Performance analysis of PIN diodes in microwave switches

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1 Journal of Vectorial Relativity JVR 4 (2009) Performance analysis of PIN diodes in microwave switches M A Medina-Plata 1, G Leija-Hernández 2 and L A Iturri-Hinojosa 3 ABSTRACT: A numerical analysis of the series resistance, R s, and the union capacitance, C j,of p-i-n diodes is presented. These p-i-n diode parameters are studied in order to reach adequate levels of insertion loss and isolation in switching devices of microwave signals designed with p-in diodes. KEYWORDS: PIN Diodes, series resistance, union capacitance, microwave switches, insertion loss and isolation. I. INTRODUCTION Microwave switches are used as control elements in a variety of applications in Microsystems because they control and lead the flow of energy from the radiofrequency (RF) signal from one part of the circuit to another through external control signals. A switch in its high impedance state is characterized by the isolation it exhibits between its terminals. The level of isolation is important in every single application, thus the switch must be able to protect the receiver circuit s sensibility from the power of the transmitted RF signal. In general, switches may be operated manually or electronically, nevertheless, many applications that uses microwave integrated circuits require switching times that can t be achieved manually, consequently electronic control must be used. Electronically controlled switches can be manufactured using p-i-n diodes or generally MESFET transistor designed with GaAs. II. PIN DIODES FOR MICROWAVE SYSTEMS The p-i-n diode is commonly manufactured with a silicon semiconductor base that is composed of two regions, a type P and a type N region. Between these regions is an intrinsic region, I, of very high resistivity. The diodes work with signals of frequencies in the microwave region (> 1 GHz). The diode at these frequencies has very high resistance in its reverse biased state and very low resistance in its forward biased state. For all purposes, the p-i-n diode can behave as a short circuit or as an open circuit under forward and reverse bias state, respectively. It is also used to switch very intense currents and/or very high tensions. II.1 PIN Diode in forward and reverse polarization In low impedance state, the p-i-n diode behaves as an inductance L in series with a resistance. Its equivalent circuit model is shown in Fig. 1. Assuming that the RF signal does not affect the stored charge, the forward bias series resistance is obtained with the following expression: 1, 2, 3 ESIME-Zacatenco/Instituto Politécnico Nacional, Anexo Edif. 5, Col. Lindavista CP México DF December, aiturri@ipn.mx, moises1603@hotmail.com

2 where µ n is the electron mobility, and µ p is the hole mobility. The charge Q [C] is the result of the carrier`s recombination (electrons and holes) in the intrinsic region, it is found using [1]: Under reverse bias state, the p-i-n diode behaves as an inductance L in series with the parallel circuit of a conductance C T and a resistance R P, as shown in Fig. 1b. where is the dielectric constant of the diode s material, and A is the area of the diode s union. (1) (2) (3) Fig. 1. Equivalent circuits for the p-i-n diode in forward and reverse bias state The resistance R P is considered to be infinite. III. MICROWAVE SWITCH CIRCUITS USING PIN DIODES Nowadays, p-i-n diodes are mainly used in wireless communication systems as switching elements for the control of RF signals [2]. In Fig. 2, Single Pole Single Through (SPST) series and shunt switches are shown. Fig. 2 SPST Diode based switches. series, shunt Another type of microwave switches are compound switches. These have better isolation responses compared to switches with a single p-i-n diode. Compound switches are combinations of p-i-n diodes JVR 4 (2009) Journal of Vectorial Relativity 111

3 in series and in shunt [2]. Figure 3 shows two compound switches of single pole single throw (SPST). In a compound switch, when the p-i-n diodes in series are in forward bias and the diodes in parallel are in reverse bias (state zero), we talk about insertion losses. In the opposite case, we talk about isolation. In these cases, control circuits for the diodes are more complex when compared to simple switches. Fig. 3 SPST Switches series shunt ELL and TEE design III.1 Insertion Losses and isolation of switches The main performance parameters of switches are insertion losses, low impedance state of the diodes by forward bias, and isolation, high impedance state of the diodes by reverse bias. Insertion losses is defined as the relationship, generally in decibels, of the power given by an ideal switch in its conduction state and the real power given by the switch in its conduction state [2]. In other words: Isolation is a measure of the switch s performance in its open state. It is defined as the relationship between the power of the microwave signals provided to the load by an ideal switch in the conduction state and the real power provided to the load when the switch is in its open state [2]. In other words: The following Table shows the mathematical expressions used to estimate the performance parameters of p-i-n diode based switches. (4) (5) Table 1. Expressions used to calculate insertion loss and isolation of SPST switches [2] Type Isolation (db) Insertion Loss (db) Series Shunt Series - Shunt TEE JVR 4 (2009) Journal of Vectorial Relativity 112

4 IV. INSERTION LOSS RESULTS AS A FUNCTION OF OPERATING FREQUENCY Consider the p-i-n diode HPND4038 from AVAGO Technologies, whose equivalent circuit parameters under forward and reverse bias states are 1.5 Ohms of series resistance and pF union capacitance. Figure 4 shows the Insertion loss response of series-shunt, shunt and TEE switches using the HPND4038 diode. Fig. 4. Insertion loss vs. operating frequency As we can see, TEE type switches and series-shunt switches have a linear and constant behavior of insertion loss of 0.4 and 0.2 db, respectively, until approximately 10 GHz. Insertion loss for frequencies under 10 GHz is less in the shunt switch. As the operating frequency increases, insertion loss in the three switches tends to 3 db. The insertion loss response for series, shunt, series-shunt, and TEE switches were analyzed for p-i-n diodes described in Table 2 [3-6]. Table 2 Series resistance and capacitance characteristics of the p-i-n diode s union. Diodo Rs (Ω) Cj( pf) HPND HPND HPND HPND The insertion loss obtained from the series switches using p-i-n diodes of Table 1 is detailed in the following Table. JVR 4 (2009) Journal of Vectorial Relativity 113

5 Table 3. Insertion loss values of the series switches for each of the diodes used Diodo IL(dB) HPND HPND HPND HPND Fig. 5 shows the insertion loss in db of the shunt, series-shunt and TEE switches as a function of the operating frequency until 10 GHz, using the p-i-n diodes from Table 2. The characteristic of insertion loss response of the switches is exponential and the shunt switch has the lower insertion loss at 1 GHz, using either diode. The lowest insertion loss perceived is using the p-i-n diode HPND4005 except in series switch. The diode is the most recommended for use in a series switch, because it has the lowest forward bias resistance with respect to the other diodes that were analyzed. Fig. 5. Insertion Loss for shunt series-shunt (c) and TEE switches (c) JVR 4 (2009) Journal of Vectorial Relativity 114

6 V. ISOLATION RESULTS AS A FUNCTION OF THE OPERATING FREQUENCY Figure 6 shows the isolation results of the switches as a function of the operating frequency, from 1 GHz to 100GHz. The diode HPND4005 in the series switch exhibits an isolation response superior to 19 db for frequencies below 10 GHz. The p-i-n diodes , HPND4028 and HPND4038 exhibit better isolation in the shunt switch, superior to 21 db. All four diodes can be used in series-shunt and TEE switches due to their isolation responses superior to 18 db. (c) (d) Fig. 6. Isolation of commutators series, parallel, (c) series-parallel and (d) TEE VI. CONCLUSIONS An analysis of the p-i-n diode s parameters that influence insertion loss and isolation of four types of microwave signal switches was presented. A strong dependency exists between insertion loss and isolation with the operating frequency. A low value of series resistance (R s ) of the p-i-n diode in forward bias state does not ensure, by itself, a good performance of the diode in a switch. It s JVR 4 (2009) Journal of Vectorial Relativity 115

7 necessary calculate the level of isolation exhibited by the switch using the diode. The level of isolation depends, in some switches, on the union capacitance (Cj) that represents the diode in the reverse bias state. ACKNOWLEDGMENT This work was partially supported by the project SIP REFERENCES [1] White, J,, Semiconductor Control, Artech House, Dedham, MA, 1977, págs [2] Gerald Hiller, Design with PIN diodes, Alpha Industries. [3] Data sheet, PIN Diode Chip for Hybrid MIC Switches/Attenuators, [4] Data sheet, HPND-0002, Small Signal RF PIN Diode Chips for Hybrid Integrated Circuits, [5] Data sheet, HPND4005, Beam Lead PIN Diode, [6] Data sheet, HPND-4028, HPND-4038, Beam Lead PIN Diodes for Phased Arrays and Switches, JVR 4 (2009) Journal of Vectorial Relativity 116

Study, Design and Simulation of a Pointing Controller Switch for Beam Switching Antenna Systems

www.ijcsi.org 143 Study, Design and Simulation of a Pointing Controller Switch for Beam Switching Antenna Systems Tomader MAZRI 1, Fatima RIOUCH 2, Najiba El AMRANI EL IDRISSI 3 1 Systems Engineering Laboratory,