# What is injection level (semiconductor physics)?

I am currently reading journal articles about semiconductor physics in solar cells. What is injection level?

I'll try to start off with what I understand. Photons hitting the silicon cause its electrons to jump to a higher energy state. Some of these electrons jump high enough that they cross from the valence band into the conduction band, thus contributing to the flow of electric current. Now, how long do these electrons stay in the conduction band and keep flowing? I believe that is what is called carrier lifetime.

I have just read that carrier lifetime depends on injection level. I think that injection level is the rate at which electrons jump into the conduction band. Am I on the right track?

1. What is injection level?
2. What is its unit of measure?
3. What are some common ways of measuring it?
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May I propose the tags semiconductor-physics and photovoltaics? – Kit Jan 15 '11 at 2:48
sure, good idea. I also added solid-state-physics, but if that tag doesn't really apply to this question, you can remove it. (I'm in particle physics myself so I'm a little fuzzy on exactly what constitutes solid state/condensed matter physics) – David Z Jan 15 '11 at 3:33
@David: Thanks. solid-state-physics fits quite well. – Kit Jan 15 '11 at 3:53
@David: it's just as the names suggest, condensed matter physics investigates all forms of condensed matter (but also some related physical systems) while solid state physics studies just the solid phase (crystals for the most part). Of course this is not exactly true, but for the most part: SSP is a subset of the CMP. – Marek Jan 15 '11 at 9:43

1. the injection level is defined as $\delta n/p_0$ where $\delta n$ is the minority carrier (e.g. electrons') density excess at non-equilibrium while $p_0$ is the equilibrium density of the majority carriers (e.g. holes).