Zero bias detector diodes for the RF/ID market - HP's HSMS-285x - includes related article on backscatter RF/ID systems - Product Announcement

Hewlett-Packard Journal, Dec, 1995 by Rolando R. Buted

Hewlett-Packard's newest silicon detector diodes were developed to meet the requirements for receiver service in radio frequency identification tags. These requirements include portability, small size, long life, and low cost.

Tracking of products and services is critical in today's highly competitive and rapidly growing world of manufacturing and service industries. To succeed in these industries, accurate and timely information is required.

Two widely used tracking methods are bar code readers and magnetic stripe. Although commonplace, they are both limited in their range and their operating environment. For example, bar codes require a direct line of sight within a few inches and a relatively clean and benign environment to operate reliably.

In contrast, a radio frequency identification (RF/ID) system uses radio signals to communicate. Line of sight is not needed and the system can operate in hostile environments characterized by water, oil, paint, and dirt. It can even be used for communication through cement, glass, wood, or other nonmetallic materials. These wireless systems are being successfully used to identify and track cattle, household pets, cars passing through toll booths, supermarket carts, railroad cars, and personnel entering and leaving secure facilities.

An RF/ID system is composed of two components: a reader (interrogator), which contains both transmitter/receiver and decoder/control modules, and a tag (transponder), which typically contains an antenna and a receiver circuit. Since a system normally has only a few interrogators but many tags, the most severe design constraints are on the tag. These constraints include portability, small size, long life, and low cost. Hewlett-Packard's newest silicon detector diodes (HSMS-285x) were developed to address these constraints.

RF/ID Technology

RF/ID tags can be active or passive. Active tags have an onboard power source (a battery) so that less power is needed from the reader, and usually have a longer read range. However, they have a limited life span and are generally more expensive to manufacture.

Passive tags do not need a separate external power source. They derive their operating power from the energy sent by the interrogator. Passive tags are lighter and cheaper than active tags and have virtually unlimited lifetime. Some passive tags contain a battery to maintain internal memory information in read/write applications. The trade-off is that passive tags have a shorter read range than active tags and require a much higher-powered reader to supply the energy needed to operate them.

RF/ID tags can be read-only or read/write. Read-only tags, as the name implies, can only be read, but can be read millions of times. Read/write tags allow the data stored in them to be altered in addition to being read.

Whether the tag is passive or active, read-only or read/write, it requires a receiver circuit. Receiver circuits can be of two types: superheterodyne or crystal video (Fig. 1). Because the superheterodyne receiver contains RF and low noise amplifiers, its detection sensitivity is typically -150 dBm. The crystal video receiver, on the other hand, is limited to only about -55 dBm. However, it is simpler and much cheaper than the superheterodyne receiver, so the RF/ID industry has adopted it for use in tags. The superheterodyne receiver is used in interrogators.

The crystal video receiver of Fig. 1 can take different forms, depending on the application. Four common configurations are shown in Fig. 2. The single-diode circuits offer simplicity and low cost, whereas the voltage doubler circuits provide a higher output for a given input. Each type can be designed with conventional n-type Schottky diodes or zero biased p-type Schottky diodes. If n-type diodes are used, an external dc bias source is needed for detection operation at low input power levels (<1 mW) because of the low saturation current. The p-type zero bias diode does not need a bias source because it has a relatively high saturation current. In addition, it offers the lowest possible cost, size, and complexity, and usually exhibits the lowest flicker noise. It is therefore the diode of choice for RF tag applications.

The performance of an RF/ID system is directly related to the frequency range in which it is used. The higher the frequency, the faster the data transfer rate and the longer the read/write range. The tag's capture window is more focused at higher frequency. Metals absorb low-frequency signals more than high-frequency signals, whereas obscuring materials such as dirt and grease absorb high-frequency signals more than low frequency signals. Most RF/ID systems operate in three basic frequency ranges. The high-frequency ranges include 850 to 950 MHz and 2.4 to 2.5 GHz. The low frequency range is 100 to 500 kHz, close to the range of AM radio stations. Some applications, such as auto toll collection, also use 5.86 GHz and 10.5 GHz.

Device Theory

A Schottky diode is simply a metal layer deposited on a semiconductor such as silicon. To improve its performance and reliability, it can be passivated with silicon dioxide or silicon nitride or both.