Majority Carrier Injection Research Articles

Eliminating Leakage Current in Thin‐Film Transistor of Solution‐Processed Organic Material Stack for Large‐Scale Low‐Power Integration

AbstractFor organic thin‐film transistors (OTFTs) made of solution processed stacks of organic semiconductor and dielectric materials, it is a grand challenge to eliminate the leakage current paths. With a top‐gate bottom‐contact structure, this work introduces a strong dipole interfacial layer made of self‐assembled monolayer (SAM) molecules at metal‐semiconductor contacts to suppress minority carrier injection for low leakage and stable operation, while not affecting majority carrier injection. Both gate insulator (GI) leakage and parasitic leakage in the device architecture are also effectively suppressed with a sputtering‐resistant polymer GI layer and photolith patterned OSC islands, respectively. The devices present a decent mobility with a typical value of 1.98 cm2 V–1 s–1, record‐low leakage current at 10–18 A µm−1 and large ON/OFF ratio (>1010) in a wide gate voltage range (100 V), reaching the theoretical limit and also the best level of inorganic counterparts despite much lower processing temperature (120 °C). Manufacturability of the material stack is verified on a 200 mm × 200 mm substrate and the fabricated 4.7 in. active‐matrix organic light‐emitting diode display, integrating more than 150 000 OTFTs, can be operated at ultra‐low frame‐rate (0.1 Hz) for power saving.

(Invited) Fermi Level Pinning Controls Photochemical Charge Separation in Particles of n-SrTiO3, n-SrTiO3:Al, and p-GaAs:Te

The space charge region (SCR) is a majority-carrier-depleted region near the interface of a semiconductor. The width of this region and the size of the potential drop across it influence the charge separation at the semiconductor interface. The space charge region is difficult to evaluate for photocatalyst particles due to their small dimensions and poor electric contact. Surface Photovoltage Spectroscopy (SPS) is a highly sensitive spectroscopic technique which can be used to monitor charge transfer and polarization in particle films. Here we apply SPS to observe the SCR in thin films of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) particles on gold substrates. Negative photovoltages correspond to light-induced majority carrier injection into the gold substrate while positive photovoltages correspond to majority carrier repulsion away from the particle-gold interface. From the inverted photovoltage size, potential barriers are estimated between 0.005 V and 0.189 V and SCR widths between 2.4 μm and 2.8 μm. The small size of the observed barriers and the large SCR widths are attributed to charge trapping in particle surface states (Fermi Level pinning), likely from chemisorbed oxygen. This suggests that depletion layers in particles are mainly controlled by surface states. Figure 1

Impacts of Minority Charge Carrier Injection on the Negative Capacitance, Steady‐State Current, and Transient Current of a Single‐Layer Organic Semiconductor Device

AbstractSpace charge limited current theory is often used to analyze electrical characteristics of single‐layer organic semiconductor devices using Ohmic and Schottky contacts for injection of majority and minority carriers, respectively. However, significant deviations from this theory in the current density–voltage and capacitance–frequency characteristics of the devices have been frequently observed. For instance, the increase in current density is not proportional to the voltage‐square and the capacitance becomes negative at low frequencies. Here, it is presented that the minority carriers in a single‐layer device despite a large Schottky barrier give rise to the negative capacitance at low frequencies and the drastic increase in steady‐state current density at high voltages. The leaky electrons at the hole‐dominant device are demonstrated by inserting an electron scavenger layer. The transient current of the device shows the injection and transport properties of electrons by low and dispersive mobility. The negative capacitance of the device at low frequencies is a response of the minority carrier injection. The current density increases drastically at high voltages due to the increasing minority carriers that redistribute the electric field, which is not shown at low voltages. Finally, the device model incorporating minority charge carriers fully describes the electrical characteristics of the device.

Bias Tunable Photocurrent in Metal-Insulator-Semiconductor Heterostructures with Photoresponse Enhanced by Carbon Nanotubes.

Metal-insulator-semiconductor-insulator-metal (MISIM) heterostructures, with rectifying current-voltage characteristics and photosensitivity in the visible and near-infrared spectra, are fabricated and studied. It is shown that the photocurrent can be enhanced by adding a multi-walled carbon nanotube film in the contact region to achieve a responsivity higher than under incandescent light of . The optoelectrical characteristics of the MISIM heterostructures are investigated at lower and higher biases and are explained by a band model based on two asymmetric back-to-back Schottky barriers. The forward current of the heterojunctions is due to majority-carrier injection over the lower barrier, while the reverse current exhibits two different conduction regimes corresponding to the diffusion of thermal/photo generated carriers and majority-carrier tunneling through the higher Schottky barrier. The two conduction regimes in reverse bias generate two plateaus, over which the photocurrent increases linearly with the light intensity that endows the detector with bias-controlled photocurrent.

On the Quasi-Saturation in State-of-the-Art Power MOSFETs

The quasi-saturation (QS) effect in a power MOSFET has been discovered several decades ago. As the field has evolved and the dimensions in power MOSFETs have changed from tens of micrometers to submicrometer levels, and the gate geometry has evolved from planar to trench, the origin and the effect of the QS need to be re-assessed. This letter first discusses the origin of the QS effect in the state-of-the-art power MOSFETs and subsequently analyses the phenomenon of majority carrier injection (or diffusion) and its contribution to the increased carrier concentration in the drift region, specific to quasi-saturation.

(Invited) Quantum Confinement Controls Effective Band Gap, Photocatalytic H2 Evolution and Photovoltage in CdSe Nanocrystals

Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor–solution interface. We find that the hydrogen evolution rate from suspended mercaptoethanol-ligated CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler–Volmer equation for reaction kinetics. In parallel, Surface photovoltage spectroscopy (SPS) was used to photochemical charge transfer at the CdSe quantum dot -indium doped tin oxide (ITO) interface. Negative photovoltages are observed under super band gap illumination from majority carrier (electron) injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (−125 to −750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe–ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron–hole (polaron) pairs that are polarized by a space charge layer at the CdSe–ITO boundary. The space charge layer is 70–150 nm wide, based on thickness-dependent photovoltage measurements. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction and electron transfer with semiconductor nanocrystals. Figure 1

Effects of asymmetric Schottky contacts on photoresponse in tungsten diselenide (WSe2) phototransistor

We present an investigation of the effects of asymmetric contacts on the photoresponse of a thin film tungsten diselenide (WSe2) phototransistor. We observe different scenarios in photoresponse during gate modulation depending on the metal-semiconductor contacts through which majority carrier (hole) injection occurs. Under illumination, a peak in drain current is observed during gate modulation when hole injection occurs from the higher Schottky barrier contact. On the other hand, regular behavior in photoresponse during gate modulation is observed when hole injection occurs from the opposite direction, the lower Schottky barrier contact. Further, we analyze the possibilities of realizing WSe2 phototransistors with improved performance in terms of responsivity, response time, and detectivity by utilizing asymmetric contact engineering and proper gating. In addition, an interesting shift of the aforementioned peak is detected, with increasing incident light intensity during gate modulation. We demonstrate that this peak shift can be explained by the photogating effect caused by trapped charges.

Use of Surface Photovoltage Spectroscopy to Measure Built-in Voltage, Space Charge Layer Width, and Effective Band Gap in CdSe Quantum Dot Films.

Surface photovoltage spectroscopy (SPS) was used to study the photochemistry of mercaptoethanol-ligated CdSe quantum dot (2.0-4.2 nm diameter) films on indium doped tin oxide (ITO) in the absence of an external bias or electrolyte. The n-type films generate negative voltages under super band gap illumination (0.1-0.5 mW cm(-2)) by majority carrier injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (-125 to -750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe-ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron-hole (polaron) pairs that are polarized by a space charge layer at the CdSe-ITO boundary. The space charge layer is 70-150 nm wide, based on thickness-dependent photovoltage measurements. The ability of SPS to directly measure built-in voltages, space charge layer thickness, sub-band gap states, and effective band gaps in drop-cast quantum dot films aids the understanding of photochemical charge transport in quantum dot solar cells.

A recombination center in p-type GaAsN grown by chemical beam epitaxy

The double carrier pulse deep level transient spectroscopy (DLTS) technique is used to characterize recombination centers in p-type GaAsN grown by chemical beam epitaxy. The DLTS peak height of a shallow hole trap H1, at 0.052 eV from the valence band edge of GaAsN, decreases by the injection of both majority and minority carriers for various values of voltage and duration of the second pulse. Although the trap is shallow, its capture cross section is relatively large to capture electrons and holes. The decrease in the number of traps is explained by the electron–hole recombination process. This confirms, for the first time, that H1 is a recombination center in p-type GaAsN.

Study of minority carrier injection phenomenon on Schottky and plasma deposited p–n junction diodes

Both temperature dependent ac capacitance ( C) together with ac conductance ( G) under various frequencies ( ω) and dc current ( I) measurements as functions of bias ( V) were employed to investigate majority as well as minority carrier injection phenomenon on chromium-p-type crystalline silicon (Cr/p-cryst. Si/ohmic contact) Schottky and aluminum/plasma deposited n-type hydrogenated amorphous silicon/p-c-Si (Al/n-a-Si:H/p-c-Si/ohmic contact) p–n structures. Ambipolar transport process was eventual for both diodes and behaved differently according to the mentioned techniques: for a well defined forward bias interval, there was an increase in capacitance towards maximum that interpreted as majority carrier injection from the back electrode. On the other side, minority carrier injection from front electrode begun when the bias went beyond the critical voltage (i.e., built-in voltage) that corresponded to the peak position and caused sharp decrease in measured capacitance, leading to observation of a hump in C– V measurement. Moreover, shape and peak position of the hump were frequency/temperature dependent. Remarkably, in the bias range where capacitance pronounced, space charge limited current (SCLC) was discerned as enrolled carrier flow mechanism according to I– V measurement, confirming further majority carrier injection. In addition, both dc conductivity and ac conductance followed the same dependence with bias voltage in forward direction. On the other side, reverse current seemed proportional to square root of reverse bias, implying generation current in the depletion region. These experimental identifications convinced us of the existence of conductivity modulation issue for the structures at hand.

Investigation on surface damage phenomena induced by flashover across semiconductor

The flashover across semiconducting materials disables many high power semiconductor devices from their further application under high electric field. Up to now, the physical mechanism of the flashover was still not understood clearly. Surface flashover experiments of silicon and gallium arsenide were performed under pulsed high voltage. The filament current channels on their surface were observed in the infrared photographs, and the infrared radiation bright spot was found in the central region between the electrodes. On the surface of different samples undergone flashover events for several times, the filament damage phenomena were observed, and for the n100 silicon sample, some circular pits were found around the filament channel, with a conical jut in the center of each pit. According to the damage phenomena at the edge of electrodes, the thermal injection and relaxation characteristics of majority carriers were discussed. An injection model induced by minority carriers was proposed, which emphasizes the surface field enhancement by non-equilibrium carrier channel, which possibly leads to the ionization of the ambient gas above the filament channel. The model was consistent with the observed experimental results.

Impact of hot-carrier stress on gate-induced floating body effects and drain current transients of thin gate oxide partially depleted SOI nMOSFETs

The impact of hot-carrier (HC) stress on thin gate oxide PD SOI nMOSFETs is investigated by analyzing the front and back channel current–voltage characteristics and the switch-off drain current transients. A particular hot-carrier degradation mode, characterized by a turn-around behavior for front gate threshold voltage degradation, is analyzed for devices with different geometries, bias conditions and source/drain architecture. A significant positive back gate threshold voltage shift is also observed after long HC stress. The maximum hot-carrier degradation (HCD) is obtained for the highest front gate biases, corresponding to the conditions of maximum electron valence band injection of majority carriers into the floating body. The presence of the HCD is found to reduce both the generation and the recombination switch-off drain current transient times. Unbiased thermal annealing in the range of 200–250 °C significantly reduces the hot-carrier-induced damage affecting the back channel characteristics. Whereas front gate direct tunnel stress is not causing any significant degradation for stress biases up to about two times the power supply for this technology node and reasonably short stress times, for the highest stress conditions the drain current transients are found to be progressively faster till front gate dielectric breakdown occurs.

Total ionizing dose damage in deep submicron partially depleted SOI MOSFETs induced by proton irradiation

This overview discusses the impact of ionizing irradiation on the static, the transient and the low-frequency noise characteristics of deep submicron partially depleted (PD) silicon-on-insulator (SOI) transistors. It is demonstrated that, while from a total ionizing dose (TID) damage viewpoint the technology is suitable for space applications, there is still a marked performance degradation. Evidence is provided that at least two basic mechanisms are involved. One is related to the radiation-induced hole trapping in the field oxide, giving rise to a subthreshold leakage current. In addition, the occurrence of majority carrier injection by electron valence-band (EVB) tunneling gives rise to some unexpected front–back channel coupling effects. A second yet unknown mechanism gives rise to a length-dependent response of the static device parameters, pointing to a laterally non-uniform charging of the gate oxide and/or the silicon body.

Explaining the parameters of the electron valence-band tunneling related Lorentzian noise in fully depleted SOI MOSFETs

The behavior of the Lorentzian noise overshoot time constant /spl tau/ and plateau amplitude is investigated in fully depleted silicon-on-insulator MOSFETs with the back gate in accumulation. It is shown that /spl tau/ is identical for long-channel nand p-MOSFETs in the gate overdrive voltage range studied. For shorter lengths (L = 0.12 μm), a slight reduction of /spl tau/ for the p-MOSFETs and an increase for the short n-MOSFETs has been found. The observations will be explained by considering both the injection of majority carriers in the floating body through electron valence-band tunneling and the dynamic resistance of the source-body and gate-body junction.

New UV Light Emitter Based on AlGaN Heterostructures with Graded Electron and Hole Injectors

ABSTRACTNew ultraviolet (UV) light emitting device structures address the problems of small carrier concentrations and large band-offsets in wide bandgap Aluminum Gallium Nitride (AlGaN) heterostructures through the use of graded epilayers for electron and hole injection. For light emission at 280–290 nm, a multiple-quantum-well separate confinement heterostructure (MQWSCH) employs a graded AlGaN structure for the injection of majority carriers from the metal-semiconductor contact layers into the spacecharge region of the pn-junction with a higher bandgap energy. Sample LED mesa devices were fabricated and have shown light emission of 289 nm under a forward bias of 12V (20mA). These results provide a ‘proof-of-concept’ for this new graded device structure which can be employed for the development of both UV-LEDs and laser diodes.

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.

Onset of electroluminescence from bilayer light emitting diodes under space charge limited majority carrier injection

An analytic theory has been developed to analyze the rise of electroluminescence from bilayer light emitting diodes upon applying a rectangular voltage pulse if the current of majority carriers is space charge limited while minority carrier injection is electrode limited. The onset of electroluminescence is governed by the growth of the interfacial charge densities rather than by the transit time. This result suggests that, for appropriate interfacial energy barriers, balanced injection can be established even if only one of the electrodes is ohmic. A qualitative agreement between theory and experiment has been obtained.

Photoconductivity of C<sub>60</sub>-H<sub>2</sub>Pc Coevaporated Thin Films

Changes in electronic and photoelectronic properties of fullerene (C60) and metal-free phthalocyanine (H2Pc) coevaporated thin films were investigated by changing their mixing ratio. The influences of thermal annealing and oxygen adsorption on the properties were also investigated. The electronic and photoelectronic properties changed by incororation of H2Pc into C60, and thermal annealing changed their properties dramatically. Thin films in which one component was doped lightly into the other showed high photoconductive sensitivity in comparison with that of C60 or H2Pc alone in over all visible region. Existence of sensitization center and process caused from minority carrier trapping was confirmed from thermal quenching of photoconductivity. Mechanisms for carrier generation and transport were discussed using a model in which photons of high and low energies were absorbed in C60 and H2Pc molecules, respectively, followed with majority carrier injection and trapping.

Threshold reduction in strained-layer InGaAs/GaAs quantum well lasers by liquid phase epitaxy regrowth

When liquid phase epitaxy regrowth at 780°C for 2 h is applied to the samples after molecular beam epitaxy, a decrease of the threshold current density in strained InGaAs/GaAs quantum well lasers by a factor of 3 to 4 is obtained. We suggest that this improvement is attributed to the reduction of nonradiative centers associated with deep levels at the three regions of the active region, the graded layer and the cladding layer. Indeed, a significant reduction of deep center densities has been observed by using minority and majority carrier injection deep level transient spectroscopy measurements.

Polythienylenevinylene as promoter of hole injection from ITO into bilayer light emitting diodes

AbstractDue to the low oxidation potential of poly‐(2,5‐thienylenevinylene) (PTV) an ohmic contact for hole injection is established at an indium tin oxide (ITO)/PTV interface. It gives rise to space‐charge‐limited conduction in an ITO/PTV/Al diode. The ohmic nature of the ITO/PTV contact can be exploited to increase hole injection into multilayer organic light emitting diodes in which internal charge accumulation would otherwise lower majority carrier injection by virtue of electric field screening at the anode.