Unipolar Electron Research Articles

Proton-Conductor-Gated MoS2 Transistors with Room Temperature Electron Mobility of >100 cm2 V–1 s–1

Room temperature electron mobility of >100 cm2 V–1 s–1 is achieved for a few-layer MoS2 transistor by use of a polyanionic proton conductor as the top-gate dielectric of the device. The use of a proton conductor that inherently exhibits a cationic transport number close to 1 yields unipolar electron transport in the MoS2 channel. The high mobility value is attributed to the effective formation of an electric double layer by the proton conductor, which facilitates electron injection into the MoS2 channel, and to the effective screening of the charged impurities in the vicinity of the device channel. Through careful temperature-dependent transistor and capacitor measurements, we also confirm quenching of the phonon modes in the proton-conductor-gated MoS2 channel, which should also contribute to the achieved high mobility. These devices are then used to assemble a simple resistive-load inverter logic circuit, which can be switched at high frequencies above 1 kHz.

(Semi)ladder-Type Bithiophene Imide-Based All-Acceptor Semiconductors: Synthesis, Structure-Property Correlations, and Unipolar n-Type Transistor Performance.

Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTI n and semiladder-type homopolymers PBTI n ( n = 1-5) were synthesized, and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (μe). Due to their all-acceptor backbone, these polymers all exhibit unipolar n-type transport in organic thin-film transistors, accompanied by low off-currents (10-10-10-9 A), large on/off current ratios (106), and small threshold voltages (∼15-25 V). The highest μe, up to 3.71 cm2 V-1 s-1, is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the μe drops by 2 orders to 0.014 cm2 V-1 s-1 for PBTI5. This monotonic decrease of μe was also observed in their homologous BTI n small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor-type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.

High-Performance n-Channel Organic Transistors Using High-Molecular-Weight Electron-Deficient Copolymers and Amine-Tailed Self-Assembled Monolayers.

While high-performance p-type semiconducting polymers are widely reported, their n-type counterparts are still rare in terms of quantity and quality. Here, an improved Stille polymerization protocol using chlorobenzene as the solvent and palladium(0)/copper(I) as the catalyst is developed to synthesize high-quality n-type polymers with number-average molecular weight up to 105 g mol-1 . Furthermore, by sp2 -nitrogen atoms (sp2 -N) substitution, three new n-type polymers, namely, pBTTz, pPPT, and pSNT, are synthesized, and the effect of different sp2 -N substitution positions on the device performances is studied for the first time. It is found that the incorporation of sp2 -N into the acceptor units rather than the donor units results in superior crystalline microstructures and higher electron mobilities. Furthermore, an amine-tailed self-assembled monolayer (SAM) is smoothly formed on a Si/SiO2 substrate by a simple spin-coating technique, which can facilitate the accumulation of electrons and lead to more perfect unipolar n-type transistor performances. Therefore, a remarkably high unipolar electron mobility up to 5.35 cm2 V-1 s-1 with a low threshold voltage (≈1 V) and high on/off current ratio of ≈107 is demonstrated for the pSNT-based devices, which are among the highest values for unipolar n-type semiconducting polymers.

Thiazole Imide-Based All-Acceptor Homopolymer: Achieving High-Performance Unipolar Electron Transport in Organic Thin-Film Transistors.

High-performance unipolar n-type polymer semiconductors are critical for advancing the field of organic electronics, which relies on the design and synthesis of new electron-deficient building blocks with good solubilizing capability, favorable geometry, and optimized electrical properties. Herein, two novel imide-functionalized thiazoles, 5,5'-bithiazole-4,4'-dicarboxyimide (BTzI) and 2,2'-bithiazolothienyl-4,4',10,10'-tetracarboxydiimide (DTzTI), are successfully synthesized. Single crystal analysis and physicochemical study reveal that DTzTI is an excellent building block for constructing all-acceptor homopolymers, and the resulting polymer poly(2,2'-bithiazolothienyl-4,4',10,10'-tetracarboxydiimide) (PDTzTI) exhibits unipolar n-type transport with a remarkable electron mobility (μe ) of 1.61 cm2 V-1 s-1 , low off-currents (Ioff ) of 10-10 -10-11 A, and substantial current on/off ratios (Ion /Ioff ) of 107 -108 in organic thin-film transistors. The all-acceptor homopolymer shows distinctive advantages over prevailing n-type donor-acceptor copolymers, which suffer from ambipolar transport with high Ioff s > 10-8 A and small Ion /Ioff s < 105 . The results demonstrate that the all-acceptor approach is superior to the donor-acceptor one, which results in unipolar electron transport with more ideal transistor performance characteristics.

Theoretical analysis of nBn infrared photodetectors

The depletion and surface leakage dark current suppression properties of unipolar barrier device architectures such as the nBn have been highly beneficial for III–V semiconductor-based infrared detectors. Using a one-dimensional drift-diffusion model, we theoretically examine the effects of contact doping, minority carrier lifetime, and absorber doping on the dark current characteristics of nBn detectors to explore some basic aspects of their operation. We found that in a properly designed nBn detector with highly doped excluding contacts the minority carriers are extracted to nonequilibrium levels under reverse bias in the same manner as the high operating temperature (HOT) detector structure. Longer absorber Shockley–Read–Hall (SRH) lifetimes result in lower diffusion and depletion dark currents. Higher absorber doping can also lead to lower diffusion and depletion dark currents, but the benefit should be weighted against the possibility of reduced diffusion length due to shortened SRH lifetime. We also briefly examined nBn structures with unintended minority carrier blocking barriers due to excessive n-doping in the unipolar electron barrier, or due to a positive valence band offset between the barrier and the absorber. Both types of hole blocking structures lead to higher turn-on bias, although barrier n-doping could help suppress depletion dark current.

Different operation regimes of cylindrical triode-type electron accelerator studied by PIC code simulations

AbstractThe performance of the converging electron beam generated in cylindrical triodes is systematically studied by particle-in-cell code simulations. Depending on the cathode and grid potentials applied, different operation regimes are identified. For low voltages between cathode and grid, laminar flow and homogeneous beam energy density at the target (anode) is obtained. This applies both to the case of unipolar electron flow and to bipolar flow with counter-streaming ions. Hereby, the electron emission current is enhanced by about 50% for bipolar flow compared with unipolar flow. A further increase by about 20% is obtained when electron backscattering at the target is enhanced due to a change of target material from aluminum to tungsten. For cathode-grid voltages exceeding a critical value, laminar flow is replaced by non-laminar flow regimes. For unipolar electron beams, a virtual cathode forms between grid and target, which leads to an inhomogeneous power density at the target. For the specific geometry investigated and the cathode potential fixed at −120 kV, the cathode-grid voltage threshold for the formation of the virtual cathode is ~32 kV for Al targets and ~28 kV for W targets. For bipolar flow, the laminar flow regime already ends at cathode-grid voltages of ~23 kV (Al target) and ~20 kV (W target), respectively, and is replaced by magnetic insulation at the beam edge. For increasing cathode-grid voltage, the magnetically insulated region extends until beam pinching occurs.

A comprehensive model of gain recovery due to unipolar electron transport after a short optical pulse in quantum cascade lasers

We have developed a comprehensive model of gain recovery due to unipolar electron transport after a short optical pulse in quantum cascade lasers (QCLs) that takes into account all the participating energy levels, including the continuum, in a device. This work takes into account the incoherent scattering of electrons from one energy level to another and quantum coherent tunneling from an injector level to an active region level or vice versa. In contrast to the prior work that only considered transitions to and from a limited number of bound levels, this work include transitions between all bound levels and between the bound energy levels and the continuum. We simulated an experiment of S. Liu et al., in which 438-pJ femtosecond optical pulses at the device's lasing wavelength were injected into an In0.653Ga0.348As/In0.310Al0.690As QCL structure; we found that approximately 1% of the electrons in the bound energy levels will be excited into the continuum by a pulse and that the probability that these electrons will be scattered back into bound energy levels is negligible, ∼10−4. The gain recovery that is predicted is not consistent with the experiments, indicating that one or more phenomena besides unipolar electron transport in response to a short optical pulse play an important role in the observed gain recovery.

Unipolar Electron Transport Polymers: A Thiazole Based All-Electron Acceptor Approach

Unipolar Electron Transport Polymers: A Thiazole Based All-Electron Acceptor Approach

Current gain above 10 in sub-10 nm base III-Nitride tunneling hot electron transistors with GaN/AlN emitter

We report on a tunneling hot electron transistor amplifier with common-emitter current gain greater than 10 at a collector current density in excess of 40 kA/cm2. The use of a wide-bandgap GaN/AlN (111 nm/2.5 nm) emitter was found to greatly improve injection efficiency of the emitter and reduce cold electron leakage. With an ultra-thin (8 nm) base, 93% of the injected hot electrons were collected, enabling a common-emitter current gain up to 14.5. This work improves understanding of the quasi-ballistic hot electron transport and may impact the development of high speed devices based on unipolar hot electron transport.

Organic photovoltaic performance improvement using atomic layer deposited ZnO electron-collecting layers

Inverted organic photovoltaic (OPV) cells based on poly(3-hexylthiophene (P3HT) as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor, were fabricated and characterized. To improve the photovoltaic performance, ZnO films were used as electron collection layers, while an under-stoichiometric molybdenum oxide MoOx was employed as the hole collection layer. Two types of ZnO layers were employed; one deposited by atomic layer deposition (ALD-ZnO) and another deposited using the sol–gel method (sg-ZnO). OPV cells with a 20nm thick ALD-ZnO layer exhibited significant efficiency enhancement compared with those based on the sg-ZnO layer with the same thickness. The ALD-ZnO film exhibited reduced defect/trap concentration compared with the sg-ZnO counterpart, as confirmed by steady state photoluminescence spectroscopy, showing a promising interface layer for efficient organic photovoltaic devices exhibiting also improved temporal stability. By employing capacitance–voltage measurements we were able to identify a downward shift of the conduction band edge of ALD-ZnO film (or equivalently, an upward shift of the conduction band minimum of the sg-ZnO film), verified also by ultraviolet photoelectron spectroscopy measurements. This resulted in a significant decrease in the electron extraction barrier at the ALD-ZnO/organic active layer interface, as was also demonstrated by the increased current in unipolar (electron only) devices. This work highlights the importance of using the ALD method to develop conformal and defect free ZnO electron collection layers for high performance organic photovoltaics.

Evolution of the conductivity type in germania by varying the stoichiometry

Information regarding the conductivity type of Si/GeOx/Ni structures with various stoichiometry has been obtained using experiments on injection of minority carriers from n- and p-type silicon. Results show that non-stoichiometric GeOx films exhibit bipolar conductivity, that is, holes as well as electrons contribute to the charge transport. Stoichiometric GeO2 films exhibit unipolar electron conductivity.

Long-wave type-II superlattice detectors with unipolar electron and hole barriers

The performance of a long-wave infrared type-II InAs/GaSb superlattice photodetector with a 50% cut-off wavelength of approximately 8.7 μm is presented. The ability to lower dark current densities over traditional P-type-Intrinsic-N-type diodes is offered by way of hetero-structure engineering of a pBiBn structure utilizing superlattice p-type (p) and n-type (n) contacts, an intrinsic (i) superlattice active (absorber) region, and unipolar superlattice electron and hole blocking (B) layers. The spectral response of this pBiBn detector structure was determined using a Fourier transform infrared spectrometer, and the quantum efficiency was determined using a 6250 nm narrow band filter and a 500 K blackbody source. A diode structure designed, grown, and fabricated in this study yielded a dark current density of 1.05×10−5 A/cm2 at a reverse bias of −50 mV and a specific detectivity value of greater than 1011 Jones at 77 K. Theoretical fittings of the diode dark currents at 77 K were used in this study to help isolate the contributing current components observed in the empirical dark current data. A variable temperature study (80 to 300 K) of the dark current is presented for a diode demonstrating diffusion-limited dark current down to 77 K.

“N” structure for type-II superlattice photodetectors

In the quest to raise the operating temperature and improve the detectivity of type II superlattice (T2SL) photodetectors, we introduce a design approach that we call the “N structure.” N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Unlike the symmetrical insertion of AlSb into GaSb layers, N design aims to exploit the shifting of the electron and hole wavefunctions under reverse bias. With cutoff wavelength of 4.3 μm at 77 K, temperature dependent dark current and detectivity measurements show that the dark current density is 3.6 × 10−9 A/cm2, under zero bias. Photodetector reaches background limited infrared photodetection (BLIP) condition at 125 K with the BLIP detectivity (D*BLIP) of 2.6 × 1010 Jones under 300 K background and −0.3 V bias voltage.

Predictive modeling of the current density and radiative recombination in blue polymer-based light-emitting diodes

The results of a combined experimental and modeling study of charge transport, recombination and light emission in blue organic light-emitting diodes (OLEDs) based on a polyfluorene derivative are presented. It is shown that the measured temperature-dependent current-voltage curves and the voltage-dependent current efficiency are accurately described using an OLED device model that is based on the separately determined unipolar electron and hole mobility functions. The recombination rate is calculated using the Langevin formula, including recombination of holes with free as well as trapped electrons. The light emission is obtained from the exciton formation profile using independently determined values of the exciton radiative decay probability, the average dipole orientation, and assuming a fraction of singlet excitons ηS =​(22±3)%, close to the quantum-statistical value. No additional free parameter is used. This shows that predictive one-dimensional device modeling of OLEDs is feasible.

A planar bipolar diode with relativistic electrons

The paper presents a detailed study of the characteristics of a bipolar diode with relativistic electrons and of the influence of the intrinsic azimuthal and external longitudinal magnetic fields on the adequacy of the theory of bipolar and unipolar electron flows. Questions on the urgency of the use of bipolar beams in modern devices, including plasma microwave electronic ones, are considered.

The Bipolar Field-Effect Transistor: VIII. Longitudinal Gradient of Longitudinal Electric Field (Two-MOS-Gates on Pure-Base)

This paper evaluates the electric current terms from the longitudinal gradient of the longitudinal electric field in Bipolar Field-Effect-Transistors (BiFETs) with a pure base and two MOS gates operating in the unipolar (electron) current mode. These nMOS-BiFETs, known as nMOS-FinFETs, usually have electrically short channels compared with their intrinsic Debye length of about 25 μm at room temperatures. These longitudinal electric current terms are important short-channel current components, which have been neglected in the computation of the long-channel electrical characteristics. This paper shows that the long-channel electrical characteristics are substantially modified by the longitudinal electrical current terms when the physical channel length is less than 100 nm.

The Bipolar Field-Effect Transistor: VII. The Unipolar Current Mode for Analog-RF Operation (Two-MOS-Gates on Pure-Base)

This paper reports the DC steady-state current–voltage and conductance–voltage characteristics of a Bipolar Field-Effect Transistor (BiFET) under the unipolar (electron) current mode of operation, with bipolar (electron and hole) charge distributions considered. The model BiFET example presented has two MOS-gates on the two surfaces of a thin pure silicon base layer with electron and hole contacts on both edges of the thin base. The hole contacts on both edges of the thin pure base layer are grounded to give zero hole current. This 1-transistor analog-RF Basic Building Block nMOS amplifier circuit, operated in the unipolar current mode, complements the 1-transistor digital Basic Build Block CMOS voltage inverter circuit, operated in the bipolar-current mode just presented by us.

Sensing electron transport in a blue-emitting copolymer by transient electroluminescence

A variation of the transient electroluminescence technique is introduced which allows us to selectively study the electron transport in a thin polymer layer. It relies on the formation of an insoluble interlayer from a formerly solvable polymer and enables probing of unipolar electron transport despite of injection barriers. It opens up possibilities to gain insight into the operation of light-emitting diodes. Applicability to a blue-emitting spirobifluorene-based copolymer is shown by comparison to time-of-flight results for electron and hole transport and evidence supplied for an intermixing of electron and hole dynamics through blocking of electrons at the polymer/anode interface.

Space-charge-limited bipolar flow in a nano-gap

This paper presents a quantum model of space-charge limited (SCL) bipolar flow in a nano-sized planar gap, including the effects of electron tunneling and exchange-correlation. It is found that the classical scaling of the SCL bipolar flow is no longer valid when the gap spacing D is comparable or smaller than the electron’s de Broglie wavelength at gap voltage Vg. The classical value of the SCL bipolar electron flow is greatly enhanced due to the electron tunneling through the space-charge electric potential created by both the electrons and ions. The space-charge effect of ions is less significant (compared to electron tunneling) in the deep quantum regime that the quantum SCL bipolar flow is nearly identical to the unipolar electron flow (or quantum Child-Langmuir law).

Architectures for efficient electrophosphorescent organic light-emitting devices

We discuss several device architectures leading to high-efficiency organic electrophosphorescent (EP) light emission. An external electroluminescence efficiency (/spl eta//sub ext/) of (10.0 /spl plusmn/ 0.5) % was realized by doping fac-tris(2-phenylpyridine)iridium (Ir(ppy)/sub 3/) into a 2,9-dimethyl-4,7-diphenyl-1,10-phenenthroline (BCP) electron transport layer. Direct exciton formation on the phosphor dopant avoids exciplex formation at the interface of unipolar hole and electron transport layers. Further, triplet exciton and carrier dynamics in a double heterostructure were investigated to determine the location and width of the exciton formation zone. High-efficiency EP is also demonstrated in a simplified two layer architecture using a 4,4'-N, N'-dicarbazole-biphenyl (CBP) ambipolar carrier transport host.