Lecture #12Current Flow in a Schottky DiodeVoltage Drop across the MS DiodeDepleted Layer Width, W, for VA 0W for p-type SemiconductorThermionic Emission TheorySchottky Diode I - VApplications of Schottky DiodesSummaryEE130 Lecture 12, Slide 1 Spring 2007Lecture #12OUTLINE• Metal-semiconductor contacts (cont.)– I-V characteristicsReading: Finish 14.2EE130 Lecture 12, Slide 2 Spring 2007Current Flow in a Schottky Diode•Current is determined by majority-carrier flow across the MS junction:–Under forward bias, majority-carrier diffusion from the semiconductor into the metal dominates–Under reverse bias, majority-carrier diffusion from the metal into the semiconductor dominatesREVERSE BIASFORWARD BIASEE130 Lecture 12, Slide 3 Spring 2007Voltage Drop across the MS Diode•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.EE130 Lecture 12, Slide 4 Spring 2007Depleted Layer Width, W, for VA 0 )(2 DAbisqNVVW 202xWKqNxVSDAt x = 0, V = - (Vbi - VA)• W increases with increasing –VA• W decreases with increasing NDLast time, we found that- (Vbi - VA)EE130 Lecture 12, Slide 5 Spring 2007W for p-type Semiconductor )(2 AbiAsqNVVW 202xWKqNxVSAAt x = 0, V = Vbi + VA• W increases with increasing VA• W decreases with increasing NAEE130 Lecture 12, Slide 6 Spring 2007Thermionic 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: AbixxVVqmv 221K.E. AbinxVVmqvv *min2)(, xxvMsvnqAvIx--min)(vxxxMsdvvnvqAIEE130 Lecture 12, Slide 7 Spring 2007Schottky 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 B, so•Therefore2/20*///232*A/cm 120 where, 4kTnSkTqVSkTqVkTnMSBAABeTmmJeAJeeAThkqmI- 2*2//32*4xncFvkTmkTEEnxeehkTmvn 0-- AMSSMVIISSkTqVSAJIeIIA where)1(/EE130 Lecture 12, Slide 8 Spring 2007• IS of a Schottky diode is 103 to 108 times larger than that of a pn junction diode, depending on B . Schottky diodes are preferred rectifiers for low-voltage, high-current applications.Applications of Schottky DiodesEE130 Lecture 12, Slide 9 Spring 2007Summary•In a Schottky contact, charge is stored on either side of the MS junction–The applied bias VA modulates this charge and thus the voltage drop across the semiconductor depletion region Flow of majority carriers into the metal varies exponentially with VA2/20*/A/cm 120 where
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