Mechanisms of interface trap-induced drain leakage current inoff-state n-MOSFET's

An interface trap-assisted tunneling and thermionic emission model has been developed to study an increased drain leakage current in off-state n-MOSFET's after hot carrier stress. In the model, a complete band-trap-band leakage path is formed at the Si/SiO₂ interface by hole emission from interface traps to a valence band and electron emission from interface traps to a conduction band. Both hole and electron emissions are carried out via quantum tunneling or thermal excitation. In this experiment, a 0.5 μm n-MOSFET was subjected to a dc voltage stress to generate interface traps. The drain leakage current was characterized to compare with the model. Our study reveals that the interface trap-assisted two-step tunneling, hole tunneling followed by electron tunneling, holds responsibility for the leakage current at a large drain-to-gate bias (V_dg). The lateral field plays a major role in the two-step tunneling process. The additional drain leakage current due to band-trap-band tunneling is adequately described by an analytical expression ΔI_d=Aexp(B_it/F). The value of B_it about 13 mV/cm was obtained in a stressed MOSFET, which is significantly lower than in the GIDL current attributed to direct band-to-band tunneling. As V_dg decreases, a thermionic-field emission mechanism, hole thermionic emission and electron tunneling, becomes a primary leakage path. At a sufficiently low V_dg, our model reduces to the Shockley-Read-Hall theory and thermal generation of electron-hole pairs through traps is dominant     

Subbreakdown drain leakage current in MOSFET

Significant drain leakage current can be detected at drain voltages much lower than the breakdown voltage. This subbreakdown leakage can dominate the drain leakage current at zero VGin thin-oxide MOSFET's. The mechanism is shown to be band-to-band tunneling in Si in the drain/gate overlap region. In order to limit the leakage current to 0.1 pA/µm, the oxide field in the gate-to-drain overlap region must be limited to 2.2 MV/cm. This may set another constraint for oxide thickness or power supply voltage.     

Modeling of drain leakage current in SOI pMOSFETs

In this paper we present a compact drain leakage current model for fully-depleted (FD) SOI pMOSFETs. The analytical and physics-based model was developed using a quasi-two dimensional approach, in which the longitudinal and vertical surface channel electric fields can be calculated. It can be used to accurately calculate drain leakage current as a function of drain and gate biases. This model in conjunction with our previous published subthreshold and above threshold model forms a concrete drain current model for FD SOI pMOSFET operation in off and on states.     

Impact of interface traps on gate-induced drain leakage current in n-type metal oxide semiconductor field effect transistor

As the features sizes of metal oxide semiconductor field effect transistor (MOSFET) are aggressively scaled into the submicron domain, hot carriers generated by the very large electric fields of drain region create serious reliability problems for the integrated circuit in MOS technology. The charges trapping in the gate oxide and the defects at the Si/SiO₂ interface have also undesirable effects on the degradation and ageing of MOSFET. Among the problems caused by these effects is the band-to-band tunnelling (BBT) of hot carriers in the gate-to-drain overlap region which is the source of the gate-induced drain leakage current I _gidl. The oxide charges shift the flat-band voltage and result in an enhancement of the I _gidl current. On the other hand, the generation of interface traps introduce an additional band-trap-band (BTB) leakage mechanism and lead to a significant increase ?I _gidl in a drain leakage current. In this work we propose a new method to calculate the I _gidl current which takes into account of the BTB leakage mechanism in order to clarify the impact of interface traps located in the gate-to-drain overlap region on the I _gidl current.     

A new model for the post-stress interface trap generation inhot-carrier stressed p-MOSFETs

A new insight into the post-stress interface trap (N_it) generation in hot-electron stressed p-MOSFETs is presented. N_it generation is suppressed for positive oxide field but enhanced for negative oxide field. This observation provides strong support for a two-carrier model, involving the recombination between trapped electrons and inversion holes. While post-stress interface instability has generally been associated with hole trapping and hydrogen transport, our results clearly show the importance of electron traps on the long term stability of the Si-SiO₂ interface, and that the two-carrier model provides a consistent explanation for post-stress N_it generation in p-MOSFETs stressed under hot-electron injection     

Hot-carrier-stress effects on gate-induced drain leakage current inn-channel MOSFETs

The effects of hot-carrier stress on gate-induced drain leakage (GIDL) current in n-channel MOSFETs with thin gate oxides are studied. It is found that the effects of generated interface traps (ΔD _it) and oxide trapped charge on the GIDL current enhancement are very different. Specifically, it is shown that the oxide trapped charge only shifts the flat-band voltage, unlike ΔD _it. Besides band-to-band (B-B) tunneling, ΔD _it introduces an additional trap-assisted leakage current component. Evidence for this extra component is provided by hole injection. While trapped-charge induced leakage current can be eliminated by a hole injection subsequent to stress, such injection does not suppress interface-trap-induced leakage current     

A steady state drain current technique for generation andrecombination lifetime measurement in the SOI MOSFET

A new steady state drain current technique is developed for both the generation and recombination lifetime extraction in the fully-depleted (FD) SOI MOSFET. At all times during the measurement, the device has one silicon surface maintained in strong accumulation, and the other surface in strong inversion. The accumulation layer is modulated with a negative ramp voltage applied to the gate, so that more holes are demanded by the accumulation layer (n-channel enhancement MOSFET). When the demand for the additional holes cannot be met by generation in the bulk of the silicon film, electrons from the inversion region have to be expelled, resulting in a decrease in the external drain current. The drain current saturates when the rate of demand for additional holes is balanced by the rate of hole generation. From the measured saturation drain current, together with the given ramp rate, the generation lifetime can be easily determined. A positive ramp voltage is used to extract recombination lifetime. As a nonpulse technique, only steady state parameters are measured. The technique has the added advantage of simplicity in the interpretation of the results     

The role of electron traps on the post-stress interface trapgeneration in hot-carrier stressed p-MOSFETs

The generation of interface traps in p-MOSFETs subjected to hot-electron injection is found to proceed even after the stress has been terminated. The extent of post-stress interface trap generation is strongly dependent on the magnitude of the preceding hot-electron stress, as well as the magnitude and polarity of the gate voltage during relaxation. Trap generation is enhanced for negative gate voltage anneal, but suppressed for positive gate voltage anneal. For a given stress-induced damage, the corresponding trap generation kinetics can be completely described by a single characteristic, which is shifted in time according to the magnitude of the applied gate voltage. Existing interface trap generation models are discussed in the light of the experimental results. A new model involving the tunneling of holes from the inversion layer to deep-level electron traps is proposed. Similar post-stress effect observed for hot-electron stressed n-MOSFETs provides additional support for the model. Our work suggests that near-interface electron traps, apart from the well-known hole traps, may also significantly affect the long-term stability of the Si-SiO₂ interface     

Investigation and modeling of interface and bulk trap generation during negative bias temperature instability of p-MOSFETs

Negative bias temperature instability is studied in thick and thin gate oxide p-MOSFETs. The relative contributions of interface- and bulk-trap generation to this device degradation mode are analyzed for a wide range of stress bias and stress temperature. The effects of gate voltage and oxide field, as well as those of inversion layer holes, impact ionized hot holes, and hot electrons on interface- and bulk-trap generation, are identified. The bulk-trap generation process is interpreted within the modified anode-hole injection model and the mechanism of interface-trap generation is modeled within the framework of the classical reaction-diffusion theory. The diffusion species for interface-trap generation is unambiguously identified. Moreover, a high-temperature, diffusion-triggered, enhanced interface-trap generation mechanism is discussed for thin gate oxide p-MOSFETs. Finally, a novel scaling methodology is proposed for interface-trap generation that helps in obtaining a simple, analytical model useful for reliability projection.     

Temperature dependence of gate induced drain leakage current insilicon CMOS devices

The temperature dependence of the gate induced drain leakage (GIDL) current in CMOS devices is investigated from 20K up to 300K. It is shown that, at sufficiently high electric field, the conventional band-to-band tunnelling GIDL current law is applicable down to near-liquid helium temperatures for both nand p-channel devices. The exponential factor B of the GIDL current law is found to be nearly independent of temperature. Moreover, the decrease of the GIDL current as the temperature is lowered, is shown to originate from the temperature variation of the pre-exponential coefficient A of the GIDL current law     

Stress induced gate–drain leakage current in ultra-thin gate oxide

Ultra-thin gate oxide reliability, in large area MOSFETs, can be monitored by measuring the gate current when the substrate is depleted. When the channel length is scaled down, the tunneling current associated with the source/drain extension region (SDE) to the gate–overlap regions can dominate the gate current. In N-MOSFETs, as a function of the negative gate voltage two components of the gate–drain leakage current should be considered, the first for V_FB < V_G < 0 V and the second for V_G < V_FB. These components are studied in this work before and after voltage stresses. The aim of this work is to see whether this gate–drain current can be used to monitor the oxide degradation above or near the source and/or drain extension region in N-MOSFETs. It is important because the most serious circuit-killing breakdown occurs above or near the drain (or source) extension region. Finally, we show that it is necessary, before explaining the gate LVSILC curves obtained after stresses on short-channel devices, to verify which is the dominate current at low voltage.     

The leakage current improvement in an ultrashallow junction NMOSwith Co silicided source and drain

The leakage current characteristics of the cobalt silicided NMOS transistors with a junction depth of 800 Å have been studied. In order to minimize the junction leakage current, the thickness of the CoSi₂ layer should he controlled under 300 Å and the Si surface damage induced by the gate spacer etch should be minimized. The post furnace annealing after the second silicidation by the rapid thermal annealing (RTA) process also affected the leakage current characteristics. The gate induced drain leakage (GIDL) current was not affected by the lateral encroachment of CoSi₂ layer into the channel direction when the gate spacer length was larger than 400 Å     

Determining the drain doping in DMOS transistors using the hump inthe leakage current

The hump in the leakage current of double-diffused metal-oxide-semiconductor (DMOS) transistors observed for low drain voltages is explained. This hump is due to surface generation current of the gate-controlled diode formed by the base-drain p-n junction. The drain bias of the DMOS transistor is shown to have the same effect on the charge at the drain surface as the body bias in the conventional MOSFET. The body effect is used to develop a new method for determining the drain doping in DMOS transistors. This method is nondestructive, and does not require special test structures. Instead, electrical measurements are performed on conventional DMOS transistors. The method is ideally suited for determining the doping in the drain region of interest. Specifically, in DMOS transistors in which a surface implant is used to reduce the on-resistance, the method provides the doping concentration in the implanted region. In DMOS transistors which do not have the surface implant, the method yields the doping concentration in the drain epitaxial layer. In this study, the method is illustrated by determining the drain doping for six discrete power MOSFET device types from three different manufacturers     

Enhanced gate induced drain leakage current in HfO2 MOSFETs

The substrate current of high-κ dielectric MOSFETs has been studied using dc sweep and transient (down to 100 μs per I-V curve) electrical measurements. These measurements reveal trap-assisted substrate current components in addition to the traditional bell-shaped impact ionization current. By separating the transversal and lateral electric field contributions, the gate induced drain leakage (GIDL) is shown to dominate the substrate current at low gate biases. At high gate biases, tunneling of valence band electrons from the bulk to the gate dominates. The results show that the GIDL current is the result of band-to-band tunneling assisted by traps located at the HfO₂/SiO₂ interface and transition layer, and not the result of oxide charging.     

Solar power generation system for reducing leakage current

This paper proposes a transformer-less multi-level solar power generation system. This solar power generation system is composed of a solar cell array, a boost power converter, an isolation switch set and a full-bridge inverter. A unipolar pulse-width modulation (PWM) strategy is used in the full-bridge inverter to attenuate the output ripple current. Circuit isolation is accomplished by integrating the isolation switch set between the solar cell array and the utility, to suppress the leakage current. The isolation switch set also determines the DC bus voltage for the full-bridge inverter connecting to the solar cell array or the output of the boost power converter. Accordingly, the proposed transformer-less multi-level solar power generation system generates a five-level voltage, and the partial power of the solar cell array is also converted to AC power using only the full-bridge inverter, so the power efficiency is increased. A prototype is developed to validate the performance of the proposed transformer-less multi-level solar power generation system.     

Hot-carrier current modeling and device degradation insurface-channel p-MOSFETs

The channel field and substrate current models developed for n-MOSFETs are applicable to p-MOSFETs. The impact ionization rate extracted for holes is found to be 8×10⁶ exp (-3.7×10⁶/E), where E is the electric field. The lucky electron approach was used to model the gate current of surface-channel (SC) p-MOSFETs successfully. Device degradation in p-MOSFETs is due to trapped electrons in the oxide. p-MOSFET lifetime has good correlation with gate current in SC p-MOSFETs. The correlation is better than with substrate current. I_G can be larger in a buried-channel (BC) p-MOSFET than in a comparable SC n-MOSFET. This makes the SC MOSFET a much more reliable device. Device lifetime of a p-MOSFET under pulse stress can be predicted from DC stress data for inverterlike waveforms. For other waveforms, there is an extra degradation probably caused by the excess hot carriers generated during the gate turn-off transient     

Gate-induced drain leakage current degradation and its time dependence during channel hot-electron stress in n-MOSFETs

The effects of channel hot-electron stress on the gate-induced drain leakage current (GIDL) in n-MOSFETs with thin gate oxides have been studied. It is found that under worst case stress, i.e. a high density of generated interface states Delta D/sub it/, the enhanced GIDL exhibits a significant drain voltage dependence. Whereas Delta D/sub it/ increases significantly the leakage current at low V/sub d/, it has minor effects at high V/sub d/. On the other hand, the electron trapping was found to increase the leakage current rather uniformly over both low and high V/sub d/ regions. In addition, GIDL degradation can be expressed as a power law time dependence (i.e. Delta I/sub leak/=A.t/sup n/), and the time dependence value n varies according to the dominant damage mechanism (i.e. electron trapping against Delta D/sub it/), similar to that reported for on-state device degradation.<>     

Physical model of drain conductance, gd, degradation ofNMOSFET's due to interface state generation by hot carrierinjection

Degradation of analog device parameters such as drain conductance, g_d, due to hot carrier injection has been modeled for NMOSFET's. In this modeling, mobility reduction caused by interface state generation by hot carrier injection and the gradual channel approximation were employed. It has been found that g_d degradation can be calculated from linear region transconductance, g_m, degradation which is usually monitored for hot carrier degradation of MOSFET's. The values of g_d degradation calculated from g_m degradation fit well to the measured values of g_d degradation The dependence of the g_d degradation lifetime on L_eff has been also studied, this model also provides an explanation of the dependence on L_eff. The model is then useful for lifetime predictions of analog circuits in which g_d degradation is usually more important than g_m degradation     

Hot hole induced interface state generation and annihilation in SOIMOSFETs

The effects of pure hot hole injection in SOI MOSFET's are investigated. Pure hot hole injection is achieved by exploiting the opposite channel based carrier injection phenomenon. It is found that significant amounts of interface states are generated, some of which are annihilated by a subsequent hot electron injection pulse. A power law of the form D_it(t)=Ktⁿ with n close to 0.25 was obtained, indicating a more complex, diffusion limited, electrochemical reaction at the interface than previously reported     

The impacts of SILC and hot carrier induced drain leakage current on the refresh time in DRAM

This paper reports the temperature dependence of SILC and hot carrier induced drain leakage current, and their impact on the refresh time in Giga-bit level DRAM with practical considerations. SILC has been found to increase as the monitoring and stress temperature increases. Due to the generation of interface states, hot carrier induced pn junction leakage current and band-to-band tunneling current have been found to increase as the monitoring temperature increases.From the simulation results of a refresh circuit for Giga-bit level DRAM, it has been found that the increase of SILC with stress time is a dominant factor in refresh failure below 373K, and the pn junction leakage current will be a dominant factor at the high elevated temperature. It has been also observed that the increase of hot carrier induced drain leakage current can be a cause for the refresh failure.