Working principle of two-phase CCD device

Working principle of two-phase CCD device

Figure 2.2 Charge transfer process in three-phase CCD

(a) Initial state; (b) Charge transfers from electrode ① to electrode ②; (c) Charge is evenly distributed under electrodes ① and ②;
(d) Charge continues to transfer from electrode ① to electrode ②; (e) Charge is completely transferred to electrode ②; (f) Three-phase overlapping pulse

For two-phase devices, if each electrode has an asymmetric structure, directional charge
movement can be achieved. Take Figure 2.3 as an example. Two different oxide thicknesses are used here to achieve the asymmetry of the surface potential
. When the same voltage is applied to the metal electrode, the surface potential under the thick oxide layer
is lower than the surface potential under the thin oxide layer. Therefore, in the structure shown in Figure 2.3, there is a built-in
barrier under the thick oxide layer of each electrode to prevent the charge stored under the thin oxide layer from moving in the opposite direction. For a two-phase CCD, as long as we note
that the surface potential changes due to the stored charge and its surface potential must always be lower than the barrier height, then
we can easily calculate the charge storage capacity. That is: Qsat=ACoxBH

Where BH is the barrier height (in volts), A is the effective area of ​​the CCD electrode, and Cox is
the capacitance per unit area of ​​the oxide film. When a very low voltage is applied to one electrode of a two-phase CCD and a very low voltage is also applied to the adjacent electrode
, the electrode can hold the charge packet. This is a unique property of the two-phase CCD,
and for this reason, whether the two clock pulses of the two-phase CCD overlap or not will not seriously affect
the performance of the device. Therefore, the requirements for the clock waveform and timing relationship are very loose. This advantage is
obtained at the cost of a complex device structure. The TCD1208AP device selected for the NIRS array detection system introduced in Chapter 3
is a two-phase CCD device.

Figure 2.3 Two-phase CCD structure and clock waveform