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Why did narrow transistor base layers allow higher-frequency operation?

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NMOS transistorNegative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors. — transistor NMOS transistor
Negative-channel metal-oxide semiconductors (NMOS) employ a positive secondary voltage to switch a shallow layer of p-type semiconductor material below the gate into n-type. For positive-channel metal-oxide semiconductors (PMOS), all these polarities are reversed. NMOS transistors are more expensive, but faster, than PMOS transistors.

Narrow base layers allow for higher-frequency operation in bipolar transistors primarily because they reduce the distance that charge carriers—electrons or holes—must travel to cross the base region.1

In a junction transistor, carriers injected from the emitter into the base must reach the collector without recombining with the opposite charge carriers present in the base layer. By making the base layer extremely thin (often on the order of microns), the transit time for these carriers is significantly shortened. This reduction in transit time enables the device to respond more rapidly to changes in input signals, thereby facilitating operation at much higher frequencies, such as those exceeding 100 megahertz.21