In a diode valve, the anode would not emit electrons when heated so when the polarity of the voltage across the tube was reversed, no current flowed. Notice how the solid state transistor works in a similar way.
Why does the anode not emit electrons?
Why does no current flow when the polarity of reversed? Why reverse the polarity?
What is a solid state transistor? Just a normal transistor?
What do they mean by solid state?