Analysis and comparison of GaN-based microwave semiconductor devices

The wide bandgap semiconductor material gallium nitride (GaN) has become the first-generation elemental semiconductor silicon (Si) and the second-generation compound semiconductor gallium arsenide (GaAs) and gallium phosphide (GaP) with its good physical and chemical properties. Third-generation semiconductor materials that have been rapidly developed after indium phosphide (InP). Compared with most current semiconductor materials, GaN has unique advantages: wider band gap, higher saturation drift speed, higher critical breakdown electric field and higher thermal conductivity, making it the most attractive new semiconductor material. one. At present, the research on GaN-based light-emitting devices has made great progress [1~3], and GaN LEDs working in the green-to-violet-light-visible region have been commercialized abroad [2]; many domestic units have successfully produced Blue LEDs have been initially industrialized [3]. Numerous studies [4~14] show that GaN materials are also very advantageous in the fabrication of high-temperature microwave high-power devices. From the material point of view, the author analyzes the reasons why GaN is suitable for microwave device manufacturing, introduces the latest research trends of several GaN-based microwave devices, and analyzes the working principle and characteristics of GaN modulated doped field effect transistors (MODFETs). Compared with other microwave devices, it shows its great potential in microwave high power applications.

The good performance exhibited by GaN-based microwave devices is inseparable from their underlying material properties. It can be seen from Table 1 that compared with Si, GaAs and SiC, GaN has the widest band gap, the highest breakdown electric field and the thermal conductivity is superior to Si and GaAs, which fully indicates that GaN materials are manufactured in microwave high-power devices. The huge advantages of the aspect.

Table 1, GaN material characteristics

Figure 1 shows the relationship between the electron drift velocity of GaN, Si, SiC, and GaAs at 300 K and the electric field. It can be seen from the figure that the electron saturation drift speed of GaN is significantly higher than other materials, indicating that GaN is very suitable for manufacturing high power, high current devices [15]. On the other hand, GaN has a high electron mobility (1 000 cm/(V·s) in bulk material), which results in low parasitic and channel resistance in the fabrication of GaN microwave devices, resulting in good characteristics. Device. In addition, as a direct bandgap semiconductor material, GaN and AlN can form an alloy with a continuous band gap from 3.4 eV to 6.2 eV, forming a doped, modulatable AlGaN/GaN heterojunction structure, and quantum formed by this structure. The two-dimensional electron gas (2DEG) effect can achieve higher electron mobility and saturated electron drift speed. At present, AlGaN/GaN heterostructures are gradually being applied to the field of microwave device manufacturing. The author compares some important parameters of AlGaN/GaN heterojunction and AlGaAs/GaAs heterojunction, see Table 2. As can be seen from the table, the AlGaN/GaN heterojunction structure has a significant material advantage over the AlGaAs/GaAs heterojunction and is more suitable for microwave applications. Xi'an University of Electronic Science and Technology used MOCVD to grow high-quality AlGaN/GaN heterojunction materials on sapphire and SiC substrates. The current 2EG mobility and surface charge density product of this material obtained on sapphire has reached 2X1016/( V·s).

Figure 1. Relationship between electron drift velocity of GaN, Si, SiC and GaAs and electric field at 300K

Table 2. Comparison of key parameters of AlGaN/GaN heterojunction and AlGaAs/GaAs heterojunction

Low heat generation rates and high breakdown fields have made GaN an important semiconductor material for research and fabrication of microwave high power devices. At present, with the continuous development of growth technology and breakthroughs in key technologies for thin film growth, various GaN heterostructures have been successfully grown, including metal semiconductor field effect transistors (MESFETs), heterojunction field effect transistors (HFETs), Microwave devices such as modulated doped field effect transistors (MODFETs) and metal-insulated field effect transistors (MISFETs).

The use of wide-bandgap semiconductor GaN to fabricate MESFETs has broad application prospects for microwave power amplification. GaN MESFETs are often fabricated using the basic theory of traditional MESFETs, which is directly based on GaN's wide forbidden band and simple manufacturing process advantages.

In 1993, Khan et al. [16] first fabricated GaN MESFETs on sapphire substrates using low-voltage MOCVD. They used a thin AlN layer as a buffer layer to improve the quality of the GaN film. TI/ Au was used as the source-drain ohmic contact, and silver was used as the gate Schottky. The device was isolated by H+ ion implantation and ion beam etching. The resulting device has a gate length of 1 μm and a transconductance of 23 mS/mm at a gate bias of -1 V.

Subsequently, SCBinari et al. [4] reported GaN MESFETs with better microwave performance. The difference is that they use an organic metal vapor phase epitaxy technique to grow an unintentionally doped GaN epitaxial layer on a sapphire substrate. The cross-sectional structure is shown in Figure 2. They first grown a 40 nm thick AlN buffer layer at a substrate temperature of 450 ° C, and then grown a 3 m thick undoped high resistance GaN layer on the buffer layer at 1050 ° C. Figure 3 shows the current gain |h21| and the unilateral gain U as a function of frequency for a 0.7μm gate length device manufactured by SCBinari et al. As can be seen from the figure, the cutoff frequency f T and the maximum oscillation frequency f max are 8 GHz and 17 GHz. In 1997, SCBinari et al. [5] developed a new type of GaN MESFETs with a source-drain spacing of 5 μm, a gate width of 150 μm, and a gate length of 0.7 to 2.0 μm. When the gate length is 1.5 μm, the maximum transconductance gm of the MESFET is 20mS/mm, and the maximum leakage current IDSI is 120mA/mm. When the leakage current is 1mA/mm, the leakage bias is 75V, and SCBinari predicts that its microwave output power will be greater than 1W/mm. In addition, SCBinari et al. predicted that with the improvement of design and process technology, the f T of GaN MESFETs will reach 20~40 GHz.