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pn_junctions

pn_junctions - 1 EECE 254 Module II Dr Purang Abolmaesumi...

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Unformatted text preview: 1 EECE 254 Module II Dr. Purang Abolmaesumi Email: [email protected] Required Reading (Sedra and Smith): 3.7.1 semiconductor basics 3.7.2 – 3.7.5 pn-junction basics 4.1.1 – 4.2.5 MOSFET basics 5.1.1 – 5.2.5 BJT basics 2 Semiconductor ( pn-Junction) Diodes Physically, diodes are formed by the interface between two regions of oppositely doped semiconductor ( i.e. , pn junction) and are thus, structurally, the simplest semiconductor devices used in electronics. Circuit Symbol: Note: From the above, it follows that zero power dissipation occurs in an ideal diode! “open-circuit” “short-circuit” Ideal Diode An ideal diode is a two-terminal device defined by the following non-linear (current- voltage) iv-characteristic: 3 Semiconductor ( pn-Junction) Diodes Physically, diodes are formed by the interface between two regions of oppositely doped semiconductor ( i.e. , pn junction) and are thus, structurally, the simplest semiconductor devices used in electronics. Circuit Symbol: Real Diode A real diode exhibits the following non-linear (current-voltage) iv-characteristic: V T =25.2 mV, n=1, I S =Saturation Current i = I s e v /nV T ! 1 " # $ $ % & ’ ’ 4 Semiconductor ( pn-Junction) Diodes Physically, diodes are formed by the interface between two regions of oppositely doped semiconductor ( i.e. , pn junction) and are thus, structurally, the simplest semiconductor devices used in electronics. Circuit Symbol: 5 Pure silicon (Si): Group IV, Tetravalent Electronic Configuration: [Ne]3s 2 3p 2 Atomic number: 14 Atomic Weight: 28.09 g/mol Physical Structure & Operation of Diodes Crystal Structure: (Face-Centered Cubic) Density (300 K): 2.33 g/cm 3 Melting Point: 1687 K At 0 K, the electrons have no thermal energy and hence all bonds are intact. Since there are no free charge carriers, the conductivity of pure monocrystalline Si at 0 K is 0. Si energy band structure at T = 0 K: Si lattice structure at T = 0 K: E g ~1.12 eV Conduction Band (CB) Valence Band (VB) All CB states are unoccupied All VB states are fully occupied 6 At any temperature >0 K, the non-zero thermal energy causes some bonds to break. This results in an equilibrium concentration of free electrons n , in the conduction band (and free holes p , in the valence band) given by Si lattice structure at T > 0 K: At room temperature (RT), n i ! 1.5x10 10 carriers/cm 3 . Note that, in pure ( i.e. , intrinsic) Si, n = p = n i " np = n i 2 Such “free” charge carriers allow the Si to conduct current. The conductivity of pure crystalline Si at room temperature is relatively poor compared to a metal, but very good compared to an insulator. n i 2 = BT 3 e ! E g / kT where B ! 5.4 x 10 31 (forSi) E g = forbidden energy “band-gap” ! 1.12 eV (forSi) T =absolute temperature (K) k =Boltzmann's constant =8.62x10-5 eV /K 7 Energy Band Structure vs . Electronic Properties: The fundamental distinction between insulators and semiconductors is the magnitude of the forbidden energy gap E g ....
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pn_junctions - 1 EECE 254 Module II Dr Purang Abolmaesumi...

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