3D Time-of-flight distance measurement with custom - Universität ...

SOLID-STATE IMAGE SENSING 73
3.2 Charge coupled devices: CCD - basic principles
The underlying physical structure for charge-coupled devices is the MOS diode in
deep depletion (see Section 3.1.2 for details). The operation sequence of CCD
devices is to integrate optically generated charge carriers (photoelectrons) locally
and to bundle them into charge packets in each pixel. The charge carriers are then
physically transported to one output stage by a clever sequence of charging and
discharging neighboring photogates. Each pixel-charge packet is subsequently
transferred to the output stage, where it is converted into an analog voltage at the
output.
Collected charge
Coupling two wells
First well collapses
Charge transferred
A
0 V
0 V
0 V
0 V
B
10 V
0 V
0 V
10 V 10 V 0 V
5 V 10 V 0 V
Figure 3.17 Charge transport in charge-coupled devices. [HEK].
The CCD-transport mechanism, first demonstrated by Boyle, Smith, Amelio and
Tompsett [BOY, AME], is illustrated in Figure 3.17. After an integration period in
which the charge is collected under the only biased CCD-gate B, in a first step the
neighboring gate C is biased to the same potential as gate B. Thus the two
potential wells merge to one and the charge will equally distribute under both gates
due to self induced coulomb drift and thermal diffusion. This process is called
charge-coupling. In the next step the bias voltage of gate B is decreased and the
potential well under this gate collapses. This results in the transport of the complete
charge packet towards gate C. By repeating these steps the charge can be moved
through the semiconductor. This transport mechanism works with a very high
efficiency, the so-called charge transfer efficiency CTE. The CTE is defined as the
0 V
C
10 V
D
0 V