WO2017069663A1

WO2017069663A1 - Power transistor arrangement and method for operation a power transistor arrangement

Info

Publication number: WO2017069663A1
Application number: PCT/SE2015/051110
Authority: WO
Inventors: Rui Hou; Mats HELGÖSTAM
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2015-10-20
Filing date: 2015-10-20
Publication date: 2017-04-27
Anticipated expiration: 2018-04-20

Abstract

A power amplifier arrangement (10) for a radio transmitter comprises a transistor (20) and a light source (30). The light source (30) is arranged for, continuously or intermittently, illuminating the transistor (20) by radiation (32) under operation conditions of the transistor. The radiation (32) comprises radiation with a wavelength shorter than 500 nm. There is also presented a radio transmitter comprising such a power amplifier arrangement (10) and a radio base station comprising such a radio transmitter. A method for operating a power amplifier arrangement in a radio transmitter comprises operating a transistor to provide currents used for transmitting of radio signals. The transistor is illuminated continuously or intermittently by radiation. The radiation comprises radiation with a wavelength shorter than 500 nm.

Description

POWER TRANSISTOR ARRANGEMENT AND METHOD FOR OPERATION A POWER TRANSISTOR ARRANGEMENT

TECHNICAL FIELD

The proposed technology generally relates to power amplifiers for a radio transmitter and methods for operating power amplifiers in a radio transmitter and in particular to power amplifier arrangements comprising field-effect transistors and methods for operating power amplifier arrangements comprising field-effect transistors in a radio transmitter.

BACKGROUND

Radio base-stations (RBS) employ power amplifiers (PA) to boost signal power for downlink transmission. Signals with high data- rate demand RBS PAs to have large bandwidth and deliver high power. These demanding applications are enabled by gallium nitride (GaN) high-electron-mobility transistors (HEMT) because of their high power-density and good thermal performance.

However, GaN HEMTs have significant trapping effects, see e.g. "Trapping Effects in GaN and SiC Microwave FETs", S. C. Binari, P.B. Klein, T. E. Kazior, Proceedings of the IEEE, Vol. 90, No. 6, June 2002 [ 1]. In an operating transistor, some high energy electrons tunnel through its barrier or buck layer and are trapped by impurities and defects in the semiconductor crystal and surface. The trapped electrons reduce the effective gate voltage and consequently the quiescent bias current. This is commonly denoted a current collapse. In extreme cases, collapsed current shuts a PA down completely and interrupt an RBS transmission.

Existing RBS PAs with GaN HEMT recover from current collapses by thermal relaxation. In this process, trapped electrons gain enough thermal energy, by interaction with thermally induced phonons in the material, to be emitted from the filled traps. This process is facilitated by high device temperature. The trapped electrons negative influence to gate voltage is removed and nominal quiescent bias current is restored. Since thermal relaxation depends on crystal vibrations in the material, thermal relaxation of trapping states is less effective at lower device temperature. The mean time for a trapped electron to escape is longer and the total trapped charges aggregate to a larger number at lower temperatures. In consequence, current collapse is more severe and last longer.

To make the point quantitative, a 10 kHz 10% duty-cycle pulse-modulated 2140 MHz carrier with 46 dBm average-power activated for 10 seconds on a RBS Band 1 GaN HEMT PA can shut the PA down completely at -40°C heat sink temperature. Any low-power RBS downlink transmission is not possible in this state. Almost full recovery of biasing current by thermal relaxation takes about 60 seconds. Current RBS products powered by GaN HEMTs are functional in low temperature because test signals in current wireless standards are forgiving enough not to shut PA down entirely. These low- temperature-GaN-friendly signals are not guaranteed in future high-data- rate communication standards.

Trapped electrons are captured by traps at different energy states within the forbidden bandgap, i.e. they require different amounts of energy to be released. Shallowly trapped electrons may be bound by an energy of 1-2 eV, while deeply trapped electrons may require as much 5 eV to be released. The shallowly trapped electrons may relatively easy be recovered by thermal relaxation, whereas deeply trapped electrons may require very high temperatures to be released. Such high temperature treatments are in general impossible to perform on amplifiers in site, i.e. incorporated in a radio transmitter, without exposing other surrounding components for risks to be damaged. Therefore, radio transmitters in the field may suffer from long-term gradual performance degradation. Parts of the problems with trapped electrons can be solved by heating up the transistors, at least moderately. Heating up transistors has, however, a drawback that it is power inefficient in cold temperature. Increasing temperature difference between transistor core and heat sink demands a proportional increase of power dissipation. Furthermore, sound PA building practice minimizes the thermal conductance from transistor core to heat- sink. Halving the thermal conductance doubles the required heat power consumption. Last but not least, trapping effects in GaN causes long-term memory effects for RBS PAs. PAs with long-term memories are difficult to linearize and require complicated, expensive and power consuming digital pre-distortion (DPD). SUMMARY

It is thus an object to provide power amplifiers suitable for being operated in a radio transmitter having less degradation, being more energy-efficient and having shorter memory effects than prior art power amplifiers.

This and other objects are met by embodiments of the proposed technology. According to a first aspect, there is provided a power amplifier arrangement for a radio transmitter. The power amplifier arrangement comprises a transistor; and a light source. The light source is arranged for, continuously or intermittently, illuminating a transistor die of the transistor by radiation under operation conditions of the transistor. The radiation comprises radiation with a wavelength shorter than 500 nm.

According to a second aspect, a radio transmitter comprises a power amplifier arrangement according to the first aspect.

According to a third aspect, a radio base station comprises a radio transmitter according to the second aspect. According to a fourth aspect, a method for operating a power amplifier arrangement in a radio transmitter comprises operating a transistor to provide currents used for transmitting of radio signals. A transistor die (21) of the transistor is illuminated continuously or intermittently by radiation. The radiation comprises radiation with a wavelength shorter than 500 nm.

An advantage of the proposed technology is that it is more effective and energy efficient to de-trap than thermal relaxation, especially in cold temperature. Another advantage of the proposed technology is that it enables the recovery from long-term degradation of field -deployed radio transmitters, avoiding impractical yet risky extreme high-temperature heat treatment. The proposed technology also reduces associated memory effects in power amplifiers in radio base stations, thereby reducing the complexity, cost and power consumption of required digital pre-distortion arrangements.

Other advantages will be appreciated when reading the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a wireless communication system;

FIG. 2 is a schematic illustration of an embodiment of a power amplifier arrangement for a radio transmitter;

FIG. 3 is a schematic illustration of another embodiment of a power amplifier arrangement for a radio transmitter;

FIG. 4 is a schematic illustration of yet another embodiment of a power amplifier arrangement for a radio transmitter;

FIG. 5 is a schematic illustration of an embodiment of a power amplifier arrangement for a radio transmitter with reflected light;

FIG. 6 is a schematic illustration of an embodiment of a power amplifier arrangement for a radio transmitter with an optical conductor; FIG. 7 is a schematic illustration of a digital pre-distortion arrangement; FIG. 8 is a schematic illustration of an embodiment of a power amplifier arrangement for a radio transmitter with a radiation control unit;

FIG. 9 is a flow diagram of steps of an embodiment of a method for operating a power amplifier arrangement in a radio transmitter;

FIG. 10 is a flow diagram of steps of another embodiment of a method for operating a power amplifier arrangement in a radio transmitter;

FIG. 1 1 is a schematic illustration of an embodiment of a power amplifier arrangement for a radio transmitter with a radiation control unit and transistor performance detector;

FIG. 12 is a flow diagram of steps of yet another embodiment of a method for operating a power amplifier arrangement in a radio transmitter;

FIG. 13 is a diagram illustrating measured drain current recovery without light illumination in a RBS Band 1 PA powered by a GaN HEMT at -30°C heat-sink temperature; and

FIG. 14 is a diagram illustrating measured drain current recovery under 10 W 400 nm UV illumination in a RBS Band 1 PA powered by a GaN HEMT at -30°C heat-sink temperature. DETAILED DESCRIPTION

Throughout the drawings, the same reference designations are used for similar or corresponding elements. The proposed solution introduces a light source of short wavelength visible light or ultraviolet (UV) light into an operating RBS PA module. When the PA is disabled by trapped electrons in cold temperature, the light source briefly illuminates the GaN HEMTs. The photons from the light source transfer energy to the trapped electrons, which results in that the trapped electrons gain enough energy to be emitted from filled traps to the conduction band instantly. In consequence the GaN PA can recover from a total current collapse swiftly. RBS transmission can be restored orders of magnitude faster in cold temperature, in comparison with thermal relaxation. The optical generation of excess electron-hole pairs in semiconductor crystals is, as such, a well-known physical process. UV light has furthermore been used during analysis procedures, investigating the nature of electron trapping, see e.g. [ 1], [2]. However, so far, light illumination has not been utilized for operating units in operating environments, i.e. as a means for continuously improving the actual operation of a power amplifier in a radio transmitter.

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of the technical context in which it is intended to be applied. Fig. 1 illustrates schematically a wireless communication system 1. A radio base station 2 is in communicational contact with a wireless communication device 3. The radio base station 2 comprises a radio transmitter 4 connected to an antenna 5. A power amplifier arrangement 10 in the radio transmitter 4 provides currents used for transmitting radio signals 6 to the wireless communication device 3.

As used herein, the non-limiting terms "User Equipment" (UE) and "wireless communication device" may refer to a mobile phone, a cellular phone, a Personal Digital Assistant (PDA) equipped with radio communication capabilities, a smart phone, a laptop or Personal Computer (PC) equipped with an internal or external mobile broadband modem, a tablet PC with radio communication capabilities, a target device, a device to device UE, a machine type UE or UE capable of machine to machine communication, iPAD, Customer Premises Equipment (CPE), Laptop Embedded Equipment (LEE), laptop mounted equipment (LME), Universal Serial Bus (USB) dongle, a portable electronic radio communication device, a sensor device equipped with radio communication capabilities or the like. In particular, the term "UE" and the term "wireless device" should be interpreted as non-limiting terms comprising any type of wireless device communicating with a radio network node in a cellular or mobile communication system or any device equipped with radio circuitry for wireless communication according to any relevant standard for communication within a cellular or mobile communication system. As used herein, the non-limiting term "radio base station" may refer to base stations, network control nodes such as network controllers, radio network controllers, base station controllers, and the like. In particular, the term "base station" may encompass different types of radio base stations including standardized base stations such as Node Bs, or evolved Node Bs, eNBs, and also macro/ micro/ pico radio base stations, home base stations, also known as femto base stations, relay nodes, repeaters, radio access points, base transceiver stations (BTS), and even radio control nodes controlling one or more Remote Radio Units (RRU), or the like.

Fig. 2 illustrates schematically a power amplifier arrangement 10. The power amplifier arrangement 10 is intended to be used for a radio transmitter, see e.g. Fig. 1. The power amplifier arrangement 10 comprises a transistor 20 and a light source 30. In this embodiment, the transistor 20, having a transistor die 21, is mounted on a substrate 12, in this embodiment a printed circuit board 14. In this particular embodiment, the light source 30 is a light generating device 34. The light source 30 is arranged for illuminating the transistor 20 by radiation 32 under operation conditions of the transistor 20. The illumination is in different embodiments continuous or intermittent. The radiation 32 comprises radiation with a wavelength shorter than 500 nm.

The transistor 20 may in different embodiments be of different kinds. Non- limiting examples are a high-electron-mobility transistor, a heterojunction bipolar transistor, a metal semiconductor field-effect transistor, a metal- oxide-semiconductor field-effect transistor, and a bipolar junction transistor. Many of these example transistors may suffer from deterioration caused by trapped electrons, at least during certain operation conditions. The transistors are manufactured starting from a basic semiconductor material. Non-limiting examples of materials used for transistors applicable to the currently presented technology is gallium nitride, GaN, gallium arsenide, GaAs, indium phosphide, InP, silicon carbide, SiC, silicon germanium, SiGe, and silicon, Si. In a particular embodiment, the transistor is a GaN high-electron-mobility transistor. Fig. 3 illustrates schematically another embodiment of a power amplifier arrangement 10. A transistor 20, in this particular embodiment a ceramic- air packaged GaN transistor, is mounted on a substrate 12, in this embodiment a printed circuit board (PCB). The active portion of the transistor 20, i.e. the transistor die 21 is packaged by a package 22 enclosing the transistor 20. The package 22 of the present embodiment is a transparent package 24, at least partially transparent to UV light or short wavelength visible light. A light source 30, in this embodiment a UV lamp, is mounted above the transistor 20. The light source 30 is thus provided outside the package 22. In a de-trapping process, the UV light emitted from the light source 30 goes through the partially transparent package 24 to illuminate the transistor 20, i.e. the GaN HEMT die. UV photons transfer energy to trapped electrons in the GaN HEMT die, so that the trapped electrons gain enough energy to be emitted from filled traps to the conduction band. The process finishes when most trapped electrons are released.

Fig. 4 illustrates an alternative solution, where the light source 30, e.g. a light-emitting-diode (LED), is integrated into the transistor package 22. In other words, the light source 30 is provided inside the package 22. The package 22 can in this embodiment be a non-transparent package 26. The de-trapping process is identical as described before.

Fig. 5 illustrates yet another embodiment of a power amplifier arrangement 10. Here, the light source 30 is mounted on the same substrate 12 as the transistor 20 inside the package 22. The package 22 is in this embodiment a non-transparent package 26. The inside surface 28 of the non-transparent package 26 has furthermore the property of reflecting at least a part of the radiation emitted from the light source 30. The light source 30 is thus arranged for illuminating the transistor die 21 by radiation that is reflected at the inside surface 28 of the non-transparent package 26, under operation conditions of the transistor 20.

As mentioned above, the light source 30 can be a light generating device, in which the light is actually generated. Non-limiting examples of such light sources are a Light Emitting Diode (LED), a gas discharge lamp, and an incandescent lamp. Presently, it is believed that a LED is the most practical and cost efficient choice. However, for particular applications, also other light generating device could advantageously be used.

Furthermore, the radiation to be used for illuminating the transistor might also be generated elsewhere and transported to the site of the transistor. In Fig. 6, such an embodiment of a power amplifier arrangement 10 is scheduled. The light source here comprises a sunlight collection arrangement. The light source 30 here comprises an optical conductor 36. The (not shown) upper end of the optical conductor 36 is arranged to collect radiation from the sun, and the so collected sun light, which comprises radiation of wavelengths below 500 nm, is conducted by the optical conductor 36 into the interior of the package 22, and there illuminating the transistor die 21.

The required wavelength of the radiation that is used for illumination depends on the semiconductor material and device structure. A larger bandgap in the semiconductor material typically calls for shorter wavelengths. E.g., for GaN HEMT, the radiation preferably comprises radiation with a wavelength shorter than 500 nm. A wavelength of the radiation that illuminates the transistor die of 500 nm, will be able to de- trap electrons trapped with an energy of about 2.5 eV. This will be enough for de-trapping some of the electrons influencing the operation of the transistor. However, deeper trapping states cannot be neutralized by such wavelengths. By using radiation that comprises ultra-violet radiation with a wavelength of 365 nm, electrons trapped anywhere within the 3.4 eV forbidden bandgap in a GaN crystal can be reached. If the radiation has a distribution of different wavelengths, low energy photons may de-trap shallowly trapped electrons, while photons having higher energies may de-trap electrons that are more strongly bound. By having at least a part of the radiation presenting a wavelength of less than 5 250 nm, electrons trapped at an energy of 5 eV may be released.

UV radiation may also be hazardous for living organisms. UV light with wavelengths below 120 nm are considered as relatively dangerous, which is why radiation with shorter wavelengths than 120 nm preferably should be0 avoided.

In one embodiment, the light source could continuously illuminate the transistor. This is advantageous, concerning the associated memory effects in power amplifiers in radio base stations. A continuous illumination of the5 transistor maintains the transistor in a relatively constant state concerning trapped electrons. The gain behavior of the transistor then becomes relatively constant over time, which means that arrangements for handling memory effects may be designed simpler and for a lower cost. This is the case, e.g. when applying digital pre-distortion. Fig. 7 schematically0 illustrates such a digital pre-distortion arrangement 70. An RF signal for transmission 52 is input to a digital pre-distortion unit 50. The digital pre- distortion unit 50 applies a distortion, illustrated by the diagram 60, on the input signal and outputs a pre-distorted RF signal 54 to the power amplifier arrangement 10. The power amplifier arrangement 10 amplifies the RF5 signal, but at the cost of introducing a distortion, illustrated by the diagram 62. An output RF signal 56 is provided to the antenna. The goal for the pre- distortion unit 50 is to provide a pre-distortion that corresponds to the invers of the distortion caused by the power amplifier arrangement. To this end, a feed-back 58 of the output RF signal is brought back to the pre-0 distortion unit 50 to enable such adaptive pre-distortion. The ideal result is a linear dependence between the input RF signal 52 and the output RF signal 56, as illustrated by the diagram 64. If the behavior of the power amplifier arrangement 10 changes with time, the pre-distortion unit 50 has to compensate for such memory effects. The lowering of the memory effects thereby reduces the required complexity, cost and power consumption of digital pre-distortion arrangements.

A drawback by a continuous illumination is that the light source requires a constant provision of energy. In alternative embodiments, different kinds of intermittently used illumination are presented.

Fig. 8 illustrates schematically a power amplifier arrangement 10 having a radiation control unit 38. The radiation control unit 38 may be provided integrated into the light source 30 or as a separate unit at least communicationally connected to the light source 30. The radiation control unit 38 is arranged for turning on and turning off the radiation from the light source. These principles can be applied to all previously presented embodiments.

In one embodiment, the radiation control unit is arranged for turning on and turning off the radiation from the light source according to a predetermined schedule. Such an arrangement may keep the operational conditions of the transistor on a relatively constant level. This opens up for a certain reduction of the required complexity of digital pre-distortion arrangements, even if they are not as significant as for a constant illumination. The predetermined schedule, e.g. a regular illumination, also reduces the risks for encountering a current collapse. At the same time, the non-continuous illumination reduces the power need for the light source. However, in order to minimize the risks for a current collapse, the illumination is typically allowed to be on during longer periods than would be absolutely necessary. In experiments, of which a few are discussed below, one has found that an illumination of 5 s often is more than necessary for regaining optimal operation conditions even in the most hostile operating environment (-40°C). Thus, in a particular embodiment, the radiation control unit is arranged for limiting radiation emitting periods of the light source to be less than 5 s. Fig. 9 illustrates a flow diagram of steps of a method for operating a power amplifier arrangement in a radio transmitter. The method starts in step 200. In step 210, a transistor is operated to provide currents used for transmitting of radio signals. In step 220, a transistor die of the transistor is illuminated by radiation. The radiation comprises radiation with a wavelength shorter than 500 nm. The illumination takes place in between operation periods of the power amplifier arrangement. However, the illumination takes place at the operation site of the transistor, i.e. under operation conditions of the transistor. In other words, the illumination takes place when the transistor is biased, i.e. turned on, but not transmitting, i.e. no RF input. The illumination is intermittent. The procedure is repeated as illustrated by the arrow 230. When operation is to be ended, the procedure follows the broken arrows 240 to step 299, where the procedure is ended. Fig. 10 illustrates a flow diagram of steps of another method for operating a power amplifier arrangement in a radio transmitter. The method starts in step 200. In step 210, a transistor is operated to provide currents used for transmitting of radio signals. In step 220, a transistor die of the transistor is illuminated by radiation. The radiation comprises radiation with a wavelength shorter than 500 nm. In this embodiment, the steps 210 and 220 take place at least partially simultaneous. The illumination may in a particular embodiment be continuous. In other words, the step of illuminating comprises continuous illuminating of the transistor die of the transistor by the radiation at least partially simultaneously to the step of operating the transistor. When the operation of the transistor is ended, also the illumination ends, as illustrated by the broken arrow 241.

In another particular embodiment, the illumination may be intermittent. In other words, the step of illuminating comprises intermittent illuminating of the transistor die of the transistor by the radiation. When the step 220 is finished, the flow returns, as illustrated by the arrow 231 to start the step 220 again when appropriate. This step of illuminating is performed at least partially simultaneously to the step of operating the transistor. In a particular embodiment, the step of illuminating comprises illuminating according to a predetermined schedule.

In order to further improve the power efficiency and life of a light source, the periods in which the illumination is performed is preferably dependent on the actual operation characteristics of the power amplifier. Fig. 1 1 illustrates schematically a power amplifier arrangement 10 having both a radiation control unit 38 and a transistor performance detector 40. The transistor performance detector 40 may be provided integrated into the transistor 20 or as a separate unit at least communicationally connected to the transistor 20. The radiation control unit 38 is arranged for detecting non-normal operation conditions of the transistor 20. The transistor performance detector 40 is connected to provide control signals to the radiation control unit 38 when such non-normal operation conditions are detected. The radiation control unit 38 is then arranged to turn on the light source 30 as a response to the control signal. These principles can be applied to all previously presented embodiments.

By monitoring the transistor performance, it is possible to gain knowledge about the status of e.g. the amount of trapped electrons. A decrease of e.g. quiescent drain current is an indication that electron trapping has occurred, and that a current collapse may be approaching. Depending on the time constants, also short and long term effects may be distinguished. Based on such information, decisions about appropriate counteractions may be taken. In particular, time instant and duration of light illumination of the transistor may be decided.

The transistor performance detector 40 may monitor different quantities associated with the operation of the transistor. Non-limiting examples are linearity, bias, temperature, memory effects, output power, and gain of the transistor. The transistor performance detector 40 may in different embodiments also or alternatively monitor a linearity quantity of the transistor. Non-limiting examples of such linearity quantities are intermodulation distortion, adjacent channel leakage ratio, and error-vector modulus. The transistor performance detector 40 may in different embodiments also or alternatively monitor a bias quantity of the transistor. Non-limiting examples of such bias quantity are operating drain current, and quiescent drain current. The transistor performance detector 40 may in 5 different embodiments also or alternatively monitor a memory effect quantity of the transistor. Non-limiting examples of such memory effect quantities are magnitude and phase scattering of amplifier gain.

In a particular embodiment, the transistor performance detector monitors at 10 least a percentage quiescent drain current drop from a nominal value of the transistor.

Fig. 12 illustrates a flow diagram of steps of a method for operating a power amplifier arrangement in a radio transmitter. The step 210 presents as

15 discussed before operation of the transistor in order to produce radio signals. A step 215, comprises detecting of non-normal operation conditions of the transistor. In this particular embodiment, this detecting step 215 comprises the step 214 of monitoring the transistor. In different embodiments, at least one of the quantities linearity, bias, temperature, memory effects, output

20 power, and gain of the transistor is monitored. In step 216, it is decided whether or not the monitored quantities are outside normal operation conditions. If the monitored quantities are within normal conditions, the flow returns, as illustrated by the arrow 233, to the monitoring step 214. If it is considered that non-normal operation conditions are detected in step

25 216, the flow continues to the step 220. In other words, the step of illuminating 220 is initiated as a response to the detection of a detected non-normal operation condition. In a particular embodiment, the step 215 of detecting non-normal operation conditions of the transistor may comprises detecting of a current collapse.

30

After illumination, the flow returns back to the monitoring step 214, as illustrated by the arrow 232. When the operation of the transistor is ended, also the illumination ends, as illustrated by the broken arrow 242.

In particular embodiments, the monitoring of a linearity quantity of the transistor comprises monitoring of at least one of intermodulation distortion, adjacent channel leakage ratio, and error-vector modulus.

In particular embodiments, the monitoring of a bias quantity of the transistor comprises monitoring of at least one of operating drain current, and quiescent drain current.

In particular embodiments, the monitoring of a memory effect quantity of the transistor comprises monitoring of at least one of: gain magnitude scattering, and gain phase scattering.

In a particular embodiment, the step of detecting non-normal operation conditions of the transistor comprises monitoring of at least a percentage quiescent drain current drop from a nominal value of the transistor. The de-trapping process can by advantage be used for low-temperature trapping recovery or memory-effect mitigation. In different embodiments the de-trapping process is activated on demand, and light illumination is intermittent. In other embodiments the light illumination can be continuous.

The effectiveness of short wavelength light, and in particular UV light, to recovery of current collapse in RBS PAs is demonstrated by measurement results shown in Figs. 13 and 14. A RBS Band l GaN HEMT PA operating at -30 Celsius heat-sink temperature outputs a 10 kHz 10% duty-cycle pulse modulated 2140 MHz carrier with 46 dBm average-power for 3 seconds. The current collapse and recovery for three repetitions of the procedure without any illumination is plotted in Fig. 13. The recovery time, determined as the 90% settling, is 20 seconds without any illumination. The current collapse and recovery for three repetitions of the procedure with 10 W 400 nm UV illumination is plotted in Fig. 14. The recovery time, determined as the 90% settling, is 2 seconds with UV illumination. The measurements thus show that the collapsed current recovers an order of magnitude faster in cold temperature with UV illumination than without.

Compared with thermal energy that has to be spread and dissipated by heat sink, photon energy can be concentrated on a GaN transistor and efficiently absorbed by its semiconductor material. Significantly less power is thus needed to stimulate trapped electrons for an order of magnitude shorter time.

Since short wavelength light, in particular UV light, illumination mitigates trapping effects in GaN transistors, it also reduces associated memory effects in RBS PAs. PAs with shorter and less intensive memories may be linearized by simpler, less expensive DPD with lower power consumption.

The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the present scope as defined by the appended claims. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

ABBREVIATIONS

Abbreviation Explanation

BTS Base Transceiver Stations

CPE Customer Premises Equipment

DPD Digital Pre-distortion

GaN Gallium Nitride

HEMT High Electron Mobility Transistor LED Light Emitting Diode

LEE Laptop Embedded Equipment

LME Laptop Mounted Equipment

PA Power Amplifier

5 PC Personal Computer

PCB Printed Circuit Board

PDA Personal Digital Assistant

RBS Radio Base-station

RRU Remote Radio Units

10 UE User Equipment

USB Universal Serial Bus

UV Ultraviolet

REFERENCES

15

[ 1] "Trapping Effects in GaN and SiC Microwave FETs", S. C. Binari, P.B. Klein, T. E. Kazior, Proceedings of the IEEE, Vol. 90, No. 6, June 2002.

Claims

1. A power amplifier arrangement ( 10) for a radio transmitter (4), comprising:

a transistor (20); and

a light source (30), arranged for, continuously or intermittently, illuminating a transistor die (21) of said transistor (20) by radiation (32) under operation conditions of said transistor (20);

said radiation (32) comprising radiation with a wavelength shorter than 500 nm.

2. The power transistor arrangement according to claim 1, characterized by a package (22) enclosing said transistor (20).

3. The power amplifier arrangement according to claim 2, characterized in that said light source (30) is provided inside said package (22).

4. The power amplifier arrangement according to claim 2, characterized in that said light source (30) is provided outside said package (22), wherein said package (22) being at least partially transparent for said radiation (32).

5. The power amplifier arrangement according to any of the claims 1 to

4, characterized in that said transistor (20) being one of:

a high-electron-mobility transistor,

a heterojunction bipolar transistor,

a metal semiconductor field-effect transistor,

a metal-oxide-semiconductor field-effect transistor, and

a bipolar junction transistor.

6. The power amplifier arrangement according to any of the claims 1 to

5, characterized in that a semiconductor material in said transistor (20) is selected from:

gallium nitride, GaN,

gallium arsenide, GaAs, indium phosphide, InP,

silicon carbide, SiC,

silicon germanium, SiGe, and

silicon, Si.

7. The power amplifier arrangement according to any of the claims 1 to 4, characterized in that said transistor (20) being a GaN high-electron- mobility transistor.

8. The power amplifier arrangement according to any of the claims 1 to 7, characterized in that said radiation (32) comprises ultra-violet radiation.

9. The power amplifier arrangement according to claim 8, characterized in that said ultra-violet radiation comprises radiation with a wavelength between 120 nm and 375 nm.

10. The power amplifier arrangement according to any of the claims 1 to 9, characterized in that said light source (30) being one of:

a light-emitting diode,

a gas discharge lamp,

an incandescent lamp, and

a sunlight collection arrangement.

1 1. The power amplifier arrangement according to any of the claims 1 to 9, characterized in that said light source (30) being a light-emitting diode.

12. The power amplifier arrangement according to any of the claims 1 to 9, characterized by further comprising a radiation control unit (38), arranged for turning on and turning off said radiation (32) from said light source (30).

13. The power amplifier arrangement according to claim 12, characterized in that said radiation control unit (38) is arranged for turning on and turning off said radiation (32) from said light source (30) according to a predetermined schedule.

14. The power amplifier arrangement according to claim 12, characterized by further comprising a transistor performance detector (40) arranged for detecting non-normal operation conditions of said transistor (20), said transistor performance detector (40) being connected to provide control signals to said radiation control unit (38) when such non-normal operation conditions are detected, wherein said radiation control unit (38) being arranged to turn on said light source (30) as a response to said control signal.

15. The power amplifier arrangement according to claim 14, characterized in that said transistor performance detector (40) monitors at least one of:

linearity,

bias,

temperature,

memory effects,

output power, and

gain

of said transistor.

16. The power amplifier arrangement according to claim 15, characterized in that said transistor performance detector (40) monitors a linearity quantity of said transistor (20), said linearity quantity being at least one of:

intermodulation distortion,

adjacent channel leakage ratio, and

error-vector modulus.

17. The power amplifier arrangement according to claim 15 or 16, characterized in that said transistor performance detector (40) monitors a bias quantity of said transistor (20), said bias quantity being at least one of: operating drain current, and

quiescent drain current.

18. The power amplifier arrangement according to any of the claims 15 to 17, characterized in that said transistor performance detector (40) monitors a memory effect quantity of said transistor (20), said memory effect quantity being at least one of:

gain magnitude scattering, and

gain phase scattering.

19. The power amplifier arrangement according to claim 17, characterized in that said transistor performance detector (40) monitors at least a percentage quiescent drain current drop from a nominal value of said transistor (20).

20. The power amplifier arrangement according to any of the claims 12 to 19, characterized in that said radiation control unit (38) is arranged for limiting radiation emitting periods of said light source (30) to be less than 5 s.

21. A radio transmitter (4) comprising a power amplifier arrangement ( 10) according to any of the claims 1 to 20.

22. A radio transmitter according to claim 21, characterized by further comprising a digital pre-distortion arrangement (70) connected to said power amplifier arrangement (10).

23. A radio base station (2) comprising a radio transmitter (4) according to claim 21 or 22.

24. A method for operating a power amplifier arrangement in a radio transmitter, wherein said method comprises the steps of:

- operating (210) a transistor to provide currents used for transmitting of radio signals; and - illuminating (220) a transistor die of said transistor continuously or intermittently by radiation;

said radiation comprising radiation with a wavelength shorter than 500 nm.

25. The method for operating a power amplifier arrangement in a radio transmitter according to claim 24, characterized in that said step of illuminating (220) comprises continuous illuminating of said transistor by said radiation at least partially simultaneously to said step of operating said transistor.

26. The method for operating a power amplifier arrangement in a radio transmitter according to claim 24, characterized in that said step of illuminating (220) comprises intermittent illuminating of said transistor by said radiation.

27. The method for operating a power amplifier arrangement in a radio transmitter according to claim 26, characterized in that said step of illuminating (220) is performed at least partially simultaneously to said step of operating (210) said transistor.

28. The method for operating a power amplifier arrangement in a radio transmitter according to claim 26 or 27, characterized in that said step of illuminating (220) comprises illuminating according to a predetermined schedule.

29. The method for operating a power amplifier arrangement in a radio transmitter according to claim 26 or 27, characterized by the further step of detecting (215) non-normal operation conditions of said transistor, wherein said step of illuminating (220) being initiated as a response to detection of a detected said non-normal operation condition.

30. The method for operating a power amplifier arrangement in a radio transmitter according to claim 29, characterized in that said step of detecting (215) non-normal operation conditions of said transistor comprises detecting of a current collapse.

31. The method for operating a power amplifier arrangement in a radio transmitter according to claim 29 or 30, characterized in that said step of detecting (215) non-normal operation conditions of said transistor comprises monitoring (214) of at least one of the quantities:

linearity,

bias,

temperature,

memory effects,

output power, and

gain

of said transistor.

32. The method for operating a power amplifier arrangement in a radio transmitter according to claim 31, characterized in that said monitoring (214) of a linearity quantity of said transistor being monitoring of at least one of:

intermodulation distortion,

adjacent channel leakage ratio, and

error-vector modulus.

33. The method for operating a power amplifier arrangement in a radio transmitter according to claim 31 or 32, characterized in that said monitoring (214) of a bias quantity of said transistor being monitoring of at least one of:

operating drain current, and

quiescent drain current.

34. The method for operating a power amplifier arrangement in a radio transmitter according to any of the claims 31 to 33, characterized in that monitoring (214) of a memory effect quantity of said transistor being monitoring of at least one of: gain magnitude scattering, and

gain phase scattering.

35. The method for operating a power amplifier arrangement in a radio 5 transmitter according to claim 33, characterized in that said step of detecting (215) non-normal operation conditions of said transistor comprises monitoring (214) of at least a percentage quiescent drain current drop from a nominal value of said transistor.

10 36. The method for operating a power amplifier arrangement in a radio transmitter according to any of the claims 24 to 35, characterized by the further step of:

performing a digital predistortion of a signal to be amplified by said transistor.