When beta gets large, the base current Ib gets small. When the base current is small compared to the other currents in the base circuit (as happens in modern high-speed circuits), it is useful to consider the transistor as a voltage-controlled device: Collector current Ic is exponentially dependent on base-to-emitter voltage Vbe, and transconductance = Ic/Vbe = Gm = Ic/Vt. Vt is the thermal voltage, proportional to absolute temperature. I recall Vt is 26 millivolts at room temperature, but I may be recalling incorrectly.