[NLRS] Transistor thermal runaway?

[Scott]

Can't exactly remember from tech school...what does heat in a transistor 
do that causes thermal runaway?  As the PN junction heats, does its 
"resistance" decrease, causing more current to flow in the base circuit 
thus causing more heat, thus causing lower resistance, etc.???  Being 
that transistors are current controlled devices, would using a current 
source such as an LM334 to drive the base, help alleviate thermal 
runaway by supplying a constant current?  I tried the diode (1N4001) 
from base to emitter of the PA transistor (with the two bodies touching 
through heatsink compound), but it isn't working as well as I'd like...

-- 
Scott N0EDV
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)