First, the principle and structure of PIN diode
A general diode is formed by a semiconductor material doped with an N-type impurity and a semiconductor material doped with a P-type impurity to form a PN junction. The PIN diode is a thin layer of low-doped Intrinsic semiconductor layer between the P-type semiconductor material and the N-type semiconductor material.
The structure of the PIN diode is shown in Figure 1. Since the intrinsic semiconductor approximates the dielectric, this is equivalent to increasing the distance between the two electrodes of the PN junction capacitance, making the junction capacitance small. Second, the width of the depletion layer in the P-type semiconductor and the N-type semiconductor is widened as the reverse voltage increases, and as the reverse bias increases, the junction capacitance also becomes small. Due to the existence of the I layer, the P region is generally made thin, the incident photons can only be absorbed in the I layer, and the reverse bias is mainly concentrated in the I region, forming a high electric field region, and the photogenerated carriers in the I region are Accelerated motion under the action of a strong electric field, so the carrier transit time constant decreases, thereby improving the frequency response of the photodiode. At the same time, the introduction of the I layer increases the depletion region and broadens the effective working area of ​​the photoelectric conversion, thereby improving the sensitivity.
Figure 1 Schematic diagram of the PIN diode
There are two basic structures of the PIN diode, namely a planar structure and a mesa structure, as shown in FIG. For Si-pin 133 junction diodes, the carrier concentration of the I layer is very low (on the order of 10 cm), the resistivity is very high (on the order of k-cm), and the thickness W is generally thick (between 10 and 200 m). The doping concentration of p-type and n-type semiconductors on both sides of the I layer is usually high.
Both the planar structure and the I-layer of the mesa structure can be fabricated by epitaxial techniques, and the highly doped p+ layer can be obtained by thermal diffusion or ion implantation techniques. Planar structure diodes can be conveniently fabricated using conventional planar processes. The mesa structure diode also needs to be fabricated (by etching or trenching). The advantages of the mesa structure are:
1 The curved portion of the planar junction is removed, and the surface breakdown voltage is improved;
2 Reduce the edge capacitance and inductance, which is beneficial to increase the operating frequency.
Figure 2 Two structures of the PIN diode
Second, the working state of PIN diode under different bias
1. Right down
When the PIN diode is applied with a forward voltage, many of the P and N regions are injected into the I region and recombined in the I region. When the injected carriers and the composite carriers are equal, the current I reaches an equilibrium state. The intrinsic layer has a low resistance due to the accumulation of a large number of carriers, so when the PIN diode is forward biased, it exhibits a low resistance characteristic. The larger the forward bias voltage, the larger the current injected into the I layer, and the more carriers in the I layer, the smaller the resistance. Figure 3 is an equivalent circuit diagram under positive bias. It can be seen that it is equivalent to a small resistor with a resistance between 0.1Ω and 10Ω.
Figure 3 equivalent circuit diagram of PIN diode under forward bias
Forward bias current and forward impedance characteristic curve
2, under the bias
When no voltage is applied across the PIN diode, since the actual I layer contains a small amount of P-type impurities, at the interface of the IN, the holes in the I region diffuse toward the N region, and the electrons in the N region diffuse into the I region, and then form. Space charge zone. Since the impurity concentration in the I region is very low compared to the N region, most of the depletion region is in the I region. At the PI interface, due to the concentration difference (the hole concentration in the P region is much larger than the I region), diffusion motion also occurs, but its influence is much smaller than the IN interface and can be ignored. Therefore, when the zero bias occurs, the PIN diode exhibits a high resistance state due to the presence of the depletion region in the I region.
3, reverse
The reverse bias is very similar to the zero bias. The difference is that the built-in electric field is strengthened. The effect is to widen the space charge region of the IN junction and mainly to the I region. The PIN diode at this time can be equivalent to a resistor plus capacitor, the resistance is the remaining intrinsic region resistance, and the capacitor is the barrier capacitance of the depletion region. Figure 4 is an equivalent circuit diagram of the reverse biased PIN diode. It can be seen that the resistance range is between 1 Ω and 100 Ω, and the capacitance ranges from 0.1 pF to 10 pF. When the reverse bias is too large, the depletion region fills the entire I region, and the I region punch-through occurs at this time, and the PIN tube cannot work normally.
Fig. 4 Equivalent circuit diagram and reverse bias current and reverse capacitance characteristic curve of PIN diode under reverse bias
Third, PIN diode as a radio frequency switch
3.1 Working principle
Because the PIN diode's RF resistance is related to the DC bias current, it can be used as an RF switch and attenuator. Series RF switch circuit: When the diode is positively biased, it is turned on (short circuit); when the diode is zero-biased or reverse-biased, not only the maximum operating frequency of the switch will be limited, but also the minimum operating frequency will be limited, such as the PIN tube. Controls the switching of DC or low frequency signals. The switch also has an upper operating frequency due to the tube cutoff frequency. The frequency band of the switch is required to be as wide as possible because the frequency band of the signal source is wider and wider.
3.2 Performance parameters
Insertion Loss and Isolation: Insertion attenuation is defined as the ratio of the maximum capital power P produced by the source to the actual power P obtained by the load when the switch is turned on, ie P / P . If the actual power on the load when the switch is off is P, then the isolation is written in the form of a component:
According to the definition of the network scattering parameters, there are:
The ideal switch, the attenuation is infinite when disconnected, and the attenuation is zero when turned on. Generally, only the ratio of the two should be as large as possible. Since the impedance of the PI N tube cannot be reduced to zero, it cannot be increased to infinity. Therefore, the actual switch is not infinitely attenuated when it is turned off, and is not zero when it is turned on. Generally, the ratio of the two should be required to be as large as possible. Large, the conduction attenuation of the switch is called insertion loss, and the attenuation at the time of disconnection is called isolation. Insertion loss and isolation are the basic indicators for measuring the quality of the switch. The goal is to design switches with low insertion loss and high isolation.
Power capacity: The power capacity of a switch is the maximum microwave power it can withstand. The power capacity of the PIN diode is mainly limited by the following two aspects: the maximum power dissipation allowed when the tube is turned on; the maximum reverse voltage that the tube can withstand when it is turned off, that is, the reverse breakdown voltage. If the switch exceeds these limits when it is working, the former will cause the temperature rise inside the tube to be too high and burn out; the latter will cause avalanche breakdown in Zone I. It is determined by the smaller of the microwave signal power allowed in the on and off states of the switch. The nonlinear effect at high power (IIP3) is also a major factor in the power handling of the switch, especially in mobile communication base stations.
Driver requirements: The drive circuit of the PI N tube switch and the FET switch is different. The former needs to provide current bias, while the latter requires bias voltage. The driver is one of the main factors affecting the switching speed.
Switching speed: refers to the speed at which the switch is turned on and off, which is an important indicator in fast devices. The current equations in Zone I can be listed as follows:
The switching speed is increased to the order of ns. Usually, a very thin PIN tube of I layer is used. Because the number of carriers stored in the thin layer I is small, the switching time is greatly shortened. In this case, the switching time basically depends on the carrier. The time of the I layer is independent of the carrier lifetime. Increasing the switching speed also allows the use of tubes with short carrier lifetimes to increase the pulse amplitude of the control current, but the latter is limited by the maximum power and reverse breakdown voltage of the PIN tube.
Voltage standing wave ratio (VSWR): Any component on the high frequency signal path will not only generate insertion loss, but also increase the standing wave on the signal transmission line. The standing wave is formed by the interference of the transmitted electromagnetic wave and the reflected wave. This interference is often caused by the impedance mismatch of different parts of the system or the impedance mismatch of the connection points in the system.
Switching ratio: A PIN tube, when the package parasitic parameters are not considered, its forward state can be represented by a forward resistor R1, and the reverse state can be represented by a series resistor R2 and a layer I capacitive reactance jXc. Because of >>R2, the reverse state can be approximated by jXc. We call the ratio of impedance Xc/R1 in the positive and negative states as the switch ratio to measure the merits of the PIN switch. If the switching ratio is to be increased, then C and R2 must be relatively small. It can be seen that when the frequency is increased, the switching performance is degraded.
Fourth, summary
This paper introduces the structure and working principle of PIN diode, and analyzes its working state and equivalent circuit under various bias voltages. Finally, the PIN diode is introduced as a radio frequency switch. The PIN diode adds an intrinsic layer (I layer) compared to a conventional diode, making it useful for a wide range of applications, especially in the RF field and in photodetection. Therefore, it is meaningful to study the principle and characteristics of PIN diodes in depth.
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