全文获取类型
收费全文 | 145416篇 |
免费 | 1546篇 |
国内免费 | 487篇 |
专业分类
化学 | 68541篇 |
晶体学 | 2035篇 |
力学 | 4955篇 |
综合类 | 5篇 |
数学 | 12108篇 |
物理学 | 39265篇 |
无线电 | 20540篇 |
出版年
2016年 | 1396篇 |
2015年 | 1061篇 |
2014年 | 1478篇 |
2013年 | 5061篇 |
2012年 | 3599篇 |
2011年 | 4602篇 |
2010年 | 3010篇 |
2009年 | 3018篇 |
2008年 | 4562篇 |
2007年 | 4849篇 |
2006年 | 4833篇 |
2005年 | 4771篇 |
2004年 | 4250篇 |
2003年 | 3872篇 |
2002年 | 3789篇 |
2001年 | 4340篇 |
2000年 | 3450篇 |
1999年 | 2869篇 |
1998年 | 2592篇 |
1997年 | 2566篇 |
1996年 | 2466篇 |
1995年 | 2415篇 |
1994年 | 2228篇 |
1993年 | 2230篇 |
1992年 | 2444篇 |
1991年 | 2413篇 |
1990年 | 2298篇 |
1989年 | 2263篇 |
1988年 | 2188篇 |
1987年 | 1948篇 |
1986年 | 1823篇 |
1985年 | 2399篇 |
1984年 | 2497篇 |
1983年 | 2123篇 |
1982年 | 2414篇 |
1981年 | 2202篇 |
1980年 | 2254篇 |
1979年 | 2227篇 |
1978年 | 2338篇 |
1977年 | 2253篇 |
1976年 | 2277篇 |
1975年 | 2204篇 |
1974年 | 2039篇 |
1973年 | 2291篇 |
1972年 | 1410篇 |
1971年 | 1095篇 |
1970年 | 1030篇 |
1969年 | 987篇 |
1968年 | 1138篇 |
1967年 | 1176篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
71.
Streit D.C. Hafizi M.E. Umemoto D.K. Velebir J.R. Tran L.T. Oki A.K. Kim M.E. Wang S.K. Kim C.W. Sadwick L.P. Hwu R.J. 《Electron Device Letters, IEEE》1991,12(5):194-196
The authors have fabricated n-p-n GaAs/AlGaAs heterojunction bipolar transistors (HBTs) with base doping graded exponentially from 5×1019 cm-3 at the emitter edge to 5×1018 cm-3 at the collector edge. The built-in field due to the exponentially graded doping profile significantly reduces base transit time, despite bandgap narrowing associated with high base doping. Compared to devices with the same base thickness and uniform base doping of 1×1019 cm-3 , the cutoff frequency is increased from 22 to 31 GHz and maximum frequency of oscillation is increased from 40 to 58 GHz. Exponentially graded base doping also results ill consistently higher common-emitter current gain than uniform base doping, even though the Gummel number is twice as high and the base resistance is reduced by 40% 相似文献
72.
Zupac D. Baum K.W. Kosier S.L. Schrimpf R.D. Galloway K.F. 《Electron Device Letters, IEEE》1991,12(10):546-549
The effect of noncatastrophic positive human body model (HBM) electrostatic discharge (ESD) stress on n-channel power MOSFETs is radically different from that on p-channel MOSFETs. In n-channel transistors, the stress causes negative shifts of the current-voltage characteristics indicative of positive charge trapping in the gate oxide. In p-channel transistors, the stress increases the drain-to-source leakage current, probably due to localized avalanche electron injection from the p-doped drain 相似文献
73.
Frank W. Schmidt 《Linear and Multilinear Algebra》1991,30(3):229-230
This section appears from time to time. Contributions are invited, and should be submitted to R. C. Thompson, Mathematics Department, University of California, Santa Barbara, CA 93106. 相似文献
74.
The application of optical amplifiers (OAs) to fiber-optic delay line signal processing is demonstrated. Both erbium-doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs) are applicable. Analytical results are presented for both amplified and unamplified fiber-optic recirculating delay lines (AFORDLs and UFORDLs). In the AFORDL an OA is inserted in the fiber loop. It is shown that the active AFORDL structure is capable of realizing all-fiber filters not possible with the passive UFORDL. This result is significant because it shows that the OAs can function beyond just the trivial optical loss compensation to provide extra flexibility not available in passive designs. An AFORDL design is presented which has a pole near +1 and a zero at -1 in the Z -plane resulting in useful characteristics in the magnitude and phase responses. The results suggest that extension of the concept to higher-order filters has the potential to lead to the realization of more complex optical processors 相似文献
75.
Yan R.-H. Lee K.F. Jeon D.Y. Kim Y.O. Park B.G. Pinto M.R. Rafferty C.S. Tennant D.M. Westerwick E.H. Chin G.M. Morris M.D. Early K. Mulgrew P. Mansfield W.M. Watts R.K. Voshchenkov A.M. Bokor J. Swartz R.G. Ourmazd A. 《Electron Device Letters, IEEE》1992,13(5):256-258
The authors report the implementation of deep-submicrometer Si MOSFETs that at room temperature have a unity-current-gain cutoff frequency (f T) of 89 GHz, for a drain-to-source bias of 1.5 V, a gate-to-source bias of 1 V, a gate oxide thickness of 40 Å, and a channel length of 0.15 μm. The fabrication procedure is mostly conventional, except for the e-beam defined gates. The speed performance is achieved through an intrinsic transit time of only 1.8 ps across the active device region 相似文献
76.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献
77.
W. G. Habashi G. Baruzzi M. F. Peeters M. M. Hafez 《Numerical Methods for Partial Differential Equations》1991,7(2):193-207
Finite element solutions of the Euler and Navier-Stokes equations are presented, using a simple dissipation model. The discretization is based on the weak-Galerkin weighted residual method and equal interpolation functions for all the unknowns are permitted. The nonlinearity is iterated upon using a Newton method and at each iteration the linear algebraic system is solved by a direct solver with all unknowns fully coupled. Results are presented for two-dimensional transonic inviscid flows and two- and three-dimensional incompressible viscous flows. Convergence of the algorithm is shown to be quadratic, reaching machine accuracy in very few iterations. The inviscid results demonstrate the existence of nonunique numerical solutions to the steady Euler equations. 相似文献
78.
W. Boltz 《e & i Elektrotechnik und Informationstechnik》2003,120(10):309-311
Ohne Zusammenfassung
Kurzfassung eines Vortrags der 41. Fachtagung der ?sterreichischen Gesellschaft für Energietechnik (OGE) im OVE, die am 5.
und 6. November 2003 in Salzburg stattfindet. 相似文献
79.
80.
It is shown that the dephasing which suppresses the weak localization correction to the conductivity has the same physical origin than pair-breaking in superconductors and thus-following de Gennes-may be expressed in terms of the correlation function of the operator for time reversal. 相似文献