Yu-Hsin Liu

Discovery of a photoresponse amplification mechanism in compensated PN junctions

We report the experimental evidence of uncovering a photoresponse amplification mechanism in heavily doped, partially compensated silicon p-n junctions under very low bias voltage. We show that the observed photocurrent gain occurs at a bias that is more than an order of magnitude below the threshold voltage for conventional impact ionization. Moreover, contrary to the case of avalanche detectors and p-i-n diodes, the amplified photoresponse is enhanced rather than suppressed with increasing temperature. These distinctive characteristics lead us to hypothesize that the inelastic scattering between energetic electrons (holes) and the ionized impurities in the depletion and charge neutral regions of the p-n junction in a cyclic manner plays a significant role in the amplification process. Such an internal signal amplification mechanism, which occurs at much lower bias than impact ionization and favors room temperature over cryogenic temperature, makes it promising for practical device applications.

High efficiency silicon 1310 nm detector without defect states or heteroepitaxy

The Si community has continued to seek low cost, fully complementary metal-oxide-semiconductor compatible optical detection techniques to overcome the interconnect bottleneck facing the electronics world. We demonstrate high internal quantum efficiency 1310 nm detectors using entirely the properties of Si crystal by employing homojunction band structure engineering to tailor the optoelectronic properties of the material. Nearly 100% internal detector quantum efficiency has been obtained. The device concept may find broad applications benefiting from the extended spectral response beyond the limit of bandgap, especially the limit associated with indirect bandgap of the material.

Single photon avalanche detectors: prospects of new quenching and gain mechanisms

Abstract While silicon single-photon avalanche diodes (SPAD) have reached very high detection efficiency and timing resolution, their use in fibre-optic communications, optical free space communications, and infrared sensing and imaging remains limited. III-V compounds including InGaAs and InP are the prevalent materials for 1550 nm light detection. However, even the most sensitive 1550 nm photoreceivers in optical communication have a sensitivity limit of a few hundred photons. Today, the only viable approach to achieve single-photon sensitivity at 1550 nm wavelength from semiconductor devices is to operate the avalanche detectors in Geiger mode, essentially trading dynamic range and speed for sensitivity. As material properties limit the performance of Ge and III-V detectors, new conceptual insight with regard to novel quenching and gain mechanisms could potentially address the performance limitations of III-V SPADs. Novel designs that utilise internal self-quenching and negative feedback can be used to harness the sensitivity of single-photon detectors,while drastically reducing the device complexity and increasing the level of integration. Incorporation of multiple gain mechanisms, together with self-quenching and built-in negative feedback, into a single device also hold promise for a new type of detector with single-photon sensitivity and large dynamic range.

Cycling excitation process: An ultra efficient and quiet signal amplification mechanism in semiconductor

Signal amplification, performed by transistor amplifiers with its merit rated by the efficiency and noise characteristics, is ubiquitous in all electronic systems. Because of transistor thermal noise, an intrinsic signal amplification mechanism, impact ionization was sought after to complement the limits of transistor amplifiers. However, due to the high operation voltage (30-200 V typically), low power efficiency, limited scalability, and, above all, rapidly increasing excess noise with amplification factor, impact ionization has been out of favor for most electronic systems except for a few applications such as avalanche photodetectors and single-photon Geiger detectors. Here, we report an internal signal amplification mechanism based on the principle of the phonon-assisted cycling excitation process (CEP). Si devices using this concept show ultrahigh gain, low operation voltage, CMOS compatibility, and, above all, quantum limit noise performance that is 30 times lower than devices using impact ionization. Established on a unique physical effect of attractive properties, CEP-based devices can potentially revolutionize the fields of semiconductor electronics.

A high-efficiency low-noise signal amplification mechanism for photodetectors

Recently we discovered a signal amplification mechanism to amplify photocurrent with high efficiency and low noise. Unlike conventional impact ionization used in avalanche photodetectors, the new amplification mechanism can produce high (>1000) gain with very low excess noise factor (<2 for Si) under very low bias voltage (3V). The new amplification mechanism offers a promising solution for light detection for Si-photonics, imaging, and sensing. Physics of this mechanism lies in two subsequent processes i) Auger excitation between mobile and highly localized electrons and ii) electron-phonon coupling. In this paper, experimental results are supported by the proposed physical model using simulations within density functional theory (DFT) framework.

An amorphous silicon photodiode with 2 THz gain‐bandwidth product based on cycling excitation process

Since impact ionization was observed in semiconductors over half a century ago, avalanche photodiodes (APDs) using impact ionization in a fashion of chain reaction have been the most sensitive semiconductor photodetectors. However, APDs have relatively high excess noise, a limited gain-bandwidth product, and high operation voltage, presenting a need for alternative signal amplification mechanisms of superior properties. As an amplification mechanism, the cycling excitation process (CEP) was recently reported in a silicon p-n junction with subtle control and balance of the impurity levels and profiles. Realizing that CEP effect depends on Auger excitation involving localized states, we made the counter intuitive hypothesis that disordered materials, such as amorphous silicon, with their abundant localized states, can produce strong CEP effects with high gain and speed at low noise, despite their extremely low mobility and large number of defects. Here, we demonstrate an amorphous silicon low noise photodio...

Cycling excitation process for light detection and signal amplification in semiconductors

An intrinsic signal amplification mechanism, namely cycling excitation process (CEP), has been demonstrated in a heavily doped and heavily compensated silicon p-n junction diode. The physical process amplifies photo-generated signal at low bias (<5V) and produces ultralow excess noise at least partially attributed to an internal stabilization mechanism via electron-phonon interactions. Auger excitation, which can be calculated with Fermi Golden rule and quasi pseudopotential, and localized carrier ionization by phonon absorption are considered two key processes responsible for the unique device characteristics. A partially compensated p-n junction silicon diode based on the proposed CEP principle has shown high gain of ~6000 at -5V and an excess noise factor as low as 3.5 at this gain level, measured at 635nm wavelength and 1KHz for potential imaging applications.

A new signal-amplification mechanism discovered in semiconductors

Preamplifiers (i.e., electronic devices that amplify signals) are required in optical imaging and detection systems to increase weak current signals.1 If the detector itself can produce sufficient gain, however, the sensitivity of such devices may be able to overcome the limitations that are imposed by the thermal noise of electronics. An internal amplification mechanism (i.e., impact ionization) has been used in photodetection for decades. In an avalanche photodiode—a reverse-biased p-n junction device that is operated at a voltage close to breakdown voltage,2, 3 APD—an ionization collision with the lattice—occurs when the photogenerated primary carriers acquire enough energy: see Figure 1(a). Secondary electron-hole (e-h) pairs are produced from this collision, which in turn cause additional ionization collisions as the pairs cross the depletion region (i.e., the ‘avalanche’ process). APD-based photoreceivers achieve sufficient sensitivity for fiber-optic communications. However, they require a high operation voltage (over 20V) and suffer from high excess noise with increasing gain. In devices with internal gain, interference originates mainly from shot noise that is amplified with the signal.4 The noise of these systems is best characterized by the excess noise factor (ENF), which is calculated from the fluctuation of the amplification gain. In our work, we are proposing a new internal amplification mechanism called the cycling excitation process (CEP). This process relies on the transitions involving localized states, which are formed via dopant compensation within a p-n junction diode. The Coulomb interactions that occur between energetic carriers and these localized states have stronger efficiency Figure 1. Schematic illustration of (a) the avalanche process and (b) the cycling excitation process (CEP). The former is based on impact ionization between the hot and the bound electron in the valance band. In contrast, CEP occurs as a result of the Auger process between a hot electron and an electron in the localized state in the dopant within the n-type region. Eg : Energy bandgap. 0: Primary carrier from direct photo absorption. 1: Carrier produced by Auger excitation.

Approaching the Quantum Limit of Photodetection in Solid-State Photodetectors

Many aspects of life involve sensitive photodetection, which is now widely implemented in solid-state devices made from semiconductor materials due to their relatively low cost, high scalability, and better compatibility with the existing CMOS technology. State-of-the-art Geiger-mode avalanche detectors and the challenges they faced in single-photon detection efficiency and timing resolution are covered. Emerging classes of solid-state detectors are then reviewed, including bandgap engineering and the use of a heavily doped emitter. A novel class of detector is introduced exhibiting monolithic incorporation of impact ionization and bipolar gain in a single device structure by using bandgap engineering. Finally, we discuss a recent detector concept which involves using disorder in solid-state material to substantially increase the probability of carrier excitation and reduce noise. The recent breakthroughs in solid-state photodetection can significantly impact areas of quantum information processing, optic communications, and imaging.

An ultra-efficient internal mechanism to amplify photoresponse for Si and compound semiconductor devices

We have demonstrated a light detection mechanism, cycling excitation process (CEP) using a heavily doped and heavily compensated silicon p-n junction that can amplify the photoresponse at low bias and produces ultra low excess noise. The CEP effect has been observed in both Si and GaAs, showing that it may exist in a large family of semiconductors.