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ECE 875: Electronic Devices Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University [email protected] Lecture 21, 26 Feb 14 Chp. 03: metal-semiconductor junction: Schottky barrier Ideal barrier Effective barrier VM Ayres, ECE875, S14 Example from Exam: For the equilibrium condition: Junction Equilibrium: metal contact to n-type Si when work functions qFm > qFs metal n0= 1017 cm-3 Although the charges are balanced, the layer on the metal side is very thin: similar to p+: ionized acceptors EF qybi EC -- P+ P+ P P P P P P P EF Ei E (x) Neutral region n-side Depletion region W ~ WDn EV Answer: (a) qV0 = --N + N + n --N + N + d d d d (b) Band-bending diagram: q FB = 4.0 eV – 3.8 eV = 0.2 eV W = 0.14 mm q ybi =qV0 = 0.057 eV Ideal Schottky barrier: q ybi = 0.057 eV q fBn0 = 0.2 eV EC ECm = EF -- EF P+ P+ W ≈ WDn = 0.14 mm More accurately to scale: q ybi = 0.057 eV EC q fBn0 = 0.2 eV ECm = EF -- P+ P+ W ≈ WDn = 0.14 mm EF Ideal Schottky barrier: q ybi = 0.057 eV EC q fBn0 = 0.2 eV ECm = EF -- q fn ND+ ND+ EF W ≈ WDn = 0.14 mm EC – EF = q fn : Chp. 01: NC = effective DOS at the conduction band edge (eq’n (18)). Practical: use Appendix G n ≈ ND assumes fully fully ionized donors in saturation Temp range qfBn0 = height of ideal Schottky barrier as seen from the metal = qybi + [qfn = (EC - EF)] n-type semiconductor qfn = EC - EF p-type semiconductor qfp = EF - EV Assume: 300 K Equilibrium condition Draw band-diagrams with the following values marked on it: Barrier height on metal side: qfBn0 = 0.8 eV = given Barrier height on semiconductor side: qybi Barrier width W: primarily on n-side: WDn Draw band-diagrams with the following values marked on it: Barrier height on metal side: qfBn0 = 0.8 eV = given Barrier height on semiconductor side: qybi Barrier width W: primarily on n-side: WDn 0 Two factors change as a function of doping concentration Competing effects Fixed: 0.0259V @ r.t. q ybi = ? EC q fBn0 = 0.8 eV ECm = EF -- q fn ND+ ND+ W ≈ WDn = ? EC – EF = q fn : Chp. 01: EF Lecture 21, 26 Feb 14 Chp. 03: metal-semiconductor junction: Schottky barrier Ideal Schottky barrier Effective Schottky barrier VM Ayres, ECE875, S14 During a C-V voltage sweep, real current is flowing. Current flowing lowers the ideal barrier height Lowered barrier height is called the effective barrier height Any active measurement will give you the effective barrier height not the ideal one Same as abrupt junction Pr. 02, Chp. 02 Intercept ybi Slope N Pr. 8(a), Chp. 03: VR to VF is plotted right to left Not left to right You take a C-V curve data in F/V. But Units in eqn’s and Fig 30 are C = es/WD in F/cm2 The extra factor 1012 or 13 came from normalizing to a contact area to get the F/cm2 unit. Why: During a C-V voltage sweep, real current is flowing. Current flowing lowers the ideal barrier height Lowered barrier height is called the effective barrier height Any active measurement will give you the effective barrier height not the ideal one Consider an e- in a current reaching a previously established Schottky barrier as shown: e- --N + N + n --N + N + d d d d The e- is leaving the metal. e- --- An e- that leaves a metal induces a re-arrangement of charge in the metal. Keep in mind that metallic electrons can rearrange very easily. The real re-arrangements of e- and + that develops can be described as an image charge. Mathematical equivalent REAL -++ -- e- + e- The image charge field also does work on the e-, independent of anything the depletion region field E is doing. e0 => es as the e- is really transporting into a semiconductor. Combined potential energy of an e- entering the semiconductor from image charge field AND depletion region field : e0 => es as the e- is really transporting into a semiconductor. During a C-V voltage sweep, real current is flowing. Current flowing lowers the ideal barrier height and shifts the max to xm Lowered barrier height is called the effective barrier height qfBn Any active measurement will give you the effective barrier height not the ideal one