|M.Sc Student||Ramon Dan|
|Subject||Tunneling through the Base Collector Energy Barrier in|
Abrupt InP/GaInAs/InP Double Heterojunction
|Department||Department of Electrical and Computer Engineering||Supervisor||PROF. Dan Ritter|
The goal of this work is to understand and model the tunneling mechanism through the base-collector energy barrier in abrupt double heterojunction bipolar transistors (DHBTs). In this type of devices, the electrons are injected from a wide bandgap emitter into a base with a narrower bandgap. The electrons, after crossing the base by diffusion, must proceed into the wider bandgap collector. The transfer mechanism into the collector is quantum mechanical tunneling. The tunneling process reduces the electron transfer rate into the collector, and hence the current gain of DHBTs.
The model developed in this work predicts the current gain of abrupt DHBTs with a single unknown parameter - the electron temperature in the base. The value of this parameter must hence be larger than room temperature, but smaller than the temperature corresponding to the kinetic energy of the electrons injected to the base. The model is based upon two assumptions. The first assumption is that the energy distribution function of the electrons in the base is the Maxwell-Boltzmann function, with a temperature that is not necessarily the lattice temperature. In other words, we assume that the electrons in the base are at quasi-equilibrium. The justification for this assumption is that the average time that electrons remain in the base is their lifetime, which is longer than their momentum relaxation time.
The second assumption of the model is that the concentration of the electrons in the base does not depend on their position in the base. This assumption is very good in the case of DHBT, because the energy barrier impedes the path for electrons to the collector.
An excellent fit between model calculations and experimental results was obtained for some of the transistors we have investigated. For each base-emitter bias point, the electron temperature in the base was obtained by fitting this single parameter to the measurements. It was found that the electron temperature in the base is higher that the lattice temperature, and that it increases with base-emitter voltage. Increase of the injection energy of electrons to the base with base-emitter can explain this result. Comparing the measured electron temperature in the base to the temperature of the electrons injected from the wide bandgap emitter to the base, we estimate that the energy relaxation time is of the order of the 2-3 times the lifetime, namely about 16-25[pSec].