Philips Semiconductors - Newsroom; Backgrounders;

1998-09-21, BG_ic859,
Technical Backgrounder From Philips Semiconductors

Philips Semiconductors' Silicon RF Transistors and MMICs

The mobile phone market is driven by four factors - size, weight, useability and cost. Pocket sized mobile phones, which ten years ago existed only in the realms of science fiction, are already an affordable reality.

Small is Better
The size and weight of a mobile phone are determined largely by the size of its battery pack. Smaller and lighter battery packs mean smaller and lighter handsets. However, the smaller you make the battery pack the less energy it can store. As a result, the handset's standby and talk times (the times before its battery needs recharging) are reduced. Small battery size and long standby/talk time are conflicting requirements that must be overcome by the design engineer.

One way of extending standby and talk times is to reduce the amount of power that the handset draws from its batteries. However, although the microphone, earpiece, keypad and display functions in a mobile phone can all be implemented with very low power monolithic integrated circuits, the RF sections of the phone (particularly its RF transmitter) remain power hungry.

To communicate reliably with a local basestation the phone must be capable of transmitting at a specified power level. In the case of GSM phones the required antenna power is 2 Watts. While the current drawn from the batteries by the rest of the phone can be as little as a few thousandths of an Amp, the peak current drawn by its RF transmitter may be as high as 1.3 Amps or more. Consequently, one of today's greatest challenges for the design engineer is to increase the efficiency of the RF power amplifier so that as little as possible of the battery power is wasted.

More Users, Less Air Space
The useability of a mobile phone is not only a question of standby and talk times. It is also influenced by the number of users that can be accommodated on the system at any one time. Early 'analog' mobile phones required each call to be allocated to a single frequency channel (typically in the range 900 MHz to 1 GHz) that had to remain open between the handset and a local basestation throughout the duration of the call. As a result, these early systems very soon became 'bandwidth' limited. Once all the channels available to a cellular basestation were being used, no further calls could be made from within that cell. With the rapid increase in subscribers in the late 80's and early 90's analog cell-phone systems frequently became overloaded.

In later systems, such as GSM, the bandwidth problem has largely been overcome by going digital. Digital systems allow a single frequency channel to carry a large number of calls, each one being allocated a short time-slot in the total transmission by a process called time division multiple access (TDMA). The time slots are repeated so fast that the process is undetectable by the user. However, these digital systems have to operate at higher radio frequencies (typically around 2 GHz), mainly because lower frequencies are fully utilised by existing communication systems. Because of the performance limitations of silicon transistors at these higher frequencies, the RF receiver and transmitter in early digital mobile phones had to use relatively expensive Gallium Arsenide (GaAs) transistors.

New Solutions in Silicon
In February 1997, Philips Semiconductors solved the size, weight and frequency band problems by introducing its 5th Generation Silicon RF Wideband Transistors: devices which are much easier to use and lower cost than GaAs solutions. Capable of operating at supply voltages as low as 3 Volts, these high-performance silicon transistors allow handset designers to use battery packs with fewer cells. Their excellent high-frequency performance copes easily with 2 GHz operation and their high operating efficiency results in less battery power being wasted.

Philips is producing both small-signal types, for use in the phone's RF receiver, and medium power types for RF power amplification in its transmitter (see diagram 1).

In addition to cost benefits and ease of use, the new transistors offer several advantages:

high gain so fewer transistors are required to provide the required degree of signal amplification.

fewer peripheral components are required, therefore reducing handset manufacturing costs.

for small signal types: a very low noise figure (typically 1 dB) so the mobile phone can pick up very weak signals from the basestation transmitter.

for medium power types: very high power added efficiency (typically 60%), which minimises the amount of battery power wasted in the handset's RF power amplifier and reduces the amount of heat dissipated. The transistors run cooler so their reliability is enhanced.

Diagram 1

Philips Semiconductors' 5th Generation RF Wideband Transistors are suitable for small signal (receive) and medium power (transmit) amplification in a mobile phone. These high-performance silicon transistors are lower cost and much easier to use than GaAs solutions.

The 5th Generation RF Wideband Transistors are produced in a double-polysilicon diffusion process that has been specifically adapted by Philips Semiconductors for the manufacture of high-frequency transistors. To meet the high-volume production demands of the mobile phone industry, Philips Semiconductors confirmed an additional investment of US$ 15 million in the discretes wafer fab at Nijmegen, The Netherlands, at the same time as announcing the double-polysilicon technology and products.

On receiving news of this further investment David Moorhouse, Associate Director at DataQuest Europe said, "The current level of investment that Philips is making in its Nijmegen facility puts the company in a strong position to meet the rapidly growing market demand for high-performance low-voltage RF transistors."

Philips Semiconductors has now further developed its double-polysilicon transistor technology so that many of the discrete components that normally surround the transistors are included on-chip, creating small-scale integration MMIC (Monolithic Microwave Integrated Circuits) solutions. By integrating inductance loops, resistors and capacitors onto the chip, the company has added active bias circuitry to create the first 'smart' transistors that automatically compensate for temperature and process variations.

The company has taken the small-scale RF integration route to create several small building blocks rather than integrating several chips into one. Effectively, designers can now access the advantages of discretes - flexibility, ease of circuit customisation, very small SMD packaging and good price/performance ratios - while saving on development resources because several difficult to design functions are now provided on-chip (see accompanying press release, 'Double Poly advance simplifies mobile communications design').

These silicon MMICs bring several advantages to designers and manufacturers of mobile phones:

reduction in external component count

easier and faster circuit design because biaising and temperature compensation circuitry is integrated on-chip. In addition, the designer doesn't have to perform detailed design of the RF function or compensate for process spread effects.

the MMIC is about the same size as the discrete it replaces, so the board size can now be reduced by about 75%

reduced bills of materials, board assembly cost and logistics

Philips Semiconductors can now offer mobile phone designers a portfolio of high performance silicon RF devices including discrete transistors and MMICs.

The Discrete Semiconductors Business Group of Philips Semiconductors is the fourth largest supplier of discrete semiconductors in the world, with an annual turnover in excess of US$ 1 billion.

The group's three main areas of expertise are: RF products for mobile communications, video amplifiers for monitors, and RF products for use in infrastructure such as basestations and CATV; power semiconductors for automotive, industrial, lighting and EDP applications; and commodity semiconductors with a broad range of diodes and transistors. This broad product range makes the Discrete Semiconductors Business Group a global supplier to all major segments of the electronics industry. Sales offices are located in all major markets around the world and are supported by systems labs.