Types of Image Sensors - Essential Principles of Image Sensors - page 68

A A′

Readout part

φ off

p n -substrate

Photodiode

VCCD
channel

φ PD

φ read

V-sub

(a)

p

Readout

φ PD

φ read

(b)

Without electric field (move by random thermal motion)

p

φ PD

With electric field (drift by electric field)

(c)

p

completely depleted read out
= complete transfer read out

φ PD

Electric field

Depleted photodiode

(d)

FIGURE 5.21
Schematic diagram of transfer mechanism in depleted PD: (a) before readout; (b) early state of readout opera-
tion; (c) final state of readout process; (d) after completion of readout.

and the charge amount in the PD decreases rapidly. But if there is a potential barrier,
as shown in Figure 5.22b, at the stage that charge quantity reduces to the level that
( V g ‒ V s ) decreases to the same degree with several-fold of kT, I s decreases very much
as follows:

( )

I ∝ exp q VV − kT1 − (5.2)

s g s

This is the weak inversion mode or subthreshold region of a MOSFET. 13 In this situation,
the leading mechanism of electron movement is thermoelectron emission mode, and it