The Lithium-Drifted Silicon p-i-n Junction as an X-Ray and Gamma-Ray Dosimeter

The p-i-n junction radiation detector is a solid-state analog of an ionization chamber. A complete review covering the operation of semiconductor junctions as radiation detectors has been given by Brown (1). In general, all types function in the same basic manner and, therefore, are fabricated in the same fashion with minor variations suited to the particular type under discussion. In the case of the p-i-n structure, impurities are introduced in a single crystal of p-type (conduction by holes) silicon, and a highly compensated, or intrinsic, region is thereby established within the bulk. This region, because of its poor conductivity, can act similarly to a dielectric and is capable of sustaining a high electric field. The introduction of the impurity (lithium) requires a high concentration at the surface through which it was diffused. This surface and the volume directly below are overcompensated and are converted to n-type (electron conduction) silicon. When a potential is applied to the device, the field established is uniform across the intrinsic region going to zero at the n-i and i-p boundaries. Energetic charged particles lose energy rapidly in the bulk through interaction with the electrons of the silicon. These collisions may lift electrons from the valence to the conduction band and may also lift electrons from deeper-lying occupied electronic bands to higher-lying unoccupied bands; holes are thereby created in nominally full bands. These electron-hole interactions cause electrons to fall to the bottom of the conduction band and the holes to rise to the top of the valence band. Many more hole-electron pairs are formed during the Auger de-excitation process. Such processes result in a value of approximately 3.5 ev being expended by the incident particle for each hole-electron pair created. These carriers drift in the electric field and give rise to a current. The charge collected is, therefore, a measure of the energy deposited in the counter by the primary particle.