Simulation of Charge Transfer Process in a 2DEG-CCD

Abstract

This paper describes the modeling and simulation charge transfer process of a 2DEG charge coupled device. We have introduced a new approach to calculate the scattering rate of two-dimensional electron gas in the transport channel of a 2DEG-CCD. This technique was applied to the investigation of the charge transfer process with ensemble Monte Carlo method. I. I NTRODUCTION Charge Coupled Devices made on silicon have found wide application in imaging and signal processing. These applications have a low-speed operations but, the CCDs for microwave frequency are not suitable. Hetero-junction CCDs, have been demonstrated to have many advantages over earlier silicon and GaAs structures because of their higher low-field mobility and saturation velocity. The operation of these structures are limited only by the saturation velocity of the carriers which results in an upper frequency limit of approximately 40GHz. Microwave frequency filters such as , transversal filtering, correlation, and variable delay line could be accomplished by the CCDs based on this technology. These filters would find many uses in microwave frequency applications such as personal mobile communication devices and in the communication systems (1)-(5). In this work, charge transfer process of a 2DEG-CCD by ensemble Monte Carlo method is simulated. In the simulation we used a simplified structure that reported in (6) with difference that a cermet film is used to encapsulate the inter-electrode gaps. The cermet film results in a monotonic variation of the surface potential between the electrodes at all clock frequencies (7). As a consequence, energy troughs which would otherwise be produced in the active layer under the inter-electrode gaps are removed. This eliminates the possible performance degradation caused by the retention of passing signal charges, thus effectively increasing the charge transfer efficiency (CTE) of the device. This structure also eliminate the need of submicron (<0.5gm) inter-electrode gaps(8). Since the surface potential between the electrodes vary linear calculating the scattering in all mesh point is very complex. Therefore, a simple approximation to this problem is more convenient. Thus we propose a new approach to simulating the charge transfer in the CCD that will be described in the section III. In the following, first, charge transfer is simulated for GaAs channel material system and then effect of choice of material system upon speed of charge transfer is discussed. It was observed that transit speed will be