Transistor NPN reverse bias ?

[joseche]

When current reaches the base (Ib) it allows current going from Emiter to Collector (Iec). That is forward bias, it means it is biased, right ?

so what is reverse biasing ?

 

[duffy]

Shuts it off more, increases the depletion region. About the only time anyone intentionally does this on an NPN is with an avalanche transistor, an exotic trigger application used for its fast risetimes.

A more common use for reverse-bias is with the varactor diode. This diode is used for its capacitance, which is controlled by the thickness of the depletion region. You don't want it to conduct, so it is reverse-biased, and the more it is reverse-biased, the lower the capacitance becomes.

 

[joseche]

so having a bigger depletion region to make the current flow from base to collector.

If normally the operation is:
[latex]I_{c}=I_{b}*\beta[/latex]

When reverse biased, what is the [latex]\beta[/latex] ?, intuition tells me it should be from 0..1, is that true ?

 

[audioguru]

When the base-emitter junction is forward-biased then current begins to flow between collector and emitter.
Current flows in the collector-base junction only when the transistor is saturated (turned on hard).

Beta is hFE for DC and is hfe for AC which is the amount of base to collector current gain when the transistor is not saturated. Most little transistors have a beta of about 230 times so a base current of 0.1mA results in a collector to emitter current of 23mA.

 

[duffy]

Having a bigger depletion region shuts it OFF.

 

[joseche]

yes, after reading the chapter of transistor from the art of electronics I got it cleared out. The reverse bias is only called the current that goes from base to collector on an NPN transistor. When current goes from emiter to base it is going from N to P, that is the normal way so it is called forward bias, but when it goes from base to collector it is going from P to N, which is not normal so it's called reverse bias.