How a Silicon detector chip works (basic)

This section is a writeup of my understanding of the science behind the chip. Knowledge of the mechanism behind the software is often useful for understanding of how to use the chip itself, or merely to satiate curiosity.

Basic Science

This section deals with science not unique to the Timepix and Medipix detector chips: however, it is not on the AS or A level physics syllabus, so I thought it would be convenient for some basics to be written up here.

The P-N Silicon Semiconductor Diode.

Silicon is a "semiconductor" material: this means that there is a small band gap between the conduction band and valence band of electrons. Electrons in the conduction band are de-localised, or not attached to a specific atom or molecule within the material: hence they can act as free charge carriers and conduct electricity. In a full conductor, there is no band gap between the valence band and the conduction band, and hence there are many de-localised electrons that can act as charge carriers, and hence the material conducts electricity. In a semiconductor, it takes only a small input of energy to move electrons to the conduction band, and hence for electricity to be conducted in the material. This means that the greater the temperature of the semiconductor the greater its conductivity - the inverse of a conductor.

A P-N diode consists of two types of silicon semiconductor: one P-type and the other N-type. The P-type contains an excess of electron holes in its conduction band, with an excess of negative ions; The N-type contains an excess of electrons in its conduction band, with an excess of positive ions. Both types are neutrally charged overall. Note an "electron hole" simply means an absence of an electron, which in effect acts as a positive charge carrier.


fig.1: A PN diode. Source.

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