TABLE OF CONTENTS Most modern radio transceivers (i.e., transmitter/re ceiver) use relayless switching to go back and forth between the receive and transmit states. In many cases, this switching is done with PIN diodes. Similarly, IF filters or front-end bandpass filters are selected with a front-panel switch that handles only direct current. How? Again, PIN diodes. These interesting little components allow us to do switching at RF, IF, and audio frequencies without routing the signals themselves all over the cabinet. This chapter shows how these circuits work.
The P-I-N or PIN diode is different from the PN junction diode (see Fig. 21-1A): it has an insulating region between the P- and N-type material (Fig. 21-1B). It is therefore a multiregion semiconductor device, despite having only two electrodes. The I-region is not really a true semiconductor insulator but rather is a very lightly doped N-type region. It is called an intrinsic region because it has very few charge carriers to support the flow of an electrical current.
When a forward-bias potential is applied to the PIN diode, charge carriers are injected into the I-region from both N and P regions. But the lightly doped design of the intrinsic region is such that the N- and P-type charge carriers don t immediately recombine (as in PN junction diodes). There is always a delay period for recombination. Because of this delay phenomena, a small, but finite number of carriers...
