Charge Transfer

A thin layer of silicon dioxide is grown on a section of silicon and a conductive gate structure is applied over the oxide. Applying a positive electrical potential to the gate creates a depletion region where the free electrons generated by incoming photons can be stored. A potential well collects electronic charge from any source until the well is filled. Practical potential well capacities range up to a million electrons. Depletion depths range from a few micrometers up to tens of micrometers in specially prepared silicon.

Electrons freed by thermal agitation and by high-energy particles are indistinguishable from those generated by photon interaction. These dark electrons can have an adverse effect on the detection limits for photon-induced charge.

Charge Transfer The charge collected in a potential well must be brought to an output amplifier. When a series of oxide and conductive gate structures is fabricated with multiple phases, potential wells can be propagated through a silicon sheet.

This charge transfer concept is essential to understanding charge-coupled devices. When an appropriate sequence of potentials is applied to the gates, the potential wells are propagated in the direction shown in the figure Charge Transfer. Any charge that has been collected is carried along in the wells, and the charge packets in each potential well remain separated. Charge packets can be transferred thousands of times without significant loss of charge.

Direction of Charge Transfer

Phase 1

Phase 2

Phase 3

Gate

SiO2layer


t0		Q 2
	Q 1	Q 2
t1
	Q 1	Q 2
t2		Q 2
	Q 1	Q 2
t3
	Q 1	Q 2
t4
	Q 1	Q 2

The illustration is simplified to emphasize the concept. To ensure effective charge transfer, charge propagation actually occurs in a channel buried just below the surface, where there is no interference from interface states. The gates actually overlap, to create the drift field required for efficient charge transfer.

30Advanced Camera Operation Manual