| he study of high field carrier transport in semiconductor devices is important since the presence of high fields can either be a degrading or an enhancing effect based on the nature and operation of the device. As physical dimensions of semiconductor devices shrink and internal electric fields rise, a large number of carriers gain high kinetic energy. These hot carriers can degrade the device performance and lead to reliability problems in MOSFET's and HBT's. On the other hand, high electric fields are deliberately included in devices like the avalanche photodiode (APD) to increase the signal-gain of the device by carrier multiplication. High electric fields are also present in erasable-programmable read-only memories (EPROM) to charge a floating gate electrode. Hence understanding and modeling high-field carrier transport has become important in the construction and optimization of numerous semiconductor devices.;In this dissertation we study carrier transport at high energies with a particular emphasis on the avalanche photodiode. We have calculated the semiconductor bandstructure using the empirical pseudopotential approach and studied the impact ionization phenomenon using a wave vector dependent threshold energy. We have performed Monte Carlo simulations using real bands to study the carrier transport. APD's are increasingly attractive for use in demodulating optical signals in large-capacity, long-distance optical communication systems. The gain-bandwidth product is often used to quantify the performance of an APD and in effect this single parameter represents the cumulative effect of both the quantum efficiency and response speed of the device. We have studied the separate absorption, grading, multiplication APD's (SAGM-APD) and have derived expressions for the excess noise factor using a 'discrete ionization model' that fits the experimental results very well. To obtain high signal-to-noise ratio in an APD, the semiconductor material must be chosen such that the differences between the electron and hole ionization rate is large. For III-V semiconductor materials useful in the 1.30 and 1.55... |
