RF Characterization Of Diamond Schottky PIN Diodes By Mohammad Faizan .

1.2 PIN and Schottky PIN Diodes as Limiters RF/microwave limiters are usually based on PIN diodes or Schottky diodes. But they can also be fabricated with field-effect transistors (FETs), even in monolithic-microwaveintegrated-circuit (MMIC) form (Browne, 2012). A microwave PIN or a Schottky PIN diode acts as a variable resistor at high frequencies. The device can be used for amplitude modulation, pulse modulation, attenuation, switching as well as phase shifting of RF signals by the variation of forward-bias control current. Making use of very small levels of control power, the device can potentially control much larger amplitudes of RF signal power (Skyworks, 2008). The Schottky PIN diode presents a very high input impedance at low input voltage signals, the insertion loss typically being close to 0 dB in this scenario. The insertion loss of the limiter PIN mostly depends on the junction capacitance of the diode. As the higher input signals turn the diode on, the impedance falls to a low value after a momentary delay, leading to an impedance mismatch reflecting most of the signal and thereby minimizing the risks posed to the receiver system. There are two types of mainstream limiters: active and passive. An active limiter makes use of an external DC bias to forward-bias the limiter diode beyond their turn-on, whereas a passive limiter depends on the incoming RF signal in order to develop the voltage necessary to turn-on the PIN diodes (Smith et al., 1999). Figure 1-3 shows a simple block diagram of a conventional passive RP limiter. The circuit has a PIN diode and an RF choke connected in parallel, and in shunt with the transmission line. DC blocking capacitors are usually present at the input and output of the circuit. A single stage limiter 3

can typically attenuate the amplitude of a high-power signal by up to 30 dB (Skyworks, 2008). Figure 1-3. A Single-Stage Limiter Circuit (Redrawn from (Skyworks, 2008)) Figure 1-4 shows a typical input vs. output power response for a single-stage limiter, demarcating the low-insertion loss operation in presence of low input voltage swings and the limiting operation stage when the diode turns on due to RF signal burst. Figure 1-4. Transfer Curve for a Single-Stage PIN Limiter (Redrawn from (Skyworks, 2008)) 4

A wide range of limiter circuit configurations with the PIN diode are possible which are widely used and are enlisted in Figure 1-5. Figure 1-5. Typical Limiter Circuit Configurations (Redrawn from (Deng Et Al., 2020)) The anti-parallel configuration exhibits good linearity and having a back-to-back diode setup is generally required so that one of the diodes turns on for positive cycles of voltage swings and the other diode for negative voltage swings. In order to reduce distortion at high-frequencies, the stacked and reverse-bias configurations are more prevalent (Deng et al., 2020). 1.2.1 Device Operation Schottky diodes are majority carrier devices with low forward voltage drop and fast switching action. Hence, they respond rapidly to variations in the input signal amplitude, thereby being attractive for limiter circuits. Schottky diode limiters are also a more suitable choice at higher frequencies as they rectify high frequency microwave and mm-wave signals more effectively than conventional PIN diodes, mostly because of the absence of minority carrier storage effect (Bera et al., 2010). The structure consists of a highly doped substrate of p-type material doped with boron on which a nominally undoped intrinsic region is grown. The top layer is usually a very thin and lightly doped layer of n-type material, generally phosphorus. 5

Figure 1-6. Schottky P-Type-Intrinsic-N-Type (PIN) Diode Structure When a Schottky diode is under reverse-bias, there is an absence of charge carriers in the i-region. At 0 V dc bias, the diode impedance is at its maximum and the device generally behaves as a lossy capacitor (Doherty & Joos, 1998). Only a negligible insertion loss is incurred at this stage due to the diode’s junction capacitance, while most of the input signal power can pass by the Schottky diode limiter. When the diode is forward-biased, the majority charges (holes) from the p-region are injected into the intrinsic region due to the buildup of electric field across the junction. This usually happens when a high-power RF signal is incident on the transmission line, forcing the diode to essentially turn on and thereby reducing the diode impedance to reach its minimum value. The input signal power is attenuated by the impedance mismatch and this isolation against potentially harmful signals is provided by the diode’s low impedance. The diode size has two major constraints to tackle; one being the maximum allowed capacitance which is determined by either the bandwidth of operation or the insertion loss, and the thickness of the intrinsic layer which is determined by the rectification efficiency at the desired operating frequency (Brown, 1967). 6

Figure 1-7 shows the comparison of the power output and power input in case of a magnitude burst of an incoming RF signal. As can be seen below, the spike leakage is defined as the maximum output power emanating from the diode in a circumstance when it briefly passes more power than that under normal saturated signal conditions (Browne, 2012). When the diode starts conducting and its impedance is reduced to its minimum, then the power level is known as flat leakage. Hence, it becomes paramount to ensure a limiter diode curbs the amount of energy during this output spike, in turn protecting the subsequent stages in the receiver system (Skyworks, 2008). Moreover, it is difficult in practice to realize constant output power with the single-stage limiters discussed earlier. The flat leakage power is usually reduced by making use of multistage limiters (Deng et al., 2020). Figure 1-7. Input Power and Output Power of a Limiter Vs Time (Redrawn from (Skyworks, 2008)) Figure 1-8 details the transfer function of a single-stage limiter showing the low insertion loss operation stage, the limiting stage and the maximum insertion loss stage when the limiter diode saturates. 7

Figure 1-8. Single-Stage Limiter Transfer Function (Redrawn from (Skyworks, 2008)) A limiter is considered ideal if it doesn’t attenuate low-power input signals and at the same time has an attenuation that can maintain a constant power output when the input level increases beyond a certain value. There is always a possibility that input RF signals can propagate through the limiter diode stage to the load and not be reflected, even after the diode has reached its low impedance state. The power that gets transmitted to the circuitry following the limiter can be 2-4 dB greater than the input threshold level of the diode. Once, the impedance of the limiter diode reduces to its minimum and if the input signal amplitude level keeps on increasing, the limiter output power increases dB for dB as shown in Figure 1-8 (Skyworks, 2008). 1.2.2 Limiter Figure of Merit The most important characteristic of a high-power Schottky PIN limiter diode is that it should be highly reflective when damaging high-power signals are incident. It should also 8

have a low absorption loss which can increase the power handling manifold. The limiter device should also additionally have good thermal properties to tackle the stress due to heating (Brown, 1967). The basic metrics that help in evaluating a limiter diode performance are detailed below: RF power handling: The junction size of the limiter diode primarily determines its maximum RF power handling capability. A diode with a larger diameter can conduct larger currents and hence handle higher input signal amplitudes. But the downside is that the shunt capacitance in the circuit also increases with diode diameter, increasing the parasitics and the insertion loss and in turn degrading the diode power figure of merit. Isolation: Isolation is an important metric that determines the performance of a good limiter. It is a measure of the RF power through the limiter that doesn’t get transferred to the load when the limiter diode is in a non-conducting state (Doherty & Joos, 1998). Insertion loss: When signals less than the threshold amplitude level of the limiter are incident, they transmit through the device ideally with a very low loss known generally as the insertion loss. In an ideal case, insertion loss is zero but in most practical limiters it is typically less than 0.5 dB in a desired frequency band of operation. Leakage: The reverse-bias leakage current of the Schottky diode is also a major contributing factor in the device performance and should be minimized. Minority carrier lifetime: Minority lifetime is the average time interval between the generation and recombination of the minority carriers in the device. This brief delay in the reversal of the diode’s impedance back to its high impedance state should be minimized as any succeeding high amplitude RF signal can prove to be damaging to the following 9

circuitry. The series resistance and lifetime of a PIN diode need to be balanced with respect to each for the limiter performance as they are proportional to each other (Skyworks, 2008). Junction capacitance: The structure of heavily doped p - and n -regions and the intrinsiclayer sandwiched between them leads to the accumulation of charge at the p - and n - layer junctions. Hence, a thin layer is formed that is depleted of charge carriers and this layer acts as parallel plate capacitor. A PIN/Schottky PIN diode has a flat capacitance response as a function of reverse voltage. The small-signal insertion loss of the limiter diode depends on its junction capacitance and is given by Equation (1.1): 𝐶𝑗 𝜖𝑟 𝜖0 𝐴 𝑑 (1.1) Where Cj is the junction capacitance, εr relative permittivity, ε0 permittivity of vacuum, A the area and d the thickness of the depletion layer. The “punch-through effect” is defined as the ability of the device junction to reach its minimum capacitance when no bias is applied, and this occurs when the depletion region extends across the entire i-layer of the Schottky diode. If a certain region remains unswept by the depletion, it could act as a lossy capacitor thereby increasing the insertion loss and degrading the figure of merit (Brown, 1967). On-resistance: The on-state resistance Ron of the Schottky diode usually determines the isolation of the device. Under a shunt configuration, high-power incident RF signals need to be reflected, the device heating needs to be minimized, and the power dissipation needs to be minimized. Low Ron is hence pivotal to the limiter diode RF performance. The characteristics of the limiter are achieved through biasing. The diode on-resistance being current-dependent leads to an attenuation which is a function of the forward current; 10

saturating to the contact resistance of the diode after reaching its minimum value. The conductivity modulation leading to the low resistance can therefore be achieved by the dc bias or by the incident RF power levels. The factors discussed above are a measure of the performance of the limiter. The most widely used figure of merit of a limiter diode is the cut-off frequency FCO, given by Equation (1.2) below: 𝐹𝐶𝑂 1 2𝜋.𝑅𝑜𝑛 .𝐶𝑜𝑓𝑓 (1.2) Where Ron is the resistance of the diode in the on-state and Coff is the off-state capacitance. The dimension of this figure of merit is in frequency, which is quite suitable to estimate the high-frequency performance of these devices. This also complements the Baliga highfrequency figure of merit (BHFFOM) well (Baliga, 1989). 1.3 Conventional PIN Diode Limiters Traditional limiters available in the market today still widely make use of Si and GaAs. An X-band receiver protector made of a Si PIN diode for example presented an insertion loss of less than 1 dB for low signals under 0 V bias and an insertion loss of 34 dB at a current of just 100 mA (Leenov, 1964). Silicon PIN diodes also easily make key components for microwave control circuits as well as other limiter circuits in which ADCs require protection (Deng et al., 2020). Although GaAs PIN diodes don’t show better power handling capability than Si diodes, they started being employed in the 1980s and 1990s for better monolithic integration with various microwave circuits due to limitations of Si (Seong-Sik Yang et al., 2009). Si PIN diodes have also shown difficulties in bonding into circuits (Seymour et al., 1987). GaAs PIN diodes were perfectly suited for attenuator and 11

limiter applications due to their long transit times at high frequencies as well as their low on-resistance and diode capacitance with respect to area (Smith et al., 1999). A combination of a microstrip and a shunt-loaded monolithic molecular beam epitaxy (MBE) GaAs PIN diode limiter was designed, with a 0.2 dB small-signal insertion loss and a limiting of more than 15 dB at 10 GHz (Seymour et al., 1987). Other monolithic GaAs PIN diode attenuator circuits have shown 26 dB of variable attenuation at X-band, which can be used to significantly enhance the dynamic range of a receiver (Seymour et al., 1990). Another monolithic limiter based on 0.15 µm pHEMT GaAs process, available commercially, shows a wide bandwidth, a low-signal insertion loss of 1 dB for a frequency range 7 GHz to 12 GHz and an ability to resist up to 10 W of input power (Pham et al., 2016). 1.4 Diamond as an Alternative Commercially available limiters made of Si and GaAs diodes used in receiver protector systems currently have certain limitations at the application of high-power signals. Material technologies based on GaN, SiC and diamond have been attracting considerable interest for use in microwave control circuits and power devices, mainly due to their superior thermal conductivities, higher breakdown fields and higher threshold. SiC finds applications where high-power capability is needed such as high-level mixers or limiters (Eriksson et al., 2001). Work centered on SiC Schottky diode limiters operating at 11 GHz, with adjustable limiting power level of up to 15 dB has been shown (Bera et al., 2010). Diamond is a wide-bandgap material with high thermal conductivity (22 W/cm.K), wide band gap (5.45 eV), high breakdown field (10-20 MV/cm) and high hole saturation velocity (2.2 107 cm/s). These intrinsic properties make diamond a far superior 12

semiconductor as compared to other wide-bandgap materials like GaN and SiC and conventional ones like Si and GaAs, especially in applications that require operation at high frequencies and high-power. Table 1-1 lists the electronic properties of diamond in comparison with Si and GaAs. Table 1-1. Comparison of Intrinsic Properties of Si, GaAs and Diamond (Redrawn from (Vojs Et Al., 2008)) Property Barrier height (eV) Relative Permittivity Electron mobility (cm2/Vs) Hole mobility (cm2/Vs) Resistivity (Ω.cm) Breakdown field (V/cm) Maximum working temperature ( C) Diamond 5.45 5.5 1900 1600 1016 1 107 1000 Si 1.124 11.7 1350 480 103 3 105 225 GaAs 1.43 11.7 8800 400 109 3.5 105 470 Diamond has an approximately 60X thermal conductivity when compared to GaAs. This gives diamond Schottky diodes enhanced capability against device heating. The diamond PIN diode limiters are projected to handle more than 10x RF power when compared to Si or GaAs

Vertical diamond PIN diodes were proposed and fabricated with an n -i-p structure consisting of: a very thin and heavily phosphorus-doped n-type diamond layer and an intrinsic diamond layer grown on a heavily boron-doped diamond substrate with a (111) crystallographic orientation. Direct current (DC) and RF small-signal