Pulsed bipolar CMOS imager

This paper will describe an active pixel CMOS-compatible imager aimed at high resolution still cameras. We will discuss pixel operation, column sense circuits, serial output, and show results from existing imagers. In this abstract, we show results from a prototype 640x480 imager with 5.9x5.9 μm2 pixels built in 0.8 μm double-poly CMOS with one additional base implant. Active Capacitor-coupled Bipolar Pixel The pixel (Fig. 1) is a vertical NPN bipolar transistor with floating base capacitively coupled to common row lines, common collector, and emitter tied to common column lines. The base is reverse-biased to collector and emitter during integration. For readout, a strobe pulse is capacitively coupled to the base, forward-biasing the base-emitter junction, and dumping the bipolar-amplified base charge onto the emitter. Readout happens row by row; while each row is strobed and its pixel’s charge output is sensed by column charge-sense circuits, the previous row’s output is serially scanned out. Fabrication of the bipolar active pixel is CMOS compatible with one additional mask for p-base implant (inside n-well). The dielectric of poly to base capacitor is formed by the gate oxidation. The base capacitor is not sensitive to misalignment between poly and diffusion masks. The emitter n+ implant is self-aligned by the LDD structure and n+ S/D implant. The NLDD implant reduces leakage current at the emitter-base junction and reduces bipolar action along the emitter edge. The entire pixel array shares a common n-well collector tied to Vdd. The pixel charges in a row are dumped onto the column lines by strobing the row line momentarily high (Fig. 2). The strobe signal is generated off-chip during the sense phase of the column charge sense amplifier. This strobe signal is routed to a particular row by vertical scan circuits. The periodic pixel strobing pushes the instantaneous base current up to a level where β does not suffer low-current degradation. The peak pixel current is hundreds of times higher than it would be if the transistor were operating continuously in a forward-biased state. The strobe line capacitive coupling to the base, Csb, determines the saturation charge Qb that can be dumped from the base