at lower temperatures. A model based on the effect of electron transfer deferred by deep traps is established to explain the anomalous current dropback phenomenon. Impact ionization of trapped electrons is believed to happen at different spatial positions along the channel at different temperatures. The actual position at which impact ionization occurs depends on the thermal properties of the involved trap which is identified to have energy level at .47 ± .05 ev below the conduction band edge. The I-V characteristic of the channel is strongly affected by the excessive field strength generated through impact ionization. This model explains the observed phenomena consistently. In the second investigation method, the deep traps existing along the FET channel are examined via the Deep Level Transient Spectroscopy (DLTS) technique. Standard transient analysis is discussed and shown to be inadequate for ion-implanted samples. A new model based on a more realistic trapezoidal doping profile is derived and simulated. The simulation results are compared with experimental data and excellent agreement is obtained. A hole-like DLTS peak experimentally obtained from an n-GaAs Schottky diode is successfully simulated by the new model and shown to be an artifact due to the tail portion of the doping profile. Capacitance versus voltage (C-V) measurement confirms that the trapezoidal doping concentration is an idealized approximation for ion-implanted samples. These two approaches significantly improve the understanding of defect-related electrical properties of ion-implanted GaAs FET devices and contribute to a better knowledge of device characterization.
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