Application of GaN power devices in array radar transceiver systems

With the rapid development of the third-generation semiconductor technology, gallium nitride (GaN) power devices have entered a period of rapid development. From the development history of semiconductor devices, we can see that the several leaps in the development of semiconductor devices are closely related to the emergence of several semiconductor materials in the same period. First, the discovery of silicon (Si) materials has made breakthrough progress in the application of semiconductors in the field of microelectronics. The widespread application of daily household appliances and computers should be attributed to the emergence of silicon materials. Later, the research on gallium arsenide (GaAs) materials enabled the application of semiconductors to enter the field of optoelectronics. The light-emitting diodes and semiconductor lasers made of gallium arsenide materials and some similar compound semiconductors, such as indium phosphide (InP), have played an irreplaceable role in the fields of optical communications and optical information processing, which has also brought about the rapid development of VCD and multimedia. These two generations of semiconductor devices also occupy a pivotal position in the field of microwave power.

1 Characteristics of GaN power devices

The first generation of semiconductor power devices represented by Si materials have completely replaced vacuum tube devices in VHF, UHF, L band, and S band. Radar transmission power devices have completely replaced vacuum tube devices. The pulse output power of silicon semiconductor power devices in L band and below is several hundred watts, the power below L band has exceeded kilowatts, and the output power in S band can reach 350W. The maximum operating frequency of GaAs semiconductor power devices can reach 30GHz to 100GHz, and the output power is relatively small. The third generation of semiconductor materials, such as gallium nitride (GaN) and silicon carbide (SiC), which began to be studied in the middle of the last century. The common feature of these materials is that their energy band gap is between 3.2 and 3.4eV, which is two to three times the energy band gap of GaAs and Si.

This article focuses on the application of GaN power devices in array radar transceiver systems. The characteristics of GaN high electron mobility transistors (GaN HEMTs) are explained below in combination with the physical properties of semiconductors.

(1) High output power and high added efficiency

The development of GaN HEMT benefits from the wide bandgap semiconductor AlGaN/GaN heterojunction material system. As shown in Table 1, GaN has a high breakdown field strength, which is several times higher than that of Si and GaAs.

The characteristic carrier generation rate is very high. Due to the spontaneous polarization and piezoelectric polarization effects on the heterojunction interface, the two-dimensional electron gas concentration is very high, and the electron saturation velocity is also high. AlGaN/GaN heterojunction is epitaxially grown on a wide bandgap material SiC semi-insulating substrate, which has better thermal conductivity than metal copper. Its good heat dissipation characteristics are conducive to high-power operation [2]. GaN HEMT also has the characteristics of low parasitic capacitance and high breakdown voltage, which is very suitable for realizing high-efficiency amplifiers.

Heat dissipation efficiency of GaN devices

(2) Long pulse width and high duty cycle

GaN HEMT is usually epitaxially grown on a wide bandgap material SiC semi-insulating substrate, which has better thermal conductivity than metallic copper. Proper control of the power density of GaN HEMT can easily achieve long pulse width and high duty cycle, and can be achieved in high-power continuous wave operation.

(3) Wide operating bandwidth and high operating frequency

The cutoff frequency of GaN HEMT directly determines the operating frequency and instantaneous bandwidth of its application. It increases with the increase of channel doping concentration, while it decreases with the increase of channel thickness and gate length [3]. Due to the limitation of the bandgap energy of Si semiconductor materials, its cutoff frequency is low, so the operating frequency of Si semiconductor power devices can only operate below the S band. GaAs devices have much better carrier mobility than other devices and a high cutoff frequency, but are limited by the breakdown field strength and have a low operating voltage, resulting in low device output power. GaN HEMT has the characteristics of wide bandgap energy, high breakdown field strength and high saturated electron drift velocity, which compensates for this deficiency and obtains good high-frequency performance. GaN HEMT can operate at higher frequencies and have high output power. In addition, the inherent characteristics of GaN HEMT make its input and output impedance higher, and broadband impedance matching of the circuit is easier to achieve, making GaN HEMT suitable for broadband applications.

(4) Strong radiation resistance and environmental adaptability

GaN is an extremely stable compound with strong atomic bonds, high thermal conductivity, the highest degree of ionization among III-V compounds, and good chemical stability. This makes GaN devices more resistant to radiation than Si and GaAs. At the same time, GaN is a high melting point material with high thermal conductivity. GaN power devices usually use SiC with better thermal conductivity as the substrate. Therefore, GaN power devices have a higher junction temperature and can work in high temperature environments.

CREE Ku-band 70W GaN

2 Requirements of array radar for transceiver system

GaN power devices have the characteristics of wide operating frequency band, high output power and high efficiency, and are particularly suitable for application in array radar transceiver systems. According to the characteristics of array radar, the following points deserve special attention in the application of GaN power devices [3].

(1) Good saturation working capacity

The array radar transceiver system is usually composed of a multi-stage cascade of power amplifiers. In order to minimize the impact of the difference of each stage on the final output, each stage is required to work in the saturation region, and at the same time, attention is paid to the reliability indicators under the saturation state.

(2) Output pulse leading and trailing edges

Array radars generally work in a pulse state. The leading and trailing edges of the pulse are closely related to the timing and measurement accuracy of the radar system. If the leading and trailing edges are too large, it may cause timing confusion.

(3) Output pulse drop

The output pulse droop of the amplifier is related to many factors. Under the condition that the amplifier is in normal condition and the peripheral circuit is well designed, the main factor is the droop generated by the GaN HEMT itself when it is working.

(4) Harmonic suppression

In large-scale array radar systems, since the radiation power emitted into space is relatively large, its harmonics will also interfere with some electronic equipment, so there are strict requirements for the second harmonic and even the third harmonic.

(5) Output power flatness

Array radars usually do not operate at a single frequency, nor do they operate at several frequencies simultaneously in communication mode. Instead, they operate with broadband frequency conversion and frequency hopping. Sometimes the frequency changes linearly with time within a pulse, and the output power of the amplifier has corresponding frequency response characteristics.

(6) Phase stability

For array radar, the phase stability of the device directly affects the formation of the radar transmit lobe pattern, and phase stability is particularly important in array radar.

(7) Inter-pulse noise

Here, inter-pulse noise refers to the noise generated by the transmitter when the radar is working in the receiving state. The inter-pulse noise directly affects the detection power of the radar.

(8) Anti-standing wave capability and stability under standing wave conditions

Since GaN devices have a higher breakdown voltage, their standing wave resistance is also better. However, due to their high gain, they are prone to instability under load mismatch conditions. It is generally required that relevant indicators can still meet the requirements when VSWR ≤3.

3 Application Analysis of GaN Power Devices

From the requirements of array radar for transceiver system, it can be seen that how to apply GaN power devices in array radar transceiver system, which type of amplifier bias is selected, which peripheral circuit is used, etc. are closely related. At the same time, many requirements and implementation methods are closely linked, and sometimes compromises cannot be considered. The following mainly analyzes the selection of amplifier working type, bias circuit and modulation circuit.

(1) Selection of amplifier operating type

Silicon bipolar transistors (BJTs) in the S-band frequency range usually work in C-class self-biasing. The transistor only needs a collector voltage. When the RF swing voltage at the input exceeds the internal potential of the emitter-base junction, the transistor draws collector current, and its collector current is controlled by the current flowing between the base-emitter junction. When the radar is in the receiving state, the device is not driven. At this time, the amplifier does not draw static DC current, the amplifier has no power dissipation, and is in the cut-off state. This type of amplifier is particularly suitable for the application of array radar transceiver systems. The circuit is simple, especially the requirement of low inter-pulse noise can be achieved without much processing, but this type of amplifier also has disadvantages. Compared with GaN HEMT, the operating frequency is lower, the single-stage gain is small, the additional efficiency is not high, and it cannot be used in long pulse width and high duty cycle. Unlike BJT, GaN HEMT is usually biased in Class A or Class AB, and its bias state is controlled by the bias voltage between the gate and source, and it is a voltage-controlled device. This working mode still has a high gain for small signals. Although there is a certain degree of isolation between the transmit and receive paths of the array radar, the energy of the generated leakage signal is amplified after multiple cascade amplification, which makes the radar unable to work normally. Therefore, it is necessary to consider solving this problem from the perspective of bias state and power supply modulation.

(2) Selection of power modulation circuit

GaN HEMT is similar to GaAs FET. The gate is negatively biased. The drain power modulation method is used to shut down the power supply of the amplifier during reception to ensure the noise requirements during radar reception, that is, pulse-to-pulse noise. GaN HEMT has a high drain operating voltage and a large output power. Therefore, it is not enough to completely learn from the experience of using GaAs FET, and even the system cannot work properly. When GaAs FET is used for transmission, the transmission power is not high (usually in the watt level), the loop gain is relatively low, and better transceiver isolation can be achieved through RF switches. However, when GaN HEMT is used, the transmission power is usually very large (more than 100 watts). If each stage of amplification is biased in Class A, the loop small signal gain is very high. In a highly integrated transceiver system, the transceiver isolation is difficult to control, and a small disturbance can cause the transceiver system to be unstable. Therefore, when using GaN HEMT to implement the transmission cascade, it is necessary to properly adjust the bias state to control the small signal gain to ensure the stable operation of the radar transceiver system. When working in Class B or Class C, the quiescent current will decrease and the gain will be reduced. When using drain power modulation, the reduction in quiescent current can also cause the falling edge of the pulse to deteriorate. In radar PD processing, the deterioration of the falling edge is equivalent to increasing the fuzzy distance, which is also unacceptable to the system. Therefore, a combination of partial Class C work and partial Class A drain power modulation work can be adopted. Comprehensively considering small signal gain, falling edge time and inter-pulse noise, finding a suitable balance is the key.

4 Ways to Reliably Apply GaN Power Devices

Improving the breakdown voltage of GaN HEMT and reducing the RF current offset of the device under high operating voltage are effective ways to improve the reliable operation of GaN HEMT. An important measure to improve the breakdown voltage of the device in the production of GaN HEMT is to introduce special technologies such as field modulation plate on the gate. The introduction of field plate can effectively reduce the electric field strength of the gate near the drain end [4], thereby increasing the breakdown voltage of the device. The pulsed transmitting amplifier consumes a large amount of DC current. When designing, special attention should be paid to the parasitic inductance of the drain bias, because it can generate a very high voltage spike, which can cause damage to the GaN HEMT.

Conclusion

In summary, since GaN power devices have many excellent properties and are used in array radar transceiver systems, this article mainly explains the application considerations and analysis of GaN power devices in array radar transceiver systems from the perspective of array radar application design. Due to space limitations, it cannot cover everything, but it has strong pertinence and guidance for the design of array radar transceiver systems, and can be used as a reference for a large number of array radar transceiver system engineering designers.