Low Read Noise

A photon detector converts the energy of the incoming photons to a charge packet, and ultimately this charge must be converted to voltage or current and sent off chip. The amplifier that performs this conversion will add "read noise" to the detection process. Achieving very low read noise is a fundamental requirement of a high-sensitivity device.

Figure 1. Schematic picture of CCD output amplifierFigure 1. Schematic picture of CCD output amplifier. (Click on image to see larger version.)

We have used a number of techniques to design and build very low-noise readout amplifiers (the Sense FET in Figure 1), including designing for very low-input capacitance (therefore giving a high voltage per unit charge) and designing for low 1/f noise (taking care to avoid noise caused by the trapping of electrons as they traverse the amplifier channel).

Figure 1 shows schematically how the charge-to-voltage conversion takes place in a charge-coupled device (CCD); the charge is passed onto a very low-capacitance node (shown as an n+ diode in this picture) and a voltage is developed. This voltage is sensed with a low-noise field-effect transistor (FET) whose output is then sent off chip.

 

 

Figure 2. Layout design of output nodeFigure 2. Layout design of output node. (Click on image to see larger version.)
Figure 3. Microphotograph of output node Figure 3. Microphotograph of output node.

Figure 2 shows a typical layout design of an output node in a plane view looking down on the surface of the CCD circuit. Figure 3 is a corresponding circuit microphotograph of the above design taken from an actual finished CCD device.

The chart shown in Figure 4 is a plot of read noise for several types of output amplifiers as a function of output data rate. The noise increases for higher data rates because the filters in the output circuit must be increased in bandwidth, therefore letting through more thermal noise generated by the amplifier. Low-noise amplifiers show a slow rise in read noise as the bandwidth of these filters is increased.

Figure 4. Plot of read noise for several types of output amplifiers as a function of output data rateFigure 4. Plot of read noise for several types of output amplifiers as a function of output data rate. (Click on image to see larger version.)

The black and white dots are data taken for our normal metal-oxide-semiconductor FET (MOSFET) low-noise buried-channel low-noise amplifiers, while the red dots are data from a more recent junction FET (JFET) design. Both types of amplifiers can be constructed in our standard CCD process. The JFET amplifier has substantially lower Johnson and flicker noise and lower capacitance than the MOSFET design, and therefore performs with lower read noise than the conventional output.

 

 

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