Spring 2007EE130 Lecture 13, Slide 1 Lecture #13 ANNOUNCEMENTS 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 diodes Reading: Finish Ch. 14, Start Ch. 5

Spring 2007EE130 Lecture 13, Slide 2 Practical 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 barrier

Spring 2007EE130 Lecture 13, Slide 3 E c, E FS EvEv E FM Equilibrium Band Diagram Band Diagram for V A  0 E c, E FS EvEv E FM qV bi  Bn q(V bi -V A )

Spring 2007EE130 Lecture 13, Slide 4 Specific Contact Resistivity,  c Unit:  -cm 2 –  c is the resistance of a 1 cm 2 contact For a practical ohmic contact,  want small  B, large N D for small contact resistance

Spring 2007EE130 Lecture 13, Slide 5 Approaches to Lowering  B Image-force barrier lowering N = dopant concentration in surface layer a = width of heavily doped surface layer q  Bo EFEF  ECEC metal n+ Si A. Kinoshita et al. (Toshiba), 2004 Symp. VLSI Technology Digest, p. 168  M engineering –Impurity segregation via silicidation –Dual ( low-  M / high-  M ) silicide technology A. Yagishita et al. (UC-Berkeley), 2003 SSDM Extended Abstracts, p. 708 M. C. Ozturk et al. (NCSU), 2002 IEDM Technical Digest, p. 375 Band-gap reduction –strain –germanium incorporation  Very high active dopant concentration desired

Spring 2007EE130 Lecture 13, Slide 6 Voltage Drop across an Ohmic Contact Ideally, R contact is very small, so little voltage is dropped across the ohmic contact, i.e. V A  0V  equilibrium conditions prevail

Spring 2007EE130 Lecture 13, Slide 7 Review: MS-Contact Charge Distribution In a Schottky contact, charge is stored on either side of the MS junction –The applied bias V A modulates this charge

Spring 2007EE130 Lecture 13, Slide 8 Schottky Diode: Small-Signal Capacitance If an a.c. voltage v a is applied in series with the d.c. bias V A, the charge stored in the Schottky contact will be modulated at the frequency of the a.c. voltage  displacement current will flow:

Spring 2007EE130 Lecture 13, Slide 9 Once V bi and N D are known,  Bn can be determined: Using C-V Data to Determine  B

Spring 2007EE130 Lecture 13, Slide 10 Summary EFEF EcEc EvEv EFEF EcEc EvEv EFEF EcEc EvEv EFEF Since it is difficult to achieve small  B, practical ohmic contacts are achieved with heavy doping: EFEF EcEc EvEv EFEF EcEc EvEv EcEc EvEv Charge storage in an MS junction  small-signal capacitance:

Spring 2007EE130 Lecture 13, Slide 11 V I Reverse biasForward bias N P V I diode symbol – + pn Junctions

Spring 2007EE130 Lecture 13, Slide 12 Terminology Doping Profile:

Spring 2007EE130 Lecture 13, Slide 13 Idealized Junctions