Diamond Detectors for Ionizing Radiation - HEPHY

CHAPTER 5. DETECTOR MATERIAL COMPARISON 24
Collection Distance [µm]
400
350
300
250
200
150
100
10000
5000
Signal [e-]
50
Natural type IIa diamond
1990 1992 1994 1996 1998
Time [year]
Figure 5.3: The history of the charge collection distance of CVD diamond.
250
Collection Distance (µm)
200
150
100
50
8000
6000
4000
2000
Mean Charge (e)
0
0 0.2 0.4 0.6 0.8 1 1.2
Electric Field (V/µm)
0
Figure 5.4: The collection distance vs. the electric eld.
pairs.
In order to remove the intrinsic charge from the bulk, a pn-junction is introduced.
Usually starting with a weakly doped n-type silicon wafer, a thin layer on one side is
heavily doped with boron acceptors, and a thin layer on the opposite side with arsenic
donors, resulting in a p + nn + -diode. Alternatively, the bulk material can be of p-type,
which makes no principal dierence. Finally, the surfaces are metallized to form Ohmic
contacts. When the pn-junction is reverse-biased, all free carriers are drained from the
bulk, and the detector is sensitive to ionizing radiation. Fig. 5.5 shows such a silicon
detector with the applied bias voltage, which is above the depletion voltage 1 , and the
resulting electric eld. The implant layers are much thinner in reality, thus a the electric
1 The depletion voltage is the minimum bias voltage required to establish a space charge zone across
the whole bulk