Channel Potential

The channel potential test measures the electro-static potential under each electrode of interest on a CCD detector. To perform the test, the gate of interest must be surrounded by two diodes, with one diode statically biased and the other, the measurement node, setup to source a very small current (e.g. 20 nA). All other gates between the diodes must be biased 'on' and to ensure that the correct measurements are obtained other gates may need to be biased 'off'. Figure 1 depicts the measurement of Summing Well 'A' (SWA) on a hypothetical detector. Note that in Fig. 1 all gates, excluding SWA, within the serial register are biased 'on' and the parallel register clocks are biased 'off'.

Figure 1. Schematic diagram of channel potential setup on a typical CCD.

Figure 1. Schematic diagram of channel potential setup on a typical CCD.

The measurement proceeds as follows. Once the static voltage and current biases are applied, the voltage on the gate of interest is swept from negative to positive and the resultant voltage on the measurement node, which floats to the electro-static potential under the gate of interest, is recorded. The potential measured at the zero gate voltage is defined as the threshold potential and the voltage corresponding to the breakpoint under negative bias is defined as the inversion voltage. The slope of the channel potential curve is also a critical parameter as it represents the effective gain between external applied gate voltage and the resultant electro-static potential within the silicon detector.

The fundamental configuration of a basic MOSFET and a typical resultant channel potential curve are shown in Fig. 2. The clipping of the curve at the negative gate voltages less than V|N is a result of the Si-SiO2 interface operating in inversion, assuming an n-type buried channel in a p-type epi substrate. The clipping of the curve at positive gate voltages occurs when the potential under the gate of interest exceeds the potential under the statically biased diode.

Figure 2. (left) Basic MOSFET (right) and a typical measurement result.

Like all the DC tests described herein, the channel potential measurements can be efficiently performed using an automated DC test setup. With the threshold potential and inversion voltages in hand, a complete Channel Potential (CP) diagram can be drawn showing the entire serial register or parallel register. Including instrument design, measured part-to-part tolerances and post radiation CP shifts, the channel potential diagrams provide an easy way of verifying correct operation of the CCD detector throughout the life of the instrument.

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