International Radiation Detectors, Inc.

Time Response

The diode response time is determined by the resistance and capacitance present in the detector and measuring circuitry and the charge carrier transit time. Capacitance C of the diode is given by the silicon permittivity e, area (A), and the p- epitaxial layer thickness (D). For the AXUVHS5 diode with 25 micron silicon thickness the capacitance is:

C = εA/D = (10^-12) x (10^-2) / (25 x 10^-4) = 4 pF

The diode series resistance is mainly governed by the distance electrons need to travel in the front field free n region towards the n+ electrode. For small area diode like the AXUVHS5 this resistance is only a few ohms.

Using the 50 ohm scope impedance, 5 ohm diode resistance and 4 pF capacitance the rise time (t_RC) given by 2.2 RC equals 0.48 ns. Worst case carrier transit time t_C, is a hole traveling across the entire depletion layer thickness. The time is given by the thickness (D) divided by the hole drift velocity. For the AXUVHS5 diode at 50 volt bias the electric field is 2x10⁴ V/cm and the corresponding drift velocity is 5.3x10⁶ cm/sec [14] giving rise to a transit time of 0.47ns.

Adding the times in quadrature yields a theoretical rise time of 0.67 ns which is very close to the experimentally measured value of 0.7ns.

It should be noted that carrier motion by diffusion process is completely neglected here which is a much slower process than the motion by drift. When a high speed photodiode like AXUVHS5 is fully exposed to radiation, carriers are also generated at the diode periphery. These carriers are collected by diffusion process because of absence of the electric field in this region. This results in the diode fall time much higher than the risetime. Therefore, to achieve almost equal rise and fall times it is necessary that the radiation be limited only to the diode active area possibly using a pinhole.

Larger the diode area larger will be the distance electrons need to travel to the n+ electrode. Therefore, large area diodes like AXUV20 have a series resistance around 20 ohm. To reduce this resistance to 1 ohm, AXUV20HS1 diodes have been specially fabricated which give 1 n-sec risetime when used with 150 volts bias (to reduce the drift time) and with a high speed amplifier (to get rid of 50 Ohm scope impedance).