A new silicon chip (INVESTIGATOR) manufactured for R\&D purposes has 134 mini-matrices with various pixel designs. Each matrix consists of 8 $\times$ 8 pixels, which put out 64 analogue signals at 65 MHz. The silicon pixel design is based on the newest technology of the complementary metal-oxide-semiconductor (CMOS) monolithic active pixel sensor (MAPS), which integrates the silicon sensor and the read-out circuitry in a pixel. The MAPS has advantages of low power consumption, high granularity of a pixel, and fast read-out. In this paper, the charge collection time for different pixel designs and reverse bias voltages is studied by using the INVESTIGATOR. The charge collection time is estimated by fitting the waveform for changing pixel pitch, reverse bias voltage, diameter of collection n-well diode, and spacing. Based on these results, we discuss the dependence of the relative depletion volume and the charge collection time on the pixel geometry.

Keywords: Silicon pixel detector, Monolithic active pixel sensor, Charge collection time