At 0K, the energy states in the valence band are all occupied and the states in the conduction band are all empty. As the temperature rises, electrons in the vallance band can get excited and transition to a state in the conduction band; hence, create a free electron and an empty state in the vallance band, called a hole. Free electrons and holes can move around and contribute to conduction. Let’s see how their populations change with time and temperature.
Generation rate, G, is the number of electron-hole pair generated per unit time per unit volume. For any given semiconductor, G only depends on the temperature. Let’s assume we don’t lose any of the generated electrons or holes and see how their populations change with time. You may use the slider to change the temperature. (270K-320K)
A free electron in the conduction band can transition to the vallance band and fill an empty state. This way, a free electron and a hole both vanish and this process is called “Recombination”. The recombination rate, R, is again described as per unit volume per unit time. What do you think determines R? Here for simplicity generation is halted.
You may hover the mouse over the box to generate some electron-hole pairs and then observe them recombine.
In reality, generation and recombination happen at the same time. Let’s assume at time=0, there are no electrons and holes (T=0) and we instantaneously raise the temperature to 300K. Can you describe qualitatively why we reach equilibrium?
What happens when you change the temperature?