Masao Iwase

On the universality of inversion layer mobility in Si MOSFET's: Part I-effects of substrate impurity concentration

This paper reports the studies of the inversion layer mobility in n- and p-channel Si MOSFETs with a wide range of substrate impurity concentrations (10/sup 15/ to 10/sup 18/ cm/sup -3/). The validity and limitations of the universal relationship between the inversion layer mobility and the effective normal field (E/sub eff/) are examined. It is found that the universality of both the electron and hole mobilities does hold up to 10/sup 18/ cm/sup -3/. The E/sub eff/ dependences of the universal curves are observed to differ between electrons and holes, particularly at lower temperatures. This result means a different influence of surface roughness scattering on the electron and hole transports. On substrates with higher impurity concentrations, the electron and hole mobilities significantly deviate from the universal curves at lower surface carrier concentrations because of Coulomb scattering by the substrate impurity. Also, the deviation caused by the charged centers at the Si/SiO/sub 2/ interface is observed in the mobility of MOSFETs degraded by Fowler-Nordheim electron injection. >

On the universality of inversion layer mobility in Si MOSFET's: Part II-effects of surface orientation

For part I see ibid., vol.41, no.12, pp.2357-62 (1994). This paper reports the studies of the inversion layer mobilities in n-channel MOSFETs fabricated on Si wafers with three surface orientations ([100], [110], and [111]) from the viewpoint of the universal relationship against the effective field, E/sub eff/(=q(N/sub dpl/+/spl eta//spl middot/N/sub s/)//spl epsi/Si). It is found that the universality does hold for the electron mobilities on [110] and [111], when the value of /spl eta/ is taken to be 1/3, different from the electron mobility on [100], where /spl eta/ is 1/2. Also, the E/sub eff/ dependence of the electron mobility is found to differ among [100], [110], and [111] surfaces. This is attributed to the differences in the E/sub eff/ dependence of the mobility limited by surface roughness scattering among the orientations. The origins of E/sub eff/ and /spl eta/ are discussed on the basis of the relaxation time approximation for a 2DEG (2-dimensional electron gas). While the surface orientation dependence of /spl eta/ in phonon scattering can be understood in terms of the subband occupation, it is found that the theoretical formulation of surface roughness scattering, used currently, needs to be refined in order to explain the differences in E/sub eff/ dependence and the value of /spl eta/ among the three orientations. >

A study of photon emission from n-channel MOSFET's

It is known that an n-channel MOSFET, operating in the saturation region, is accompanied by visible light emission. The spectral distribution of this emitted light is reported in this paper for the first time. It behaves as exp (-α . hv) under various bias conditions (α: constant); the energy state of hot electrons is described as a Maxwell-Boltzmann distribution. The hot-electron temperature in an n-channel MOSFET is experimentally evaluated from the photon spectrum analysis. As compared with the electric field strength calculated by two-dimensional simulation, the hot-electron temperature is found to be determined as a function of the electric field strength in the drain avalanche region.

On the universality of inversion-layer mobility in n- and p-channel MOSFETs

The authors report studies on the inversion-layer mobility in n- and p-channel MOSFETs with 10/sup 15/ to 10 /sup 18/ cm/sup -3/ substrate impurity concentrations. The validity and limitations of the universal relationship between the inversion-layer mobility and the effective normal field (E/sub eff/) were examined. Differences have been found in E/sub eff/ dependence between electron and hole mobility. A marked deviation from the universal curve due to substrate impurity scattering has been observed at low carrier concentration. The results suggest that by adding a term for the surface roughness scattering and the deviation due to Coulomb scattering to the universal curves, a more accurate description of inversion-layer mobilities can be realized over a wide range of substrate impurity concentration. The mobility degradation caused by carrier injection also has been studied.<<ETX>>

Observation of random telegraph signals: Anomalous nature of defects at the Si/SiO2 interface

Current fluctuations with discrete levels, which are called random telegraph signals (RTSs), have been studied in small size metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) from both viewpoints of relative current change and of correlated switchings. A large relative current change of as much as 30% has been observed, even at room temperature. It behaves similarly as normal small RTSs in terms of statistics and temperature dependence. RTSs have been found also in 20‐μm channel width MOSFETs. These results require another mechanism to explain RTSs in addition to simple Coulomb scattering or number fluctuation. It is emphasized that an interaction between defects at the Si/SiO2 interface is necessary to understand the correlated RTSs. The experimental results are reasonably reproduced by a model calculation assuming interacting defects. It is also pointed out that new RTSs generated by electrical stress might be a serious concern in lower submicron devices.

Electron Mobility in Si Inversion Layers

Gate voltage dependence of electron mobility in n-channel MOSFETs is investigated using Hall effect and channel conductance measurements at room temperature and at 77 K. The electron mobilities obtained by the two different methods show a good agreement with each other and exhibit a decrease with increasing gate voltage in the region of high effective normal field. The theoretical model for electron mobility is given based on the interactions of two-dimensional electron gas confined in the inversion layer with acoustic phonons, intervalley phonons, surface-roughness and ionized impurities. An analytical expression is obtained for the electron mobility, where three types of f- and g-intervalley phonons are included. The calculated results show a good agreement with the experimental data and the decrease in the mobility at high effective normal field is interpreted in terms of surface-roughness scattering which results in Eeff-2 dependence at high normal fields.

High-performance 0.10- mu m CMOS devices operating at room temperature

The authors have fabricated 0.10- mu m gate-length CMOS devices that operate with high speed at room temperature. Electron-beam lithography was used to define 0.10- mu m polysilicon gate patterns. Surface-channel type p- and n-channel MOSFETs were fabricated using an LDD structure combined with a self-aligned TiSi/sub 2/ process. Channel doping was optimized so as to suppress punchthrough as well as to realize high transconductance and low drain junction capacitance. The fabricated 0.10- mu m CMOS devices have exhibited high transconductance as well as a well-suppressed band-to-band tunneling current, although the short-channel effect occurred somewhat. The operation of a 0.10- mu m gate-length CMOS ring oscillator has been demonstrated. The operation speed was 27.7 ps/gate for 2.5 V at room temperature, which is the fastest CMOS switching ever reported.<<ETX>>

Experimental determination of finite inversion layer thickness in thin gate oxide MOSFETS

Abstract The influence of finite inversion layer thickness on the channel current in ultra thin gate oxide MOSFETs is investigated. The dependences of the channel current on gate bias, temperature, surface doping concentration and substrate bias indicate that the carrier transport is well characterized by introducing an effective oxide thickness which is an actual oxide thickness plus an average thickness of charge distribution in inversion layer. It is also confirmed that the mobility is independent of the gate oxide thickness in the range from 360 to 31 A

Experimental determination of hot-carrier energy distribution and minority carrier generation mechanism due to hot-carrier effects

Photon emission due to hot-carrier effect in MOSFETs is investigated in detail. Bias dependent photon intensity and its energy spectrum provide new findings about hot-carrier state in drain avalanche region. In particular, hot-carrier energy distribution is found to be a Maxwellian. The carrier temperature is evaluated from the energy spectrum experimentally, which is shown as a function of drain voltage. Minority carrier generation mechanism due to hot-carrier effect is discussed based upon photon mediated carrier generation model.

Temperature Dependence of Electron Mobility in Si Inversion Layers

Temperature dependence of electron mobility in Si inversion layers is investigated using channel conductance measurements in the temperature range from 15 to 300 K. The electron mobility is analyzed by using the theoretical model in which two-dimensional electron gas confined in the inversion layer interacts with acoustic phonons, intervalley phonons, surface roughness, and ionized impurities. At higher temperatures above 100 K, the electron mobility is dominated by acoustic phonon scattering and intervalley phonon scattering. At temperatures below 100 K, ionized impurity scattering plays an important role in determining the mobility in the case of low electron sheet densities, whereas in the case of higher electron sheet densities, surface roughness scattering becomes important.