Detection Limit of Large Area Id Thin Film Position Sensitive Detectors Based in a-Si:H P.I.N. Diodes
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on both sides of the device to establish the required equipotentials (the reference and the collecting ones) 7 as indicated in Fig. 1. A-SI:H P-I-N DEVICE UNDER NON UNIFORM LIGHT ILLUMINATION.
A
A
The behaviour of a-Si:H pin devices uniform illuminated is different from the one observed in c-Si p.n junctions. For the c-Si case, Jph is determined by the carrier's
.
diffusion lengths of the photocarriers from
electrode
the low field bulk region to the high field
junction while in the a-Si:H case, Jph is
n_ electrode
determined by the drift components related to the high electric field (E) in the i-layer. To
77
understand this behaviour it is important to
Selec
i etP trode TCo
substrate
notice that the photocarriers are mainly generated within the i-layer of thickness W. Figure 1 - Structure of the ID TFPSD, showing the top If W is completely deplected by reverse and sectional views. biasing or the light is completely absorbed, the junction behaves similar to a Mott barrier 9 , leading to the establishment of an uniform E 10 , responsible by the carriers drift and so, by Jph. On the other hand, when an a-Si:H p-i-n device reverse biased is non uniform illuminated, a carrier's gradient appears between the irradiated and the non irradiated regions, dependent 791
Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
oncarriers flow distribution, carrier's transit time, relaxation time, conductivity of the collecting doped layer and on the recombination losses (R), that leads to the appearance of a AV, dependent on the carriers' lateral diffusion rate towards the non-irradiated regions, which are influenced by R. To analyse the device behaviour, we consider that the carriers are collected through the p-layer by a thin resistive layer and that the reverse bias enhances the device response time and decreases 3 the role of T on the device performances . This allows to define the boundary conditions, assuming that: i) conduction is mainly dependent on carriers accumulated in the edges of the ilayer; ii) the doped layers behave as sources of carrier's losses; iii) the net flow involves both carriers; iv) the rate of lateral diffusion depends on the average diffusion velocity of the carriers (vd) and on the spatial distribution of the generated carriers. Under these conditions, AV is dependent on the carrier's net flow distribution that leads to the build up of an E along the junction plane that facilitates the carriers transport away from the irradiated region, as sketched in Figs. 2a) and b). lightbeam
light beam
--
o0 0 0 adeplection - •- ....... ................ . -
+ +C)4
+ + + 0
~ - -
0
C CrQtp-laye eC)
Q 0 ppppae
1 electrode
0
0
+ +
resistive layer
++ -
region
pi-layer
- - -- -- --- -
+. -I
- --
--
--
+ Adepleclr...re 00000e0000000 aa aa SaD 4D
electrode resistive layer r
aD 1) 0
ea 1D
a aI
l i-ayer
trr lectre elayer
deplection region
Ge
eas 0
00 ea
0a
a
electrode
0
SO
n-layer electrode
yphorltate
phorrecllarre
b)
a)
Figure 2 - Illustration of
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