Nonlinearity of p-i-n photodetectors

At higher operating frequencies, the field dependence of the carrier velocity in p-i-n photodetectors generates harmonics and intermodulation products that can degrade the dynamic range of RF fiber-optic links. The authors present both a perturbational theory and measured harmonic data for a p-i-n photodiode operated at very high power densities which show that this and other detector nonlinear effects need not seriously compromise link performance. In particular, neither transit-time nor static nonlinearities in p-i-n photodiodes need limit the dynamic range of fiber-optic links operating below 5 GHz. The fact that the theoretical bandwidth of the photodiode, with all parasitic capacitance and inductance ideally removed, is 17 GHz, suggests that comparable spur-free performance should be achievable at X and Ku-band frequencies, once packaging parasitics are reduced     

Nitride-based p-i-n bandpass photodetectors

Nitride-based p-i-n bandpass photodetectors with semitransparent Ni-Au electrodes were successfully fabricated and characterized. The photodetectors exhibit a 20-V breakdown voltage and a small dark current of 40 pA at 4-V reverse bias. It was found that spectral responsivity shows a narrow bandpass characteristics from 337 to 365 nm. Moreover, the peak responsivity was estimated to be 0.13 A/W at 354 nm, corresponding to a quantum efficiency of 44%. The relatively high response at shorter wavelength is due to the unoptimized thickness of p-Al/sub 0.1/Ga/sub 0.9/N absorption layer. At low frequency, the noise of the photodetector is dominant by the 1/f-type noise. For our 330/spl times/330 /spl mu/m/sup 2/ device, given a bias of -3.18 V, the corresponding noise equivalent power and normalized detectivity D/sup */ are calculated to be 5.6/spl times/10/sup -12/ W and 3.34/spl times/10/sup 11/ cmHz/sup 0.5/ W/sup -1/, respectively.     

Ultrawide-band long-wavelength p-i-n photodetectors

We compare different designs for very high-speed (millimeter-wave) long-wavelength photodetectors, different materials for such detectors, and different ways of characterizing the speed of these devices. Experimental results are given, showing high-speed response with bandwidths beyond 50 GHz, impulse responses less than 10 ps, and detection sensitivities to 0.1 fJ in packaged devices. Discussed is the inherent bandwidth-efficiency limit in conventional p-i-n detectors, which is then compared to theoretical and experimental results for waveguide-geometry detectors.     

Nonlinearities in p-i-n microwave photodetectors

The nonlinearities in p-i-n photodetectors have been measured and numerically modeled. Harmonic distortion measurements were made with two single-frequency offset-phased-locked Nd:YAG lasers which provide a source dynamic range greater than 130 dB and a 1 MHz-50 GHz frequency range. Carrier transport is analytically described by three coupled nonlinear differential equations, Poisson's equation and the hole and electron continuity equations. These equations are numerically solved to investigate and isolate the various nonlinear mechanisms. The numerical solution incorporates diffusion since our treatment includes carrier generation in the highly doped p-region of the device. This p-region absorption and carrier-dependent carrier velocities associated with a perturbed electric field (due to space-charge and loading effects) are shown to dominate photodetector nonlinear behavior. The numerical model was extended to predict that maximum photodetector currents of 100 mA should be possible in 20 GHz bandwidth devices before a sharp increase in nonlinear output occurs. In addition, second harmonic distortion improvements of 40-60 dB may be possible if photodetectors can be fabricated with strictly-depleted absorbing regions     

Silicon p-i-n photodetectors with integrated transistor amplifiers

A novel silicon photodetector is presented, incorporating transistor gain in a p-i-n photodiode and its performance is analyzed. Gain and noise power frequency characteristics are analytically derived in terms of an equivalent circuit. These analytical results are in good agreement with experimental results. Noise power is estimated and it is shown that, in the video frequency range, the S/N ratio is far superior to that of APD for relatively large signal levels. Detector operational features are described.     

Speed and efficiency in multiple p-i-n photodetectors

We present a theoretical evaluation of the application of multiple intrinsic layers to enhancing the quantum efficiency of high-speed p-i-n photodetectors. It is shown that the use of multiple layers may lead to substantial improvements in the efficiency of detectors operating in the 20-100-GHz range, provided that the device area is reduced to limit the intrinsic capacitance and special care is taken to avoid large parasitic effects. Potential fabrication schemes are discussed     

4H-SiC p-i-n紫外光电探测器的电容-电压特性研究

分析并比较了4H-SiC p-i-n紫外光电探测器的电容-电压（C-V）特性随温度和偏置电压的变化情况,观测到4H-SiC p-i-n结构中的深能级缺陷。结果表明：由于近零偏压时探测器i型层已处于耗尽状态,其高频（1 MHz）C-V特性几乎不随反向偏压变化,随着温度升高,被热离化的自由载流子数量增多导致高频结电容随之增大;探测器的低频（100 kHz）结电容比高频结电容具有更强的电压和温度依赖性,原因在于被深能级缺陷俘获的载流子随反向偏压增大或随温度升高而被离化,从而对结电容产生影响。     

Optimization of high-speed metal-semiconductor-metal photodetectors

Circular-aperture Metal-Semiconductor-Metal (MSM) photodetectors have been fabricated in order to decrease the device capacitance. The frequency-response of the MSM photodetectors has been calculated using a first-order approximation. The design of the circular-aperture detectors and conventional square-aperture detectors has been optimized using this approximation. The largest 3 dB bandwidths are obtained with 33 fingers for the circular detectors and 29 for the square detectors. These numbers do not change regardless of the size of the MSM photodetectors     

An analytical small-signal bias-dependent nonuniform model for p-i-n traveling-wave photodetectors

A fully analytical small-signal model is developed for the frequency response of traveling-wave photodetectors. It takes into account the dependence of the equivalent transmission-line admittance on the position, induced by the nonuniform distribution of the optical beam along the traveling direction. Moreover, the influence of the bias voltage on the transit time has been accurately investigated. The model is applied to the design of an InAlAs-InGaAs p-i-n photodetector. Its performances are investigated in term of electrical bandwidth.     

Room temperature unpassivated InAs p-i-n photodetectors grown bymolecular beam epitaxy

An unpassivated InAs p-i-n photodetector with excellent performance at room temperature was demonstrated. The zero-bias resistance area products of the diode with 720-nm thick i-layer are 8.1 Ω-cm² at room temperature and as high as 1.3 M Ω-cm² at 77 K. At 77 K, the diode exhibits a breakdown voltage exceeding 13 V. When tested under a 500 K blackbody source, the measured detectivity limited by Johnson noise is 1.2×10¹⁰ cm-Hz^½/W at room temperature and 8.1×10 ¹¹ cm-Hz^½/W at 77 K. To our knowledge, this is the best data for a room temperature infrared detector     

Tunable superlattice p-i-n photodetectors: characteristics, theory,and application

Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response     

Large-signal response of p-i-n photodetectors using short pulses with small spot sizes

In this paper, the response of a surface-normal p-i-n photodetector that was incorporated in an optically controlled optical gate was analyzed using short-pulsed, high-energy optical inputs with small spot sizes. Simulation results based in part on diffusive conduction were compared against the experimental data, providing an understanding of the device response. Results also demonstrated that p-i-n photodetectors may be designed so that reducing the spot size, counterintuitively, results in negligible additional field screening.     

Very high-speed ultraviolet photodetectors fabricated on GaN

We report on the temporal and the frequency response of both metal-semiconductor-metal (MSM) and p-i-n ultraviolet photodetectors fabricated on single-crystal GaN. The best MSM devices show a fast 10–90% rise-time of ∼28 psec under comparatively low ultraviolet excitation of ∼0.1 W/cm² averagerirradiance. The fast-Fourier transform (FFT) of the pulse response data indicates a bandwidth, f_3dB, of ∼3.8 GHz at a reverse bias of 25 V. This agrees well with the direct frequency response measurement value of ∼3.5 GHz. For the p-i-n devices, we measured a rise-time of ∼43 psec at 15 V reverse bias for a 60 μm diameter mesa with 1 μm thick intrinsic region. The FFT of the p-i-n pulse response obtains f_3dB ≈1.4 GHz. Analysis in terms of reverse bias and geometric scaling indicates that the MSM photodetectors are transit-time limited. The p-i-n devices also show evidence of transit-time limited effects based on trends with respect to reverse bias and intrinsic region thickness. However, our larger area p-i-n devices show clear evidence of RC-limited behavior. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity. We have also found preliminary evidence of microplasmic effects in the p-i-n devices.     

A model for the performance analysis and design of waveguide p-i-n photodetectors

A model to investigate the frequency response of waveguide (WG) p-i-n photodetectors (PDs) is presented. The model is based on the 2-D position-dependent distribution of carriers in the device and the results show that the contribution to the total current mainly comes from a small length of the PD measured from its input end. The effect of carrier trapping at a heterointerface has also been considered to study the frequency dependence of the photocurrent at low-bias voltages. The frequency response and bandwidth obtained from the model are in good agreement with published experimental results. A simplified and approximate relation for the fiber-to-WG coupling efficiency has also been used to calculate the overall quantum-efficiency of WGPDs. Then, the effect of WG geometry on the bandwidth-quantum efficiency product has been analyzed and some results on the optimum design of a WG p-i-n PD for maximum bandwidth-quantum efficiency product have been tabulated.     

Comparison of GaN and 6H-SiC p-i-n photodetectors with excellentultraviolet sensitivity and selectivity

We fabricated GaN and 6H-SiC p-i-n photodetectors and compared their electrical and optical characteristics. The GaN diodes suffered from significant leakage current of 37 μA/mm² at -5 V, while the SiC diode leakage current was below the noise level at 10 pA/mm² at -20 V. The built-in potentials and the unintentional “i-layer” doping densities were obtained from capacitance-voltage (C-V) measurements. The SiC detectors exhibited a broad spectral response in contrast to the abrupt cutoff observed in the GaN detectors. The peak responsivities of the GaN and SiC photodetectors corresponded to internal quantum efficiencies of 57% at 3.42 eV and 82% at 4.49 eV, respectively. Furthermore, both detectors exhibited excellent visible rejection ratios which is needed for solar-blind applications. The response times at zero bias were 18 and 102 ns for the GaN and SiC detectors, respectively     

Cryogenic performance of a high-speed GaInAs/InP p-i-n photodiode

The cryogenic performance of a high-speed GaInAs/InP p-i-n photodiode, with graded bandgap layers at the heterostructure interfaces, was investigated for the first time. DC measurements show that the dark current of the diode decreases sharply as the temperature decreases from 300 to 200 K. A factor of 1000 in dark current reduction was found for this photodiode, when it was cooled from room temperature to about 150 K. Similar modulation bandwidths were found for this device for temperatures between 9 and 300 K, with a bandwidth greater than 20 GHz. No degradation was found in performance at cryogenic temperature compared to room temperature. This enables direct integration of high-speed photodiodes with superconductive and other cryogenic electronics     

Easily manufactured high-speed back-illuminated GaInAs/InP p-i-n photodiode

A back-illuminated planar GaInAs/InP p-i-n photodiode has been fabricated with a simple fabrication process to obtain a high-speed detector. The photodiode has a capacitance as low as 54 fF, a dark current of about 3 pA, and a quantum efficiency of 74% at a 1.55- mu m wavelength. A cutoff frequency of 31 GHz was obtained when the photocurrent was about 500 mu A and the bias voltage was -10 V.<>     

Measurement of IP3 in p-i-n photodetectors and proposed performancerequirements for RF fiber-optic links

The nonlinearities generated by wide-band, high-current photodetectors are discussed in the context of the performance of an externally modulated analog radio-frequency fiber-optic link. The quantitative impart of the detector's third-order intermodulation product intercept (IP3) on the link's spur-free dynamic range is analyzed. Waveguide p-i-n photodiodes were developed to specifically address these stringent IP3 goals. The IP3 was measured using a four laser heterodyne system resulting in an IP3 of 27 dBm measured at a center frequency of 20 GHz with 20 mA of de photocurrent. The authors believe that this is the highest photodetector IP3 reported to date at these operating conditions     

High-performance p-i-n Ge on Si photodetectors for the nearinfrared: from model to demonstration

We have investigated the integration of Ge p-i-n and n-i-p heterojunction photodiodes on Si. Recognizing the crucial role of interface defects at the Ge-Si interface on the performance of photodetectors, we have designed and fabricated high-performance n-i-p Ge photodiodes on p⁺-Si substrates. These photodiodes exhibit short-circuit responsivities of 0.3 and 0.2 A/W at 1.3 and 1.55 μm, respectively, reverse dark currents of 20 mA/cm² and response times of 800 ps     

Electron and hole traps in GaN p-i-n photodetectors grown by reactive molecular beam epitaxy

GaN p-i-n photodetectors grown on sapphire by reactive molecular beam epitaxy have been characterized by measurements of room-temperature current-voltage (I-V), temperature-dependent capacitance (C-V-T), and deep level transient spectroscopy (DLTS) under both majority and minority carrier injection. Due to what we believe to be threading dislocations, the reverse I-V curves of p-i-n photodetectors show typical electric-field enhanced soft breakdown characteristics. A carrier freeze-out due to the de-ionization of Mg-related deep acceptors has been found by C-V-T measurements. Three electron traps, B (0.61 eV), D (0.23 eV), and E₁ (0.25 eV) and one hole trap, H₃ (0.79 eV) have been revealed by DLTS measurements. The photodetectors with lower leakage currents usually show higher responsivity and lower trap densities of D and E₁.