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HMPP-3892-BLK Datasheet(PDF) 4 Page - Agilent(Hewlett-Packard) |
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HMPP-3892-BLK Datasheet(HTML) 4 Page - Agilent(Hewlett-Packard) |
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4 / 11 page  4 Applications Information PIN Diodes In RF and microwave networks, mechanical switches and attenua- tors are bulky, often unreliable, and difficult to manufacture. Switch ICs, while convenient to use and low in cost in small quantities, suffer from poor distortion performance and are not as cost effective as PIN diode switches and attenuators in very large quantities. For over 30 years, designers have looked to the PIN diode for high performance/low cost solutions to their switching and level control needs. In the RF and microwave ranges, the switch serves the simple purpose which is implied by its name; it operates between one of two modes, ON or OFF. In the ON state, the switch is designed to have the least possible loss. In the OFF state, the switch must exhibit a very high loss (isolation) to the input signal, typically from 20 to 60 dB. The attenuator, however, serves a more complex function. It provides for the “soft” or controlled variation in the power level of a RF or microwave signal. At the same time as it attenuates the input signal to some predeter- mined value, it must also present a matched input impedance (low VSWR) to the source. Every microwave network which uses PIN diodes (phase shifter, modula- tor, etc.) is a variation on one of these two basic circuits. One can see that the switch and the attenuator are quite different in their function, and will there- fore often require different characteristics in their PIN diodes. These properties are easily controlled through the way in which a PIN diode is fabricated. See Figure 9. Bulk Attenuator Diode Epi Switching Diode P+ Diffusion Bulk I-Layer N+ Diffusion Metal Contact ; ; Contact Over P+ Diffusion N+ Substrate Epi I-Layer Figure 9. PIN Diode Construction. RF COMMON RF 2 RF 1 BIAS 2 3 4 1 2 3 4 1 3 4 1 2 Figure 8. Very High Isolation SPDT Switch, Dual Bias. Diode Construction At Agilent Technologies, two basic methods of diode fabrication are used. In the case of bulk diodes, a wafer of very pure (intrinsic) silicon is heavily doped on the top and bottom faces to form P and N regions. The result is a diode with a very thick, very pure I region. The epitaxial layer (or EPI) diode starts as a wafer of heavily doped silicon (the P or N layer), onto which a thin I layer is grown. After the epitaxial growth, diffu- sion is used to add a heavily doped (N or P) layer on the top of the epi, creating a diode with a very thin I layer populated by a relatively large number of imperfections. These two different methods of design result in two classes of diode with distinctly different characteristics, as shown in Table 1. As we shall see in the following paragraphs, the bulk diode is almost always used for attenuator applications and sometimes as a switch, while the epi diode (such as the HMPP-3890) is generally used as a switching element. Diode Lifetime and Its Implications The resistance of a PIN diode is controlled by the conductivity (or resistivity) of the I layer. This conductivity is controlled by the density of the cloud of carriers (charges) in the I layer (which is, in turn, controlled by the DC bias). Minority carrier lifetime, indicated by the Greek symbol τ, is a Table 1. Bulk and EPI Diode Characteristics. Characteristic EPI Diode Bulk Diode Lifetime Short Long Distortion High Low Current Required Low High I Region Thickness Very Thin Thick |
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