Costly mechanical switches can be justified when used in precision test equipment, such as vector network analyzers. For mass-produced consumer products like cable- or satellite-television (CATV/SATV) delivery systems, however, less-expensive electronic switches are a better fit. These switches are based on either transistors or PIN diodes. The semiconductor switches have no moving parts. As a result, they provide faster response times and longer life spans than their mechanical counterparts.

PIN diodes are often employed as the switching elements in single-pole, single-throw (SPST) and single-pole, multiple-throw configurations. The PIN diode behaves like a current-controlled resistor to all signals higher in frequency than 10 times the cutoff frequency (f_c) of the diode, given by:

f_c = 1/(2πτ), where τ s the minority carrier lifetime.

The PIN diode's junction resistance, R_j, can be changed from high to low by the application of a forward bias current. In addition, PIN diodes can be used in either series or shunt switching mode. The series-connected switch has an insertion loss, A, corresponding to:

In the shunt connection, the insertion loss becomes¹:

where Zo is the characteristic impedance (typically 50 or 75 Ω in RF transmission systems).

The selection of a switch topology requires a trade-off between bandwidth and isolation requirements. Although a series switch has the benefit of low-loss transmission over a very wide frequency range, it also has poorer isolation. Shunt switches are usually used in conjunction with quarter-wave transmission lines, which are inherently narrowband. Compared to the series connection, however, these transmission lines provide superior isolation.

Both test instruments and CATV/SATV equipment demand RF switches that are capable of multi-octave operation without significant signal loss. A multicarrier environment like CATV/SATV imposes a stringent linearity demand on the switch. It must not introduce excessive distortion that could lead to interference between channels. Such interference could result in the degradation of signal quality.

To improve the isolation compared to a single PIN diode, two or more PIN diodes can be connected in series. This series connection also allows the sharing of the same bias current in order to conserve power. The beauty of a two-terminal switching element, such as the PIN diode, lies in the ease with which additional diodes can be cascaded in series. In contrast, the three-terminal transistor requires duplication of the control lines for each additional series switch element.

BULK VS. EPITAXIAL PIN DIODES
Circuit designers need to distinguish between the bulk and epitaxial (Epi) types of PIN diodes. These two different methods of constructing PIN diodes result in significant differences in RF behavior. Consequently, they affect the PIN diodes' suitability for differing applications. For their part, bulk diodes have a low doping density in the substrate. To turn on, they therefore require a high bias current. As a result, the bulk PIN diode is generally unsuitable for portable and other battery-operated applications. Its very thick and pure intrinsic (I) layer produces a long carrier lifetime, of 300 ~ 3000 ns. This long carrier lifetime is an essential parameter for low-distortion performance in both switch and attenuator applications.

In contrast, the I-layer of the Epi diode is highly doped. The Epi diode is well suited for low-current RF switching in current-constrained products. The carrier lifetime is much shorter (τ 5 ~ 300 ns). Unfortunately, this difference makes the epitaxial PIN diode much poorer in linearity than the bulk diode. As the linearity of PIN diodes generally deteriorates at low bias currents, this aspect practically rules out the Epi diodes from consideration as attenuators.

As previously mentioned, also determines the PIN switch's lower-frequency limit of usability due to its relation to the cutoff frequency, f_c. Below 10 times the cutoff frequency, the PIN diode no longer behaves like a current-controlled resistor. When

the diode's behavior is unpredictable. It alternates between a current-controlled inductor and capacitor. If the frequency is further lowered to

the PIN junction of the diode acts as a conventional PN junction. In general, the bulk diode's thicker I-layer permits operation at a lower frequency than the Epi diode.