Portland State University. Department of Electrical Engineering
Date of Publication
Master of Science (M.S.) in Electrical Engineering
Bipolar transistors -- Computer simulation, Cryoelectronics
1 online resource (2, viii, 87 p.)
The BICMOS technology which integrates the CMOS technology with bipolar technology has drawn considerable attention as an attractive VLSI technology because of the high speed performance and low power consumption of the BICMOS. However, continued down scaling of CMOS devices has caused increased concerns with problems such as latch up, hot carriers and short channel effect. Most of the above mentioned problems can be avoided by operating the CMOS at liquid-nitrogen temperature(LNT). At low-temperatures, the CMOS exhibits lower sub threshold leakage, higher carrier mobility (which yields improved speed performance), and a steeper logarithmic currentvoltage slope. On the other hand, the low-temperature operation of conventional silicon bipolar circuits has been generally dismissed as impractical because of the well known decrease in the current gain at low temperature. The present interest in integrated bipolarCMOS circuits, plus the prospect of increased reliability, lower wiring delay, and lower noise, has revised interest in low-temperature bipolar devices. In this context, it is therefore important to acquire accurate knowledge of the transistor properties at liquid nitrogen temperature. This can be done in two ways. One is through experimental lowtemperature measurements and the other by low-temperature device simulations. Existing room temperature numerical simulators are typically not useful for low temperature conditions. This is because the physical assumptions such as complete ionization, the parameter models and implementation methods for room temperature condition do not hold at low temperature. Therefore, we used BiLow - a steady state onedimensional Bipolar Low Temperature Simulator for the temperature range of 77K- 300K. This simulator, originally written in FORTRAN, was converted to C for the dual purpose of proper memory management and making further modifications easier. The focus of this research has been to model bandgap narrowing, incomplete ionization and Mott Transition at room and at low-temperature, evaluate the performance of the new BiLow and to derive conclusions on the BIT performance at LNT. It was observed that the bandgap narrowing was independent of temperature for the entire range of majority carrier concentration. The effect of Mott transition on the abrupt decrease in the electron concentration in emitter has been taken care of by smoothing out the concentration profile in the emitter thereby providing a continuity in the region of Mott transition. Both the current gain(~) and the frequency(ft) values obtained from simulating the two new profiles were found to be smaller than those obtained using the original BiLow simulator, as the doping in the base is higher and the device sizes were smaller. Most of the degradation in 13 and ft was found to occur below 150K. From the plots of the charge characteristics, we found that the total charge which is a strong function of temperature is more in the case of the profiles studied for this work than the total charge from the original BiLow simulator.
Nerurkar, Swarupa Madhav, "Modeling and Simulation of Bipolar Transistor at Low Temperature" (1993). Dissertations and Theses. Paper 4614.