Radiation hard LEDs have been manufactured by ABB HAFFO and GEC.
Both LEDs are made from GaAlAs and emit in the range 820 nm.
The following data refer to the performance of the GEC LEDs
- capacitance in range 200-400 pF
- Rise time (10-90%) 3.5-4 ns
- Response >10 microW launched power for 20 mA drive current.
Require some standing current for fast rise time
(1 mA is sufficient)
PIN diodes are silicon. The following performance numbers refers to
the current GEC PIN diodes
- responsivity is better than 0.5 microA/microW.
- Capacitance of PIN alone is about 0.5 pF.
Capacitance to ground plane is about 2 pF (comes from lines 1.5mm long,
125 micron wide on 4 micron dielectric).
Total capacitance is about 2.5 pF.
- rise/fall times are being measured. Preliminary results in the range of
6 ns (includes the 4 ns from the LED) at a bias voltage of 15V.
Thinner PINs will be tried in order to obtain fast rise/fall times at low
bias voltage.
Manufacturers data on radiation resistance indicate that the response
would drop by about 10% and the dark current would increase to about 40 nA for a
fluence of 10**14 n/cm**2. Since the initial signal is 8 microA the S/N
would not be a problem after 10 years operation at LHC. The bias voltage
would need to be raised to 10V after irradiation in order to maintain the
quantum efficiency
GEC Package
The GEC packages consist of 2 pig-tailed LEDs and one pig-tailed PIN diode
mounted on a low mass silicon package.
The total size of the package is 10mm* 5.5 mm * 1.4mm. The package includes
areas to mount the DORIC and LDC chips.
One such package could read out one ATLAS SCT module.
Optical and electrical cross talk between LEDs and PINs is minimised by the use of a
metal screen which separates the LEDs and the PIN as well as the LDC and
the DORIC chips. This screen is connected to the silicon baseplate
substrate.