RF devices are the basic components to realize signal transmission and reception, and are the core of wireless communication, mainly including filters (Filter), power amplifiers (PA), RF switches (Switch), low noise amplifiers (LNA), antenna tuners (Tuner) and dual/multiplexer (Du/Multiplexer) and other types of devices. Among them, the power amplifier is a device to amplify RF signals, which directly determines the wireless communication distance, signal quality and other key parameters of mobile terminals and base stations.
Power amplifier (PA, Power Amplifier) is the core component of the RF front-end, the use of transistor current control or field effect tube voltage control will be converted to the power of the power supply in accordance with the input signal changes in the current.PA is mainly used in the transmitter link, through the transmitter channel to amplify the weak RF signals, so that the signal successfully obtain a sufficiently high power, so as to achieve a higher communication quality and longer communication distance. The PA is mainly used in the transmitter link. Therefore, the performance of PA can directly determine the stability and strength of the communication signal.
Figure 1: Applications of RF devices
With the continuous development of semiconductor materials, power amplifiers have experienced CMOS, GaAs, GaN three major technology routes. The first generation of semiconductor materials is CMOS, mature technology and stable production capacity, the disadvantage is that there is a limit to the operating frequency, the highest effective frequency below 3GHz. The second generation of semiconductor materials mainly use GaAs or SiGe, a higher breakdown voltage, can be used for high-power, high-frequency device applications, but its device power is lower, usually less than 50 W. The third generation of semiconductor materials, GaN has a higher electron mobility, fast switching speed characteristics, to make up for the GaAs and Si-based LDMOS the defects of these two traditional technologies, in the embodiment of GaAs high-frequency performance at the same time, combined with a high-frequency LDMOS. GaAs high-frequency performance at the same time, combined with the power processing capability of Si-based LDMOS. Therefore, in terms of performance is significantly stronger than GaAs, in the field of high-frequency applications have significant advantages, in the microwave radio frequency, IDC and other areas of great potential. With the acceleration of the construction of the national 5G base station, the domestic GaN RF device market has grown exponentially, is expected to release more than 100 billion yuan of GaN PA new demand. In the next three to five years GaN RF devices in 5G base station penetration rate is expected to reach 70%.
Fig. 2:RF device applications for different materials
Fig. 3:Characteristics of RF devices with different materials
GaN HEMT devices
GaN HEMTs (High Electron Mobility Transistors), as a representative of wide-bandwidth (WBG) semiconductor devices, have higher electron mobility, saturation electron velocity, and breakdown field than Si and SiC devices. Due to the material advantages, GaN has excellent power and frequency characteristics and low power loss in high-frequency operation.
GaN HEMT (high electron mobility transistor) is a use of heterojunction between the deep barrier hoarding of two-dimensional electron gas (2DEG) as a conductive channel, in the gate, source, drain two-end voltage bias regulation to achieve the conductive properties of the device structure. Due to the formation of GaN material heterojunction there is a strong polarization effect, the heterojunction interface at the quantum well generated a large number of first bound electrons, called 2DEG. The basic structure of a typical AlGaN/Ga N-HEMT device is shown in Figure 5 below, the device is the bottom layer of the substrate layer (generally SiC or Si materials), and then epitaxial growth of N-type GaN buffer layer, epitaxial growth of P-type AIGaN barrier layer, the formation of AlGaN/GaN heterojunction. Finally, the gate (G), source (S) and drain (D) of the Schottky contact are deposited on the AIGaN layer for high doping concentration and connected to the two-dimensional electron gas in the channel to form an ohmic contact.
The drain-source voltage VDS causes a transverse electric field to be generated in the channel, and under the action of the transverse electric field, the two-dimensional electron gas is transported along the interface of the heterojunction to form the drain output current IDS. The gate is in Schottky contact with the AlGaN barrier layer, and the depth of the potential wells in the AIGaN/GaN heterojunction is controlled through the magnitude of the gate voltage VGS to change the magnitude of the surface density of the two-dimensional electron gas in the channel and thus the output current of the drain in the channel. the drain output current in the channel.
Fig. 4: Appearance and circuit schematic of GaN HEMT device
Fig. 5: Schematic of GaN HEMT device structure
The evaluation of GaN HEMT devices generally consists of DC characterization (DC l-V test), frequency characterization (small signal S-parameter test), and power characterization (Load-Pull test).
DC Characterization
Like Si-based transistors, GaN HEMT devices also need to undergo DC l-V test to characterize the DC output capability and operating conditions of the devices. The test parameters include Vos, IDs, BVGD, BVDs, gfs, etc., of which the output current lps and transconductance gm are the two most central parameters.
Frequency parametric testing of RF devices consists of measurements of small-signal S-parameter, intermodulation (IMD), noise figure and spurious characteristics. The S-parameter test describes the fundamental characteristics of the RF device at different frequencies and for different power levels of the signal, and quantifies how the RF energy propagates through the system.
S-parameter is also known as scattering parameter, S-parameter is a tool for describing the electrical behavior of a component under the excitation of a high-frequency signal with RF characteristics, and it describes the method of "scattering" from the outside of the component after the component reacts to the incident signal (i.e., "scattering"). It describes the electrical behavior of a component in terms of the measurable physical quantities "scattered" from the outside of the component in response to an incident signal (i.e., "scattering"), and the magnitude of the measured physical quantities reflects the fact that components with different characteristics "scatter" differently to the same input signal.
Using small-signal S-parameters, we can determine the fundamental RF characteristics, including voltage standing wave ratio (VSWR), return loss, insertion loss, or gain for a given frequency. Small-signal S-parameters are usually measured using a continuous wave (CW) excitation signal and applying narrowband response detection. However, many RF devices are designed to operate with pulsed signals that have a wide frequency response. This makes it challenging to accurately characterize RF devices using standard narrowband detection methods. Therefore, for device characterization in pulsed mode, so-called pulsed S-parameters are often used. These scattering parameters are obtained by special impulse response measurement techniques. Currently, some companies have adopted the pulse method to test the S-parameters, and the test specifications range from 100us pulse width, 10~20% duty cycle.
Due to the material and production process limitations of GaN devices, the devices inevitably have defects, resulting in current collapse, gate delay and other phenomena. Under RF operation, the output current of the device decreases and the knee voltage increases, which ultimately reduces the output power and deteriorates the performance. At this time, pulse testing is required to obtain the real operating state of the device in pulse mode. At the scientific research level, we are also verifying the effect of pulse width on the current output capability, the pulse width test range covers 0.5us~5ms level, 10% duty cycle.
Power Characterization Test (Load-pull Test)
GaN HEMT devices have excellent characteristics to adapt to high-frequency and high-power operating conditions, therefore, small-signal S-parameter testing is difficult to meet the testing needs of high-power devices. Load-Pull test (Load-Pull test) for power devices in the nonlinear operating state of the performance evaluation is critical, can provide assistance for the matching design of RF power amplifiers. In RF circuit design, the need to match the input and output of the RF device are matched to a common wheel matching state. When the device is in the small-signal operating state, the device gain is linear, but when the input power of the device is increased to make it work in the large-signal nonlinear state, because the device will be the power traction, which will lead to the optimal impedance point of the device is shifted. Therefore, in order to obtain the optimal impedance point of the RF device in the nonlinear operating state and the corresponding output power, efficiency and other power parameters, need to be in the device for the large-signal load traction test, so that the device in a fixed input power to change the impedance value of the device output side of the matching load, to find the optimal impedance point. Among them, power gain (Gain), output power density (Pout), and power added efficiency (PAE) are important parameters for power characterization of GaN RF devices.
DC l-V Characterization Test System Based on Purcell S/CS Series Source Meter
The whole set of test system is based on Purcell S/CS series source meter, together with the probe table and special test software, it can be used for testing the DC parameters of GaN HEMT and GaAs RF devices, including threshold voltage, current, and output characteristic curve.
S/CS Series DC Source Meter
S series source meter is the first localized source meter with high precision, large dynamic range and digital touch, which has been built by Purcells for many years, integrating voltage, current input/output and measurement, etc. The maximum voltage is 300V, the maximum current is 1A, it supports four-quadrant operation, and it supports various scanning modes such as linear, logarithmic and customized, etc. It can be used in the production and R&D of GaN, GaAs RF materials and chips for DC l-V characterization. It can be used for DC l-V characterization of GaN, GaAs RF materials and chips in production and R&D.
CS series plug-in source table (host + daughter card) is a modular test product for multi-channel test scenarios. The maximum number of optional daughter cards is 10, with voltage, current input/output and measurement functions, maximum voltage 300V, maximum current 1A, supporting four-quadrant operation, high channel density, strong synchronous triggering function and high efficiency of multi-device combination.
For DC characterization of RF devices, the gate voltage is generally within ±10V and the source and drain voltages are within 60V. In addition, because the device is a three-port type, therefore, at least two S source meters, or two-channel CS daughter cards are required.
Figure 16: S-Series Source Table
Output Characteristic Curve Test
In the gate, source voltage VGs must be the case, the source and drain current lbs and voltage Vos between the change curve, known as the output characteristic curve. With the increase of Vos and constantly, the current los also increased to saturation. In addition, a set of output characteristic curve can be obtained by testing different gate and source voltage Vcs values.
transconductance test
Transconductance gm is a parameter that characterizes the control ability of the gate on the channel of the device, the larger the value of transconductance, the stronger the control ability of the gate on the channel.
It is defined as gm=dlDs/dVgo in the source, drain voltage must be the case, test source, drain current lDs and gate, source voltage VGs between the curve, and the curve is derived, you can get the transconductance value. Among them, the maximum transconductance value is called gm,max.
Pulse I-V Characterization Test System Based on PurcellР Series Pulse Source Meter/CP Series Constant Voltage Pulse Source
The whole set of test system is based on Purcell P series pulse source meter/CP constant voltage pulse source, together with the probe table and special test software, it can be used to test the pulse I-V parameters of GaN HEMT and GaAs RF devices, especially for the plotting of the pulse l-V output characteristic curve.
P Series Pulse Source Meter
P series Pulse Source Meter is a high precision, strong output, wide test range pulse source meter introduced by Purcells, integrating voltage, current input/output and measurement functions. The product has two working modes: DC and pulse. The maximum output voltage is up to 300V, the maximum pulse output current is up to 10A, the maximum voltage is up to 300V, the maximum current is up to 1A, it supports four-quadrant operation, and it supports linear, logarithmic, customized and other scanning modes. It can be used in the production, R & D of GaN, GaAs RF materials and chip pulse l-V characterization.
CP Series Pulse Constant Voltage Source
Purcell CP series pulse constant voltage source is a narrow pulse width, high precision and wide range plug-in pulse constant voltage source introduced by Wuhan Purcell Instrumentation. The device supports narrow pulse voltage output, and synchronization of the output voltage and current measurement; support for multi-device triggering to achieve the device's pulse l-V scanning, etc.; support for the output pulse timing (such as delay. pulse width, period, etc.) adjustments can be output complex curve. Its main features are: pulse current is large, up to 10A; pulse width is narrow, the minimum can be as low as 100ns; support for DC, pulse two voltage output modes; support for linear, logarithmic and customized scanning mode. The product can be applied to the I-V test of high-speed devices made of nitride, arsenide and other materials.
For pulsed l-V characterization of RF devices, the gate voltage is generally within ±10V, source and drain voltages are within ±60V, pulse width ranges from 0.5us to 500us, and the duty cycle is 10% or 20%. In addition, since the device is a three-port type, at least two RP source meters are required, or a 2-channel CP daughter card.
Figure 26: P-Series Pulse Source Table
Pulse output characteristic curve test
Due to the material and production process limitations of GaN devices, there is a current collapse effect. As a result, the device will suffer from power degradation when operating under pulsed conditions, and will not be able to achieve the ideal high power operating state. Pulsed output characteristics are tested by applying periodic pulse voltage signals to the gate and drain of the device synchronously, and the gate and drain voltages are synchronized to alternate between the static operating point and the effective operating point. The device current is monitored when Vcs and Vos are active voltages. It is demonstrated that different static operating voltages and pulse width lengths have different effects on the current collapse.
Pulse S-parameter test system based on Purcell CP series constant voltage pulse source
The whole set of test system is based on Purcell CP series constant voltage pulse source, together with network analyzer, probe stage, Bias-tee fixture, and special test software. On the basis of DC small-signal S-parameter test, it can realize the pulse S-parameter test of GaN HEMT and GaAs RF devices.
summarize
Wuhan Purcells has been focusing on the development of electrical performance test instruments and systems in the field of power devices, RF devices and third-generation semiconductors. Based on the advantages of core algorithms and system integration and other technology platforms, the company has taken the lead in independently developing high-precision digital source meters, pulsed source meters, pulsed high-current source meters, high-speed data acquisition cards, pulsed constant-voltage source meters and other instrumentation products and a complete set of test systems. Our products are widely used in the field of analyzing and testing power semiconductor materials and devices, RF devices and wide-band semiconductors. According to the user's needs, we can provide high performance, high efficiency, high cost-effective electrical performance test comprehensive solutions.
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