How does a person decide if a BJT or a FET is the best for a particular application? Any pro's and cons on either device?

Most of what goes on around here sails right over my head, but I'm slowly absorbing .....

SP

 

Well, BJT's are generally more rugged - but consume more power.

MOSFETs can handle a lot of power, but are generally quite sensitive to overvoltage and static. With proper care and feeding, they can save energy.

 

First off, there are always exceptions to every rule, but FETs are more commonly used for digital applications because they are controlled by base voltage rather than base current.

 

Are SCRs controlled by gate current or voltage?

 

SCRs are directly equivalent to 2 BJTs connected together in a specific way. They are a diode with many characteristics of a BJT.

 

yapjack3,

Are SCRs controlled by gate current or voltage?

Both. From The Controlled Rectifier Volume 1 published by the International Rectifier Corporation, 1962, they mention of page 39 that it can be turned on by a rapid rise of forward voltage across the SCR. On page 40 they have a chapter about "TURN-ON BY GATE SIGNAL", where they describe "...a forward (gate positive) voltage and current to the gate lead." To turn a SCR off the forward voltage has to be reduced below some threshold.

An interesting couple of paragraphs on page 44. "NOTE ON THEORY

It has been customary to describe the behavior of controlled rectifiers by a theory derived from two doubly interconnected (base to collector) n-p-n and p-n-p transistors with the emitters forming the controlled rectifier cathode and anode.

This theory is false because it infers that the gate current must flow over the entire cathode or emitter junction (which it does not). Further, we might have to assume that the carrier migration time from the gate lead across the entire face of the junction determines the turn-on time; fortunately this is also incorrect"
Ratch