|M.Sc Thesis||Department of Electrical Engineering|
|Supervisor:||Prof. Ritter Dan|
Heterojunction bipolar transistors (HBTs) of the InP/GaInAs material system are promising ultra-high frequency devices. The velocity of electrons in this material system is higher than in GaAs and Si, and a considerable effort is being invested in the fabrication of ever-faster devices. In this research, several aspects of electron transport in the base of InP/GaInAs HBTs are studied.
The epitaxial layers used for device fabrication were grown by the metalorganic molecular beam epitaxy (MOMBE) method. Epitaxial layer characterization included Hall effect and transmission line measurements, in order to determine the base hole concentration and mobility. Wide area HBTs were fabricated, suitable for DC characterization.
In the first part of this work, we have studied heavy carbon doping of the base. The tradeoff between gain and base resistance is discussed and the optimal doping for maximum gain and fmax is found. In addition, we have identified the dominant recombination mechanism as Auger recombination, by the relation between the transistor gain and the base sheet resistance.
The DC current gain of InP/GaInAs heterojunction bipolar transistors with varying base thickness and composition was measured. Much larger composition grade values than previously reported were achieved using strain compensation. A simple two parameter Monte Carlo simulation was developed to interpret the results. The simulation yields an accurate plot of the base transit time versus base thickness. Clear evidence for the reduction of base transit time due to hot electron injection was observed in devices with thin uniform bases. The current gain of 45 nm thick graded base devices saturated as the grading was increased beyond standard values. Grading did not increase the gain of devices with a 20 nm thick base.