全文获取类型
收费全文 | 627565篇 |
免费 | 9075篇 |
国内免费 | 4478篇 |
专业分类
化学 | 286509篇 |
晶体学 | 8565篇 |
力学 | 26341篇 |
综合类 | 123篇 |
数学 | 66469篇 |
物理学 | 174329篇 |
无线电 | 78782篇 |
出版年
2021年 | 5800篇 |
2020年 | 6277篇 |
2019年 | 6865篇 |
2018年 | 8760篇 |
2017年 | 8751篇 |
2016年 | 12382篇 |
2015年 | 7537篇 |
2014年 | 12180篇 |
2013年 | 27076篇 |
2012年 | 21092篇 |
2011年 | 24987篇 |
2010年 | 18648篇 |
2009年 | 19328篇 |
2008年 | 24136篇 |
2007年 | 24779篇 |
2006年 | 23140篇 |
2005年 | 20721篇 |
2004年 | 18932篇 |
2003年 | 17134篇 |
2002年 | 16995篇 |
2001年 | 18715篇 |
2000年 | 14960篇 |
1999年 | 12148篇 |
1998年 | 10712篇 |
1997年 | 10538篇 |
1996年 | 9773篇 |
1995年 | 8860篇 |
1994年 | 8826篇 |
1993年 | 8372篇 |
1992年 | 9039篇 |
1991年 | 9114篇 |
1990年 | 8756篇 |
1989年 | 8286篇 |
1988年 | 7965篇 |
1987年 | 7464篇 |
1986年 | 7035篇 |
1985年 | 8922篇 |
1984年 | 9179篇 |
1983年 | 7691篇 |
1982年 | 7741篇 |
1981年 | 7405篇 |
1980年 | 7269篇 |
1979年 | 7430篇 |
1978年 | 7751篇 |
1977年 | 7623篇 |
1976年 | 7616篇 |
1975年 | 7070篇 |
1974年 | 7118篇 |
1973年 | 7314篇 |
1972年 | 5224篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
311.
Lianshan Yan Yeh C. Yang G. Lin L. Chen Z. Shi Y.Q. Willner A.E. Yao X.S. 《Lightwave Technology, Journal of》2003,21(7):1676-1684
We demonstrate the first programmable group-delay module based on polarization switching. With a unique binary tuning mechanism, the device can generate any differential group delay value from -45 to +45 ps with a resolution of 1.40 ps, or any true-time-delay value from 0 to 45 ps with a resolution of 0.7 ps. The delay varying speeds for both applications are under 1 ms and can be as fast as 0.1 ms. We evaluate both the dynamic and static performances of the device while paying special attention to its dynamic figures of merit for polarization-mode dispersion emulation and compensation applications. Our experiment shows that the device exhibits a negligible transient-effect induced power penalty (<0.2 dB) in a 10-Gb/s nonreturn-to-zero system. 相似文献
312.
Kerber A. Cartier E. Pantisano L. Degraeve R. Kauerauf T. Kim Y. Hou A. Groeseneken G. Maes H.E. Schwalke U. 《Electron Device Letters, IEEE》2003,24(2):87-89
The magnitude of the V/sub T/ instability in conventional MOSFETs and MOS capacitors with SiO/sub 2//HfO/sub 2/ dual-layer gate dielectrics is shown to depend strongly on the details of the measurement sequence used. By applying time-resolved measurements (capacitance-time traces and charge-pumping measurements), it is demonstrated that this behavior is caused by the fast charging and discharging of preexisting defects near the SiO/sub 2//HfO/sub 2/ interface and in the bulk of the HfO/sub 2/ layer. Based on these results, a simple defect model is proposed that can explain the complex behavior of the V/sub T/ instability in terms of structural defects as follows. 1) A defect band in the HfO/sub 2/ layer is located in energy above the Si conduction band edge. 2) The defect band shifts rapidly in energy with respect to the Fermi level in the Si substrate as the gate bias is varied. 3) The rapid energy shifts allows for efficient charging and discharging of the defects near the SiO/sub 2//HfO/sub 2/ interface by tunneling. 相似文献
313.
Golovchenko A. V. Pil'o S. G. Brovarets V. S. Drach B. S. 《Russian Journal of General Chemistry》2003,73(11):1832-1833
Russian Journal of General Chemistry - 相似文献
314.
A simple procedure for the synthesis of an important standard, isotopically enriched methylmercury, which is not commercially available, has been established successfully. The isotopically enriched standard synthesized is utilized in conventional isotope dilution mass spectrometry (IDMS), as well as in speciated IDMS (SIDMS), for determination of the true concentration of methylmercury in environmental samples. The CH3201Hg+ standard has been synthesized from commercially available 201HgO and tetramethyltin. The synthesis time required is 1 h at 60°C. The product is highly pure, yielding more than 90% as 201Hg in CH3201Hg+. Hazardous dimethylmercury does not occur during this synthesis procedure. The product synthesized was analyzed using high‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (ICP‐MS) and ICP‐MS alone in order to determine its concentration, isotopic composition and purity. The stability of the product was also evaluated for over 6 months and found to be stable at 4°C in the dark. The isotopically enriched methylmercury synthesized can be used in SIDMS and IDMS analyses as a standard. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
315.
G. P. Klimov 《Applied Mathematics Letters》1991,4(6):95-97
Let F be a non-arithmetic distribution on the line
, and W be the class of bounded functions w without discontinuity of the second kind such that .In this paper, we show that the solution of the homogeneous renewal equation w = w F in the class W is a constant-function. 相似文献
316.
Some flaws in a recent article by S.B. Alexander et al. (ibid., vol.7, no.1, p.11-23, Jan. 1989) on the theory of equalization of FM response of a laser diode using passive filters are noted. An error has occurred as a result of assuming a constant C as a positive instead of negative. When C is negative and large, the equalization network cannot be realized with passive networks. Also the simulated time waveform shown in the article for the optical frequency of an equalized laser does not show some spikes which are expected theoretically. The spikes occur as a result of an imperfect equalization provided by the proposed passive filters that were realized assuming C as positive. In replying the original author feels that the comments and observations result simply from attempting to extend the simple FM transfer-function model far beyond its limits while trying to introduce unnecessary theoretical rigor 相似文献
317.
318.
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
319.
Voltage switching induced by long-wavelength infrared light from a CO2 laser was observed using a double-barrier resonant tunneling diode (RTD) biased in the bistable region and the intersubband transition (IT) between the quantum confined states. Possible optoelectronic and all-optical switching applications involving hysteresis are proposed and discussed 相似文献
320.
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