Performance analysis of p-i-n diodes in microwave switches by Alejandro Iturri

Journal of Vectorial Relativity

JVR 4 (2009) 4 1-7

Performance analysis of p-i-n diodes in microwave switches M A Medina-Plata, G Leija-Hernández and L A Iturri-Hinojosa

ABSTRACT.- A numerical analysis of the series resistance, Rs, and the union capacitance, Cj,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-i-n 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.

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ESIME-Zacatenco/Instituto Politécnico Nacional, Anexo Edif. 5, Col. Lindavista CP 07738 México DF E-mail: aiturri@ipn.mx, moises1603@hotmail.com

December, 2009

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

II.1

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Diodo p-i-n en polarización directa e inversa

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) where µn is the electron mobility, and µp 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]: (2) Under reverse bias state, the p-i-n diode behaves as an inductance L in series with the parallel circuit of a conductance CT and a resistance RP, as shown in Fig. 1b. (3) where is the dielectric constant of the diode´s material, and A is the area of the diode´s union.

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Fig. 1. Equivalent circuits for the p-i-n diode in (a) forward and (b) reverse bias state

The resistance RP is considered to be infinite. III. MICROWAVE SWITCH CIRCUITS USING PIN DIODES Nowaday, 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.

JVR 4 (2009) 4 1-7

©Journal of Vectorial Relativity

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

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December, 2009

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Fig. 2 SPST Diode based switches. (a) series, (b) 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 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.

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Fig. 3 SPST Switches (a) series – shunt ELL and (b) 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: (4) 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:

JVR 4 (2009) 4 1-7

©Journal of Vectorial Relativity

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

December, 2009

(5) The following table shows the mathematical expressions used to estimate the performance parameters of p-i-n diode based switches. Tabla 1. Expressions used to calculate insertion loss and isolation of SPST switches [2]

Type

Isolation (dB)

Insertion Loss (dB)

Series Shunt Series Shunt TEE

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 0.0045pF 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

JVR 4 (2009) 4 1-7

©Journal of Vectorial Relativity

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

December, 2009

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]. Tabla 2 Series resistance and capacitance characteristics of the p-i-n diode’s union.

Diodo HPND4005 HPND0002 HPND4028 HPND4038 5082-0012

Rs (Ω) 4.7 3.5 2.3 1.5 1

Cj( pF) 0.017 0.2 0.025 0.045 0.12

The insertion loss obtained from the series switches using p-i-n diodes of Table 1 is detailed in the following table. Tabla 3 Insertion loss values of the series switches for each of the diodes used

Diodo HPND4005 HPND0002 HPND4028 HPND4038 5082-0012

IL(dB) 0.4 0.3 0.1975 0.1293 0.0864

The diode 5082-0012 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. Figure 5 shows the insertion loss in dB of the shunt, series-shunt and TEE switches as a function of the operating frequency until 100 GHz, using the p-i-n diodes from Table 2. The response of insertion loss increases exponentially above 2GHz in shunt and series-shunt switches designed with p-i-n diodes with high junction capacitance. Therefore, we recommend using p-i-n diodes with low junction capacitance when the operating frequency is greater than 10GHz.

JVR 4 (2009) 4 1-7

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

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December, 2009

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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 5082-0012, 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.

JVR 4 (2009) 4 1-7

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

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Fig. 6. Isolation for (a) series, (b) shunt, (c) series-shunt and (d) TEE switches

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 (Rs) 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 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 proyect SIP20090676.

JVR 4 (2009) 4 1-7

M A Medina-Plata et al.: Análisis de Desempeño de Diodos PIN en Conmutadores de Señales Microondas

December, 2009

REFERENCES [1] [2] [3] [4] [5] [6]

White, J,, Semiconductor Control, Artech House, Dedham, MA, 1977, págs. 39-87. Gerald Hiller, “Design with PIN diodes”, Alpha Industries. Data sheet, 5082-0012 PIN Diode Chip for Hybrid MIC Switches/Attenuators, www.avagotech.com Data sheet, HPND-0002, Small Signal RF PIN Diode Chips for Hybrid Integrated Circuits, www.avagotech.com Data sheet, HPND4005, Beam Lead PIN Diode, www.avagotech.com Data sheet, HPND-4028, HPND-4038, Beam Lead PIN Diodes for Phased Arrays and Switches, www.avagotech.com

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