|M.Sc Student||Cohen-Elias Doron|
|Subject||Control of Current Limiting Phenomena in Bipolar|
Transistor by a Non Uniform Doping Profile in the
|Department||Department of Electrical and Computer Engineering||Supervisor||PROF. Dan Ritter|
This dissertation comprises theoretical and experimental studies of current limiting phenomena in bipolar transistors with non-uniform collector doping. In the first part, we reformulate the theory of the well-known Kirk effect for the non-uniformly doped collector case. In the second part, we present an experimental study on the elimination of the energy barrier between the base and the collector of double heterojunction bipolar transistors by a doping pulse.
The Kirk effect is a space charge effect, which limits the current through a bipolar transistor. Conventionally, the doping profile in the collector is uniform. We show that weighting the doping profile toward the base increases the Kirk effect threshold current density. For example, a linear profile enhances the threshold current density by 50% compared to the uniform doping case. However, when the dopant distribution is too heavily weighted towards the base, the electric field may vanish inside the collector, and limit the current density. We have demonstrated this effect by analyzing an exponential doping profile.
In the second part of this work, we study a new method to eliminate the barrier between the base and the collector of double heterojunction bipolar transistors. The most widely used method to eliminate the barrier is composition grading. However, the disadvantage of composition grading is that the minimum thickness of the graded layer is roughly 50 nm. Thus, in thin collector devices, the grade layer may comprise 50% or more of the collector thickness. An additional drawback of composition grading is that when the graded layer is composed of GaInAsP wet etching becomes very difficult. The new approach demonstrated here is the delta doping method. We have grown a thin heavily doped n-type layer on the collector side of the abrupt base collector GaInAs/InP heterojunction. The depleted region of the heavily doped base served as the opposite delta doping. The resulting transistor characteristics clearly demonstrated that the potential barrier was fully eliminated. Microwave transistors were fabricated and characterized. Cut off and maximum oscillation frequencies of 180 GHz and 160 GHz respectively were obtained.