Shi Jie, Simon William E, Zhu Timothy C
Sun Nuclear Corporation, Melbourne, Florida 32940, USA.
Med Phys. 2003 Sep;30(9):2509-19. doi: 10.1118/1.1602171.
The sensitivity S of Si diodes generally increases with an increase in the instantaneous dose rate r of the radiation beam from clinical linear accelerators. A theoretical model is established to understand the physical origin of this dependence. During a radiation exposure, a portion of the excess minority carriers (electrons or holes) generated in the diode is captured by the R-G (recombination-generation) centers and is recombined with the majority carriers. The captured portion depends on the excess minority-carrier concentration delta p (proportional to r), the R-G center concentration N(t), and the minority-carrier capture cross-sections (sigma(n) for electrons and sigma(p) for holes) by the R-G center. When r increases, the R-G center concentration may not be sufficient to keep the recombination portion constant, which leads to an increase in diode sensitivity because a larger fraction of the charge will be collected. Larger majority-carrier concentration increases the recombination probability of the excess minority carriers and thus decreases the r dependence. The ratio of minority-carrier capture cross-sections, sigma(p)/sigma(n), influences the magnitude of the r dependence and also differentiates the r dependence between n-type and p-type diodes. A number of different circumstances can occur in diodes. When sigma(p) > sigma(n), such as for the dominant R-G center generated by electron radiation, the sensitivity is more dependent on r in an n-type diode than in a p-type diode if all the other device parameters are the same. When sigma(p) < sigma(n), the sensitivity is then more dependent on r in a p-type diode than in an n-type diode. The condition of sigma(p) < sigma(n) can occur when R-G centers with this property are generated by the foundry die process. A diode could have very small r dependence due to large R-G center concentration, generated by heavy platinum doping or radiation accumulated dose. Experimental data are compared with theory.
硅二极管的灵敏度S通常会随着来自临床直线加速器的辐射束瞬时剂量率r的增加而提高。建立了一个理论模型来理解这种依赖性的物理根源。在辐射暴露期间，二极管中产生的一部分过剩少数载流子（电子或空穴）被R-G（复合-产生）中心捕获，并与多数载流子复合。捕获部分取决于过剩少数载流子浓度Δp（与r成正比）、R-G中心浓度N(t)以及R-G中心对少数载流子的捕获截面（电子的σ(n)和空穴的σ(p)）。当r增加时，R-G中心浓度可能不足以使复合部分保持恒定，这导致二极管灵敏度增加，因为会收集到更大比例的电荷。更大的多数载流子浓度会增加过剩少数载流子的复合概率，从而降低对r的依赖性。少数载流子捕获截面的比值σ(p)/σ(n)会影响对r的依赖性大小，也会区分n型和p型二极管对r的依赖性。二极管中可能会出现多种不同情况。当σ(p) > σ(n)时，例如对于电子辐射产生的主要R-G中心，如果所有其他器件参数相同，n型二极管的灵敏度比p型二极管更依赖于r。当σ(p) < σ(n)时，p型二极管的灵敏度比n型二极管更依赖于r。当铸造芯片工艺产生具有这种特性的R-G中心时，就会出现σ(p) < σ(n)的情况。由于重铂掺杂或辐射累积剂量产生的大R-G中心浓度，二极管可能对r的依赖性非常小。将实验数据与理论进行了比较。
