#research #physics/semiconductors See also: [[Fermi Levels]], [[Streicher's General Note on Space Charge Buildup]] sourcing from: [[@luthSpaceChargeLayersSemiconductor2010]] # Space Charge in Semiconductors Motivation: Understanding what's going on in [[20231208 Waveform Analysis Wall-Hitting]] form the LINAC measurement Space charges, and their corresponding forces, are those generated from the charge distribution in a material, which includes influences from charges created from [[Method of Images (Mirror Charge Technique)|mirror image charges]]. In semiconductor materials, these occur at the surfaces of the material and act as a perturbation to the local charge balance and may carry a charge and corresponding mirror charge depending on the type of semiconducting material. ## Space Charge in CZT detectors - Per [[@yang_internal_2010]], space charge in [[CdZnTe (CZT) Detectors]] can result from hole trapping, where the positively charged holes have their movement restricted as a result of a solid material defect. This disturbs the internal electric field, creating some distortions in the field shape, which can lead to incomplete charge collection and a polarization effect. - Per [[@tangCadmiumZincTelluride2021]], the trapping can be modelled as: $q(t) = q_0 e^{-t/\tau} $ where $t$ is the time since the energy deposition, and $\tau$ is the [[Charge Carrier Lifetime]] for electrons and/or holes. from the [[@luthSpaceChargeLayersSemiconductor2010]] abstract: >"If one puts a positive point charge into a locally neutral electron plasma (electrons on the background of fixed positive cores), the electrons in the neighborhood will rearrange to compensate that additional charge; they will screen it, such that far away from the charge the electric field vanishes. The higher the electron density, the shorter the range over which electrons have to rearrange in order to establish an effective shielding. In metals with free-electron concentrations of about 10^22 cm^−3 the screening length is short, on the order of atomic distances. On the other hand, in semiconductors the free-carrier concentrations are usually much lower, on the order of 10^17 cm^−3 may be, and we thus expect much larger screening lengths, of the order of hundreds of Ångstroms. These spatial regions of redistributed screening charges are called space charge regions." ### From [[20240401 LINAC Measurement 3 Notes]] We think that it's possible that the [[20240412 Unexpected Anode Waveform Decay Problem|anode waveform decay]] comes a development of space Charge. Yuefeng outlines this in his monthly report from April 2024.