2. Si(Li) Detectors

This type of detector is essentially a reverse-biased silicon diode. X-rays are absorbed in the depletion layer and cause the formation of electron-hole pairs. The charge collected at the anode is converted to a voltage by a JFET or a similar amplifier. This results in a voltage pulse that is proportional to the number of pairs created and thus to the incident X-ray energy. The resolution is determined by the energy required to create an electron-hole pair (3.8 eV) and also by the Fano factor which is $\sim$ 0.1 for silicon.

In order to maximize the X-ray absorption cross-section the volume of the depletion region must be made as large as possible. This is done by creating an insulating layer between the anode and cathode. Such a detector is also known as a p-i-n (p-type-insulator-n-type) diode. However Si is an intrinsic p-type semiconductor so it has a slight excess of holes. This means that thermally excited charge carriers cause leakage currents across the depletion layer which make detection of the pulses produced by X-rays difficult. Doping can compensate for excess electron acceptors but it is difficult to dope such large volume uniformly. Lithium diffuses easily into silicon and can provide the required compensation. The insulating Li-drifted layer is the X-ray-sensitive region. One disadvantage of Si(Li) detectors is that they have to be stored in liquid nitrogen or the lithium will diffuse out of the depletion layer.