What is used to control the attenuation of RF signals by a PIN diode?

Explicación

At radio frequencies a forward-biased PIN diode behaves as a resistor whose value is set by the charge stored in its intrinsic layer, and that charge is set by the DC forward current, so varying the bias current varies attenuation smoothly and without distorting the signal. Reverse voltage only switches the diode off, and coupling capacitance is not a control at all.