Lecture #13Practical Ohmic ContactSlide 3Specific Contact Resistivity, rcApproaches to Lowering fBVoltage Drop across an Ohmic ContactReview: MS-Contact Charge DistributionSchottky Diode: Small-Signal CapacitanceUsing C-V Data to Determine FBSummarypn JunctionsTerminologyIdealized JunctionsEE130 Lecture 13, Slide 1 Spring 2007Lecture #13ANNOUNCEMENTS• Quiz #2 next Friday (2/23) will cover the following:– carrier action (drift, diffusion, R-G)– continuity & minority-carrier diffusion eq’ns– MS contacts (electrostatics, I-V characteristics)• Review session will be held Friday 2/16 at 12:30PM• No office hour or coffee hour today OUTLINE• Metal-semiconductor contacts (cont.)– practical ohmic contacts– small-signal capacitance• Introduction to pn junction diodesReading: Finish Ch. 14, Start Ch. 5EE130 Lecture 13, Slide 2 Spring 2007Practical Ohmic Contact•In practice, most M-S contacts are rectifying•To achieve a contact which conducts easily in both directions, we dope the semiconductor very heavily W is so narrow that carriers can tunnel directly through the barrierEE130 Lecture 13, Slide 3 Spring 2007DABnNVHnDthxDMSonnsemkTqNvqPNJmmhmH/)(*13/2*9*2/Vcm /104.5/4 whereDBnsqNW2DABnNVHeP)(y probabilit tunnelingEc, EFSEvEFMEquilibrium Band DiagramBand Diagram for VA0Ec, EFSEvEFMqVbiBnq(Vbi-VA)EE130 Lecture 13, Slide 4 Spring 2007Specific Contact Resistivity, c•Unit: -cm2– c is the resistance of a 1 cm2 contact•For a practical ohmic contact, want small B, large ND for small contact resistanceDBNHce/contactccontactAREE130 Lecture 13, Slide 5 Spring 2007Approaches to Lowering B• Image-force barrier lowering4aNqsN = dopant concentration in surface layera = width of heavily doped surface layerqBoEFECmetaln+ SiA. Kinoshita et al. (Toshiba), 2004 Symp. VLSI Technology Digest, p. 168• M engineering–Impurity segregation via silicidation –Dual ( low-M / high-M ) silicide technologyA. Yagishita et al. (UC-Berkeley), 2003 SSDM Extended Abstracts, p. 708M. C. Ozturk et al. (NCSU), 2002 IEDM Technical Digest, p. 375•Band-gap reduction–strain –germanium incorporation Very high active dopant concentration desiredEE130 Lecture 13, Slide 6 Spring 2007Voltage Drop across an Ohmic Contact•Ideally, Rcontact is very small, so little voltage is dropped across the ohmic contact, i.e. VA 0V equilibrium conditions prevailEE130 Lecture 13, Slide 7 Spring 2007Review: MS-Contact Charge Distribution•In a Schottky contact, charge is stored on either side of the MS junction–The applied bias VA modulates this chargeEE130 Lecture 13, Slide 8 Spring 2007WACs Schottky Diode: Small-Signal Capacitance•If an a.c. voltage va is applied in series with the d.c. bias VA, the charge stored in the Schottky contact will be modulated at the frequency of the a.c. voltage displacement current will flow:dtdvCiaEE130 Lecture 13, Slide 9 Spring 2007Once Vbi and ND are known, Bn can be determined:22)(21AqNVVCsDAbiDcBnFBFcBnbiln)(NNkTqEEqqV Using C-V Data to Determine B AbisDAbiDsssVVqNAVVqNAWAC22 EE130 Lecture 13, Slide 10 Spring 2007SummaryEFEcEvEFEcEvEFEcEvEFSince it is difficult to achieve small B, practical ohmic contacts are achieved with heavy doping:EFEcEvEFEcEvEcEvWACs Charge storage in an MS junction small-signal capacitance:EE130 Lecture 13, Slide 11 Spring 2007 N-typeP-type DonorsVIReverse bias Forward bias N PVIdiodesymbol – +pn JunctionsEE130 Lecture 13, Slide 12 Spring 2007TerminologyDoping Profile:EE130 Lecture 13, Slide 13 Spring 2007Idealized
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