Carrier Lifetime
In a forward-biased diode, when electrons from the n-type side of the junction are attracted to the p-type side, and vice versa for holes, the time it takes for an electron to diffuse from the junction to the point where it recombines is known as the carrier lifetime (T).
For silicon, this carrier lifetime is typically around 10 microseconds. If the distance between the junction and the metal contact is short, and electrons reach the metal contact before recombining, the diode is considered a short base diode. Conversely, if recombination occurs before reaching the metal contact, it is termed a long base diode. The time between crossing the junction and recombining is referred to as the transit time (T). Additionally, a capacitance is formed between the minority carriers, which is called diffusion capacitance or storage capacitance due to the presence of stored or diffusing minority carriers around the forward-biased pn junction.
Forward-Biased Diode
In a forward-biased diode:
- Electrons from the n-type side move towards the p-type side.
- Holes from the p-type side move towards the n-type side.
- There is a time gap before these carriers (electrons and holes) recombine.
The key terms are:
Carrier Lifetime (T): The time it takes for an electron (or hole) to diffuse from the junction to the point where it recombines. In silicon, this is typically around 10 microseconds.
Short Base Diode: If the distance between the junction and the metal contact is short, electrons reach the metal contact before recombining. This is considered a short base diode.
Long Base Diode: In contrast, if recombination occurs before reaching the metal contact, it’s called a long base diode.
Transit Time (T): The time it takes for carriers to travel from the junction to where they recombine.
Diffusion Capacitance or Storage Capacitance: A capacitance formed between minority carriers, due to the presence of stored or diffusing minority carriers around the forward-biased pn junction.
So, in simple terms, in a forward-biased diode, carriers (electrons and holes) move around. If they reach the metal contact quickly, it’s a short base diode. If they recombine before reaching the metal contact, it’s a long base diode. The time it takes for them to move and potentially recombine is called the carrier lifetime or transit time. Additionally, there’s a capacitance formed due to these carriers called diffusion or storage capacitance.