Step Function Photoconductive Signal Generation in Integrated Coplanar Test Fixtures

Abstract

Electro-optic sampling has been used in the ultrafast time-resolved characterization of active devices such as modulation-doped field- effect transistors (MODFETs) [1,2] and heterojunction bipolar transistors. One of the most interesting measurements is the large- signal response of an active device, because it gives information on an ultrafast time scale about the nonlinear response of the device. Such measurements require a photoconductive switch geometry that allows independent control of the device operating point and the photoconductive excitation; these have been developed by Frankel et. al. [1]. Ideally, to reduce and control access parasitics, the device under test should be monolithically integrated with the coplanar test fixture. In this paper we report a study of photoconductive signal generation in structures similar to those of Ref. 1, monolithically integrated with high- performance MODFETs. We show the results of photoconductive excitation, and show how positioning of the excitation beam affects the generated signal. Our results show that changes in the direction of the photogenerated current with respect to the coplanar stripline has little effect on the signal observed 1.5 mm away. We show that the photogenerated signal is step-like in form, and compare our results with a 2-D model of the carrier transport including Schottky contact effects. The process is composed of a rapid screening of the field by photogenerated carriers, followed by a slower process of charge collection.