On the Weighting Field and Admittance of Irradiated Si-sensors

In this paper the weighting field $E_W$ and the frequency dependence of the admittance Y of n$^+$p pad sensors irradiated by 24 GeV/c protons to equivalent fluences in the range $\Phi_{\rm eq} = 3$ to $13\times 10^{15}$ cm$^{-2}$ are investigated. 1-D TCad simulations are used to calculate $E_W$. For $\Phi_{\rm eq} < 10^{13}$ cm$^{-2}$ $E_W$ depends on position and time. However, for higher $\Phi_{\rm eq}$ the time constant $\tau$ is much longer than the typical electronics readout time and $E_W = 1/d$ ($d$ = sensor thickness). It is demonstrated that the increase of the resistivity of the Si bulk with irradiation is responsible for the increase of $\tau$. The admittance Y of irradiated pad sensors has been measured for frequencies between f = 100 Hz and 1 MHz and voltages between 1 and 1000 V at -20$^\circ$C and -30$^\circ$C. For f < 1 kHz the parallel capacitance C$_p$ shows a f dependence. A model with a position-dependent resistivity is able to describe the data. It is concluded: 1. The weighting field of a highly irradiated sensor is the same as the weighting field of a fully depleted sensor before irradiation. 2. Models with a position-dependent resistivity describe the frequency dependence of C$_p$ for irradiated sensors.