High Power Electromagnetic Pulse Damage Effect of a Low Noise Amplifier

Low noise amplifier (LNA) is the most sensitive component in the front end of a wireless communication system, which is very vulnerable to coupling attack of high-power electromagnetic pulse (HPEMP). This paper first analyzes the damage mechanism of a high-power electromagnetic pulse to a low noise amplifier, then designs the damage experiment of high-power electromagnetic pulse injection to the low noise amplifier, and verifies the damage mechanism and damage threshold of a certain radar low noise amplifier. The experimental results show that the LNA is not affected by the linear effect of high-power electromagnetic pulse. When the high-power electromagnetic pulse exceeds the damage threshold, thermal melting occurs between the FET grid sources in the device, causing permanent damage to the LNA and performance failure. This conclusion can provide data support for the high-power electromagnetic pulse protection design of this type of LNA.


Introduction
High-power electromagnetic pulse (HPEMP) can enter the wireless communication system in the way of "front door coupling" and "back door coupling", and cause interference, damage, and direct burning of electronic components in the form of voltage or current.Low noise amplifier (LNA) is an important component of wireless communication systems.It is the first stage after the antenna in the system and has the characteristics of low noise factor and high signal sensitivity.It is very easy to be damaged by HPEMP front door coupling attack, resulting in system paralysis.Therefore, it is of great significance to study the damage effect of HPEMP on LNA for improving the protection ability of HPEMP in wireless communication systems.
At present, a large number of theoretical and experimental studies on the damage effects of HPEMP on low-noise amplifiers have been carried out.A simulation model of the LNA electromagnetic pulse effect was established in the literature [1][2] [1,2] .In literature [3][4] , simulation research and experimental verification of the strong electromagnetic pulse effect of semiconductor devices in low-noise amplifiers were carried out [3,4] .Literature [5][6][7][8][9][10][11][12] carried out simulation analysis and experimental research on the damage effect of HPEMP in LNA [5][6][7][8][9][10][11][12] .To sum up, the current research on the damaging effect of strong electromagnetic pulse on low noise amplifiers is relatively complete, but in actual work, different low noise amplifiers are different due to different materials, different designs, different characteristics affected by HPEMP, and different damage mechanisms.To explore the HPEMP damage effect of a certain type of LNA, and provide certain data basis for the evaluation of the anti-HPEMP damage effectiveness and protection design of this type of LNA, this paper mainly carried out the HPEMP injection damage experiment of this type of LNA and explored the damage mechanism and damage threshold of this type of LNA device through theoretical analysis and experimental testing.

HPEMP effect of LNA
The effect of HPEMP on LNA can be divided into linear effect and nonlinear effect.The linear effect refers to the amplitude of HPEMP power greater than the normal signal but has not saturated or damaged the LNA, LNA still works in the normal linear range and has no impact on the physical properties and indicators of the device.The nonlinear effect refers to the fact that HPEMP power is much larger than the amplitude of the normal signal, resulting in LNA working in a nonlinear state, or causing node burnout and channel breakdown of the device, which has a serious impact on the physical properties of the device and is the basic destruction mechanism of LNA caused by HPEMP [13] .

Damage mechanism of LNA caused by HPEMP
Semiconductor devices are the core devices of LNA and the most sensitive components of HPEMP.Under the action of HPEMP, different parts of semiconductor devices may produce different damage mechanisms, but the common failure modes are mainly divided into three types: 1) Burning of metal wires; 2) Dielectric breakdown of oxide layer; 3) Secondary breakdown.

Burning of metal wires
Many metal leads inside semiconductor devices are used to interconnect modules and pins.Under the action of HPEMP, due to the high current density in the metal leads, a large amount of heat is generated, and the local temperature rises sharply to reach the melting point of the metal, causing the metal contacts and connecting lines to burn and melt to produce an open circuit, and the adjacent metal wire interconnection produces a short circuit.

Dielectric breakdown of the oxide layer
Under HPEMP, the dielectric breakdown of the oxide layer is the main factor of damage in the insulating region of semiconductor devices.The thickness of the oxide layer in a semiconductor device becomes thinner and thinner with the decreasing of device size, but the electric field strength of the device remains unchanged, resulting in a significant increase in the electric field in the channel region of the oxide layer.It is easy for carriers to form hot carriers in the strong electric field, which are captured and accumulated by the charge trap of the gate oxide layer, which will lead to the degradation of device characteristics and circuit performance with the continuous action of HPEMP.When the accumulated charge chain passes through the oxide layer to form a conductive channel, breakdown will occur, and eventually cause the failure of the device [14] .

Secondary Breakdown
The secondary breakdown of semiconductor devices can be divided into thermal secondary breakdown and electrical secondary breakdown.
The thermal secondary breakdown is related to the over-hot spot of the semiconductor device, mainly because the heat distribution inside the device is not uniform when the voltage is applied, the current density and electric field strength reaches the maximum at a certain point, and the temperature of the point rises rapidly, forming a hot spot (over-hot spot), which makes the semiconductor device appear irrecoverable damage.This phenomenon usually occurs at the us or ms level.
Under the influence of high electric field strength and high current density, the secondary breakdown is caused by the avalanche multiplication of charge carriers inside semiconductor devices, resulting in permanent damage to the devices.This phenomenon generally occurs at the ns level [14] .

Design of damage experiment platform
To study the damaging effect of HPEMP on a radar low noise amplifier, this paper designed an HPEMP injection damage experimental platform consisting of a signal source, power amplifier, adjustable DC power supply, LNA, coaxial attenuator, and vector network analyzer, as shown in Figure 1.The experimental platform makes full use of the advantages of high integration of vector network analyzer and has the characteristics of simple construction, high integration, and short test time.Compared with the conventional test scheme, the test efficiency and test accuracy of the experiment are significantly improved.

Performance specifications of the device under test
A type of LNA consists of an input and output isolator and a multistage FET amplifier, as shown in Figure 2.

Rf in Rf out
Isolator Isolator Multistage FET amplifier V cc +15V

Figure 2 Schematic diagram of a certain type of low noise amplifier
This type of LNA has the characteristics of low noise coefficient and high gain, and its basic parameters are shown in Table 1.The multistage FET amplifier is composed of gallium arsenide FET (FET) and a DC bias circuit installed on the microstrip circuit board.It has the characteristics of high sensitivity and poor power resistance, etc.The circuit principle is shown in Figure 3. 2) We connect the experimental equipment to supply power to LNA; 3) We use vector net to test S parameters and power parameters of LNA and record data; 4) The power amplifier is connected to the LNA, the microwave signal source is turned on, the pulse signal is injected into the LNA, and the gain change of the LNA is monitored in real-time through the vector net; 5) After injection, the S-parameters and power parameters of LNA were tested with vector net, and the change data of LNA parameters were recorded; 6) We step to adjust the power of the signal source, and repeat 4 to 5 steps; 7) When the gain of LNA monitored by the vector net changes significantly, the injection signal is stopped, the S-parameters and power parameters of LNA are tested and the data are recorded; 8) The LNA device is left standing for 1 minute, and S-parameters and power parameters are tested again and data are recorded.

Analysis of experimental results
When different power signals are injected, gain changes of LNA devices monitored by vector nets are shown in Figure 5.When the injection power of HPEMP is small, the gain of LNA is maintained at about 32 dB, and there is no linear or nonlinear change, which indicates that the device performance is not affected by the injection signal at this stage, and there is no linear effect, interference, degradation, and other effects.When the injection power was increased to 52 dBm, the gain of LNA suddenly decreased and the working current increased significantly.After standing for some time, the gain of LNA did not recover, indicating that the device was permanently damaged.
The comparison of scanning results of S parameters and power parameters before and after LNA damage is shown in Figure 6.When the injected power of HPEMP is lower than 52 dBm, the scanning results of S parameters and power parameters of LNA remain stable regardless of the injected power, as shown in Figure 6 (a) (b).When the injection power reaches 52 dBm, the S-parameter and power parameter scanning results of LNA show that the gain is significantly reduced and the noise increases, as shown in Figure 6 (c) (d).
The analysis of the above experimental results shows that the HPEMP damage threshold of the tested LNA is about 52 dBm.When the injected power of HPEMP is less than the damage threshold, the LNA performance is not affected, and there is no linear change or nonlinear effects such as interference and degradation.When the injected power of HPEMP is higher than the damage threshold, LNA is permanently damaged and its performance fails.

Verification of damage mechanism
When the damaged LNA was disassembled, it was found that the metal lines of the device did not melt, and all kinds of resistors and capacitors were detected normally.When the FET was disassembled, it was found that there was obvious melting between the gate and the source in the device by electron microscope observation.It can be seen that under the action of HPEMP, the FET device in this type of LNA generates a lot of heat between the source and the gate due to excessive voltage and current, and the heat accumulates rapidly in a short time, causing the device to burn out.

Conclusions
In this paper, the damage mechanism of LNA caused by HPEMP was analyzed theoretically, then the experimental platform was constructed, and the damage experiment of LNA caused by HPEMP injection was carried out.Through some improvement and optimization of the experimental platform, the test can be carried out more efficiently.The experimental analysis shows that: 1) The damage threshold of this type of LNA was 52 dBm; 2) If HPEMP injection power is lower than the LNA damage threshold, LNA performance is not affected, and there are no linear changes or nonlinear effects such as interference and degradation; 3) If the injection power of HPEMP is higher than the damage threshold of LNA, thermal melting occurs between the internal gate sources of the FET device in LNA, resulting in permanent damage to LNA and performance failure.This conclusion will provide a reference for the HPEMP protection design of this type of LNA.

Figure 1
Figure 1 LNA injection damage experimental block diagram

Figure 3 Figure 4
Figure 3 Circuit schematic diagram of multistage FET3.3 Testing Signal ParametersExperimental injection HPEMP signal source is injected with dot frequency signal in a working band.The dot frequency is 1.3 GHz, the pulse width is 1us, the repetition rate is 80 Hz, and each injection is 1 minute.The injection pulse signal waveform is shown in Figure4.

Figure 5
Figure 5 Schematic diagram of gain change of LNA

Figure 6
Figure 6 Comparison of scanning results of S-parameters and power parameters before and after LNA damage