“PIN diodes are widely used in control circuits such as limiters, switches, attenuators, and phase shifters. Compared with MESFET and PHEMT devices, PIN diodes have the characteristics of low insertion loss, high cut-off frequency and large power capacity, and are especially suitable for making broadband high-power control circuits with excellent performance.literature[1]A broadband high-power single-pole double-throw switch is made by using GaAs PIN diodes, but because it is in the form of a hybrid integrated circuit, the switch module is bulky.
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PIN diodes are widely used in control circuits such as limiters, switches, attenuators, and phase shifters. Compared with MESFET and PHEMT devices, PIN diodes have the characteristics of low insertion loss, high cut-off frequency and large power capacity, and are especially suitable for making broadband high-power control circuits with excellent performance.literature[1]A broadband high-power single-pole double-throw switch is made using GaAs PIN diodes, but because it is in the form of a hybrid integrated circuit, the switch module is bulky.
In this paper, a broadband high-power single-chip single-pole double-throw switch has been successfully developed using the GaAs PIN process of Hebei semiconductor Research Institute. The monolithic switch integrates GaAs PIN diodes, capacitors, inductors and resistive elements. In the range of 6-18 GHz, the insertion loss (IL) is less than 1.45 dB, and the isolation is greater than 28 dB; the test output power is only compressed 0.5 dB under the condition of continuous wave input power of 37 dBm and 12 GHz. Due to the single-chip manufacturing process, the circuit area is greatly reduced in the case of high power processing capability.
1 PIN diode manufacturing process
The PIN diode of this paper adopts a vertical structure. In order to make the PIN diode have better microwave characteristics, the doping concentration of the p+ layer and the n+ layer is controlled to be greater than 2.5×1018 during the epitaxial growth of the material, so as to reduce the metal-semiconductor ohmic contact resistance; the thickness of the i layer is 3 μm, the current carrying The sub-concentration is close to 3×1014, which makes the diode’s i-layer depletion capacitance and power capacity reach an optimal balance point. Figure 1 shows the final fabricated GaAs PIN diode structure diagram (a) and physical photo (b).
2 SPDT switch circuit design
An accurate model is the basis for designing a circuit. As shown in Figure 2, the GaAs PIN diode is equivalent to a resistor Rp in a positive Voltage bias state, and is equivalent to a capacitor Cr and a resistor Rn connected in series in a negative voltage bias state. Among them, Rp≈Rn, is the sum of the forward conduction resistance of the p+ layer, the n+ layer and the i layer, and Cr is the reverse bias capacitance of the i layer. Before designing the monolithic switch circuit, conduct a tape-out of the PIN diode model. Diodes are divided into two types: series and parallel, and each type has 15 sizes from small to large. By on-chip measurement and extracting the S-parameters of each diode’s forward and reverse bias states, a complete PIN diode small-signal model is established.
Single-pole double-throw switches usually have three types of structures: series, series-parallel hybrid, and parallel. Among them, the series PIN diodes in the first two structures will make the switch circuit begin to compress in a low-power state, and a parallel structure can only be used to make a high-power switch. Figure 3 is a schematic diagram of a parallel SPDT switch. The input port is connected to a 50 Ω microstrip line, and C1 is a DC blocking capacitor to prevent the bias voltages of the two output branches from interfering with each other; according to the formula Zc=1/jωC, in order to reduce the insertion loss in the on state, C1 should be Has a large capacitance. Negative voltage is applied to the bias voltage port, the diode D1 is in a reverse bias state, which is equivalent to a small capacitor. D1, microstrip lines L1 and L2 form a band-pass filter, and the entire branch is in a conducting state; the bias voltage port When positive voltage is applied, D1 is in a forward bias state, which is equivalent to a small resistance. The band-pass filter composed of D1, microstrip lines L1 and L2 is in a mismatched state, and most of the input power is reflected back, and the entire branch is in a state of mismatch. isolation state. Inductor L, capacitor C2 and microstrip line L3 form an output matching circuit. The whole switch circuit is designed by the combination of AdvancedDesign System software, schematic diagram simulation and electromagnetic field simulation.
3 Small signal and power characteristic test
Figure 4 is a photo of the chip after processing, and the chip area is 2.3 mm × 1.4 mm. Figure 5 is a block diagram of the microwave on-chip test system.Under the condition of ±5 V, after microwave on-chip small-signal testing, the SPDT switch is stable in the range of 6 to 18 GHz.
The microwave power characteristics of the switch need to be tested by placing the chip into a fixture. Figure 7 shows the assembled switch DUT. Figure 8 is the block diagram of the power test platform. The continuous wave signal provided by the signal source is amplified by the traveling wave tube amplifier and applied to the input port of the switch. The isolator prevents the amplifier from being burned by the power reflected by the switch. The output port of the switch is connected to an attenuator for Protect the power meter probe, the power characteristics of the switch can be obtained through the power meter. Figure 9 is the power characteristic test curve under the condition of 12 GHz, it can be seen that the output power is only compressed by 0.5 dB at 37 dBm.
4 Conclusion
The broadband high-power single-chip single-pole double-throw switch chip reported in this paper was tape-out at the Hebei Semiconductor Research Institute. Under the condition of ±5 V, the test insertion loss within 6~18 GHz is less than 1.45 dB, the isolation is greater than 28 dB, the return loss is greater than 7.5 dB, and the 12 GHz frequency point test P1dB is greater than 5 W. On a 4-inch (100 mm) wafer, the switch yield can reach more than 70%, which has a very good engineering application prospect.
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