Re: gEDA-user: LED in reverse

[Wojciech Kazubski <wk@xxxxxxxxxxxxx>]

>
> What happens when there are gold atoms? The electrons bouce against these
> atoms and they produce noise in the process? Does these gold increase
> only 1/f noise or also the broadband (white) noise?
Gold reduces lifetime of carriers and thus increase switching speed of the 
device. If the junction is forward biased then it is filled with free 
minority carriers (electrons or holes). If the junction is rebiased to 
reverse bias, carries from deleption layer have to be removed. Without gold 
doping they have to be removed trough external circuit and the diode conducts 
for a moment (up to miliseconds for sislicon - see switching characteristis 
of the diode). Gold makes minority carriers to recombine fast and less charge 
is stored during forward bias. Such diode recovers quickly at turn off.
In saturated transistor B-C get forward biased and charge is stored here.
>
> My application is noise-critical between 1MHz and 10MHz. Would BC547C be
> better than 2N3904?  I assume it's not a switching transistor.
>
Rather use BC549 or BC550, they have tighter spec on noise.
Or use BF240 if gain has to be higher.
PNP transistors can have lower noise due to lower base resistance.

Wojciech Kazubski


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