Lecture 16 - Metal-Semiconductor Junction (cont.) October 9, 2002

Transcription

1 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-1 Lecture 16 - Metal-Semiconductor Junction (cont.) October 9, 2002 Contents: 1. Schottky diode 2. Ohmic contact Reading assignment: del Alamo, Ch. 6, Announcement: Quiz 1: October 10, Rm (Walker), 7:30-9:30 PM; lectures #1-13 (up to metal-semiconductor junction, no space-charge-region transport). Open book. Calculator required.

2 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-2 Key questions What is the basic structure of a Schottky diode? What are its most important parasitics? What are key technological constraints in the design and fabrication of Schottky diodes? How do Schottky diodes switch? What sets their time response? What does one have to do for a metal-semiconductor junction to become an ohmic contact? Why do ohmic contacts look as S = for minority carriers?

3 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Schottky diode Key uniqueness: fast switching from ON to OFF and back. Widely used: in analog circuits: in track and hold circuits in A/D converters, pin drivers of IC test equipment in communications and radar applications: as detectors and mixers, also as varactors Technological constraint: Schottky diodes engineered using process modules developed for other circuit elements demands resourcefulness and imagination from device designer. Typical implementations: ohmic contact Schottky junction ohmic contact Schottky metal n n+ n n+ n+ semi-insulating GaAs p

4 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-4 Parasitics Series resistance due to QNR ohmic drop Voltage across junction is reduced and I-V characteristics modified: I = I S [exp q(v IR s) kt 1] ideal with series resistance I log I IRs 1/Rs 0 0 V 0 V linear scale semilogarithmic scale R s bad because: for given forward current, V increased and harder to control it degrades dynamic response of diode

5 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-5 Substrate capacitance n n+ Is Rs C n+ p parasitic substrate p-n diode Also degrades dynamics of diode.

6 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-6 Breakdown In reverse bias, as V E max At a high-enough voltage, avalanche breakdown takes place breakdown voltage I 1000 Si 300 K BV V breakdown voltage (V) E+15 1E+16 1E+17 1E+18 doping level (cm -3 ) Computation of BV: Triggering current: I S (all electron current for SBD on n-type semiconductor). Computation difficulty: E non-uniform in SCR. For moderate doping levels, BV function of N D alone (independent of ϕ Bn ):

7 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-7 Technology, layout and design considerations Often, no special metal is available ohmic metal must be used Premature breakdown may occur at edges of diodes. Premature breakdown mitigation requires edge engineering : high field at edge n direct edge contact n metal overlap n moat p n p guard ring If p guard ring used, must make sure that p-n junction never turns on. In essence, this is a 2D or 3D problem.

8 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-8 Design issues: log I I f I S BV 0 V f V Metal selection: ϕ Bn V f (for fixed I f ) I S more T sensitivity Doping level selection: N D R s C BV Vertical extension of QNR: minimum value of t required to deliver BV (beyond that, R s ) Diode area: A j C I S R s V f (for fixed I f ) more expensive

9 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture 16-9 Dynamics Uniqueness of Schottky diodes: they switch fast! Large-signal example: V I Vf 0 t + Vr V Vf Vr I RsC - 0 RsC t -switch-off transient: C charges up through R s time constant: R s C -switch-on transient: C discharges through R s time constant: R s C for fast switching minimize R s and C

10 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Switch-off transient: I I R s R s - I f (V f ) V f -I f R s -V r V f -I f R s C V f V f -I f R s C V r - - t=0 - t=0 + I(0 )=I f (V f ) I(0 + )= ( V f V r R s I f ) note: in this notation, V r is negative

11 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Switch-on transient: I I R s R s + V f -V r V r C V r V r C V f - - t=0 - t=0 + I(0 )= I s I(0 + )= V f V r R s

12 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture HSPICE example of large-signal switching: SPICE Exercise: V f =0.45 V, V r =3V. Diode model parameters: IS= 5.5e 13, N= 1.03, EG= 0.89, RS= 11, CJO= 3.24e 13, VJ= 0.5, M= 0.339, XTI= 2, and TT= current (A) ps 2 ps E E E E E E-11 time (s) parameter SPICE hand calculation τ off 4 ps 7.8 ps τ on 2 ps 1.9 ps I r (pk) 0.31 A 0.31 A I f (pk) 0.30 A 0.31 A

13 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Ohmic contact Ohmic contacts: means of electrical communication with outside world. Key requirement: very small resistance to carrier flow back and forth between metal and semiconductor. Ohmic contact = MS junction with large J S V small linearize I-V characteristics: J A T 2 exp qϕ Bn kt Figure of merit for ohmic contacts: qv kt = V ρ c ρ c ohmic contact resistivity (Ω cm 2 ) Good values: ρ c 10 7 Ω cm 2

14 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture How does one make a good ohmic contact? Classically, use metal that yields small qϕ Bn Increase N D until carrier tunneling is possible Ev EF Ec EF E v E fe E c E fh Ev Efe Efh Ec ohmic contact to n-type semiconductor ohmic contact to p-type semiconductor

15 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Experimental measurements in n-si: Ohmic contact resistance: A c R c R c = ρ c A c

16 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Justify two assumptions made earlier about ohmic contacts: 1) Through a good ohmic contact, outside battery grabs majority carrier quasi-fermi level. In good ohmic contact, R c is very small V very small negligible difference in E f across M-S interface. 2) S = at ohmic contact Strong electric field at M-S interface sucks minority carriers towards it where they recombine: E f Ec E fe E fh Ev In vicinity of ohmic contact, negligible concentration of excess minority carriers.

17 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Key conclusions Main parasitics of Schottky diode: series resistance and substrate capacitance. Ideal BV of Schottky diode entirely set by doping level. Junction edge effects in Schottky diode may cause premature reverse breakdown. No minority carrier storage in Schottky diode fast switching. Dominant time constant of Schottky diode: R s C. Typical design goals for Schottky diode: small time constant, high forward conduction, low reverse conduction, high breakdown voltage and small area. All without a dedicated process! Good ohmic contacts fabricated by increasing doping level carrier tunneling. ρ c, specific contact resistance (in Ω cm 2 ), proper figure of merit for ohmic contact. Order of magnitude of key parameters in Si at 300K: Desired specific contact resistance: ρ c < 10 7 Ω cm 2 (depends on metal and doping level).

18 6.720J/3.43J - Integrated Microelectronic Devices - Fall 2002 Lecture Self study Equivalent circuit models of Schottky diode Small-signal dynamics of Schottky diode