Lecture 8 OUTLINE MetalSemiconductor Contacts contd Current flow

Lecture 8 OUTLINE • Metal-Semiconductor Contacts (cont’d) – Current flow in a Schottky diode – Schottky diode applications – Small-signal capacitance – Practical ohmic contacts Reading: Pierret 14. 2 -14. 3; Hu 4. 17 -4. 21

Voltage Drop across the M-S Contact • Under equilibrium conditions (VA = 0), the voltage drop across the semiconductor depletion region is the built-in voltage Vbi. • If VA 0, the voltage drop across the semiconductor depletion region is Vbi - VA. EE 130/230 M Spring 2013 Lecture 8, Slide 2

Depletion Width, W, for VA 0 Last time, we found that At x = 0, V = - (Vbi - VA) • W increases with increasing –VA • W decreases with increasing ND EE 130/230 M Spring 2013 Lecture 8, Slide 3

W for p-type Semiconductor p-type semiconductor At x = 0, V = Vbi + VA • W increases with increasing VA • W decreases with increasing NA EE 130/230 M Spring 2013 Lecture 8, Slide 4

Current Flow FORWARD BIAS REVERSE BIAS EE 130/230 M Spring 2013 • Current is determined by majority-carrier flow across the M-S junction: o Under forward bias, majoritycarrier diffusion from the semiconductor into the metal dominates o Under reverse bias, majoritycarrier diffusion from the metal into the semiconductor dominates Lecture 8, Slide 5

Thermionic Emission Theory • Electrons can cross the junction into the metal if • Thus the current for electrons at a given velocity is: • So, the total current over the barrier is: EE 130/230 M Spring 2013 Lecture 8, Slide 6

Schottky Diode I - V For a nondegenerate semiconductor, it can be shown that We can then obtain In the reverse direction, the electrons always see the same barrier FB, so Therefore EE 130/230 M Spring 2013 Lecture 8, Slide 7

Applications of Schottky Diodes • IS of a Schottky diode is 103 to 108 times larger than that of a pn junction diode, depending on FB. Schottky diodes are preferred rectifiers for low-voltage, high-current applications. Block Diagram of a Switching Power Supply EE 130/230 M Spring 2013 Lecture 8, Slide 8

Charge Storage in a Schottky Diode • Charge is “stored” on both sides of the M-S contact. – The applied bias VA modulates this charge. EE 130/230 M Spring 2013 Lecture 8, Slide 9

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: EE 130/230 M Spring 2013 Lecture 8, Slide 10

Using C-V Data to Determine FB Once Vbi and ND are known, FBn can be determined: EE 130/230 M Spring 2013 Lecture 8, Slide 11

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 EE 130/230 M Spring 2013 Lecture 8, Slide 12

Tunneling Current Density Equilibrium Band Diagram EFM q. Vbi FBn Ec, EFS Ev EE 130/230 M Spring 2013 Lecture 8, Slide 13 Band Diagram for VA 0 EFM q(Vbi-VA) Ec, EFS Ev

Example: Ohmic Contacts in CMOS EE 130/230 M Spring 2013 Lecture 8, Slide 14

Specific Contact Resistivity, rc • Unit: W-cm 2 – rc is the resistance of a 1 cm 2 contact • For a practical ohmic contact, want small FB, large ND for small contact resistance EE 130/230 M Spring 2013 Lecture 8, Slide 15

Approaches to Lowering FB • Image-force barrier lowering FBo EF metal DF N = dopant concentration in surface region a = width of heavily doped surface region Ec n+ Si Very high active dopant concentration desired • FM engineering – Impurity segregation via silicidation A. Kinoshita et al. (Toshiba), 2004 Symp. VLSI Technology Digest, p. 168 – Dual ( low-FM / high-FM ) silicide technology • Band-gap reduction – strain A. Yagishita et al. (UC-Berkeley), 2003 SSDM Extended Abstracts, p. 708 C. Ozturk et al. (NCSU), – germanium incorporation M. 2002 IEDM Technical Digest, p. 375 EE 130/230 M Spring 2013 Lecture 8, Slide 16

Voltage Drop across an Ohmic Contact • Ideally, Rcontact is very small, so little voltage is dropped across the ohmic contact, i. e. VA 0 Volts equilibrium conditions prevail EE 130/230 M Spring 2013 Lecture 8, Slide 17

Summary • Charge is “stored” in a Schottky diode. – The applied bias VA modulates this charge and thus the voltage drop across the semiconductor depletion region The flow of majority carriers into the metal depends exponentially on VA small-signal capacitance EE 130/230 M Spring 2013 Lecture 8, Slide 18

Summary (cont’d) EF Ec Ec Ev EF EF Ev Ec Ev Since it is difficult to achieve small FB in practice, ohmic contacts are achieved with heavy doping, in practice: EF EE 130/230 M Spring 2013 Ec Ec Ev EF Lecture 8, Slide 19 Ev

Summary of Key Points • Schottky barrier height: – Energy barrier that must be surmounted in order for a carrier in the metal to enter the semiconductor • Built-in potential: FBn-(EC-EF)FB for n-type S, FBp-(EF-Ev)FB for p-type S – Ideally is equal to the work function difference between M and S • But in practice (for Si) FBn (2/3)EG and FBp (1/3)EG • In equilibrium the flow of carriers from M to S (IM S) equals the flow of carriers from S to M (IS M) • Under forward bias IS M increases exponentially and dominates • Under reverse bias IS M decreases exponentially so that IM S (which is independent of VA) dominates