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  • 1. Majority CarrierDiode By Dr. Ghanshyam Singh
  • 2. Junction Breakdown• When a huge reverse voltage is applied to a p-n junction, the junction breaks down and conducts a very large current.• Although, the breakdown process is not naturally destructive, the maximum current mus t be limited by an external circuit to avoid exces sive junction heating.• There are mechanisms dealing with the breakdown: zener diode, tunneling diode and av alanche diode.
  • 3. Tunneling Effect • If a very high electric field is applied to a p-n junction in the reverse direction, a valence electron can make a transition from the valence band to the conduction band by penetrating through the energy bandgap called tunneling. • The typical field for Si and GaAs is about 106 V/cm or higher.
  • 4. Tunneling Effect• To achieve such a high field, the doping concentration for both p- and n-regions must be very high such as more than 5 x 1017 cm-3.• The breakdown voltage for Si and GaAs junctions about 4Eg/e is the result of the tunneli ng effect. With the breakdown voltage is more t han 6Eg/e, the breakdown mechanism is the resu lt of avalanche multiplication.• As the voltage is in between 4Eg/e and 6Eg/e, the breakdown is due to a mix of both tunneling effect and avalanche multiplication.
  • 5. Backward Diode• a backward diode (also called back diode) is a variation on a Zener diode or tunnel diode having a better conduction for small reverse biases (for example –0.1 to –0.6 V) than for forward bias voltages.• The schematic symbol for the backward diode, annotated to show which side is P type and which is N; current flows most easily from N to P, backward relative to the arrow.
  • 6. Current–voltage characteristics• The forward I–V characteristic is the same as that of an ordinary P–N diode.• The breakdown starts when reverse voltage is applied. In the case of Zener breakdown, it starts at a particular voltage. In this diode the voltage remains relatively constant (independent of current) when it is connected in reverse bias.• The backward diode is a special form of tunnel diode in which the tunneling phenomenon is only incipient, and the negative resistance region virtually disappears. The forward current is very small and becomes equivalent to the reverse current of a conventional diode.
  • 7. Energy Band diagramElectron energy is on the vertical axis, position within thedevice is on the horizontal axis. The backward diode has theunusual property that the so-called reverse bias directionactually has more current flow than the so-called forwardbias.
  • 8. Applications of backward diodes• Detector Since it has low capacitance and no charge storage effect, and a strongly nonlinear small-signal characteristic, the backward diode can be used as a detector up to 40 GHz.• Rectifier A backward diode can be used for rectifying weak signals with peak amplitudes of 0.1 to 0.7 V.• Switch Backward diode can be used in high speed switching applications.
  • 9. Schottky diode• The Schottky diode also known as hot carrier diode is a semiconductor diode with a low forward voltage drop and a very fast switching action.• When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts, while a Schottky diode voltage drop is between approximately 0.15– 0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.
  • 10. Schottky effect• In electron emission devices, especially electron guns, the thermionic electron emitter will be biased negative relative to its surroundings. This creates an electric field of magnitude F at the emitter surface. Without the field, the surface barrier seen by an escaping Fermi-level electron has height W equal to the local work-function. The electric field lowers the surface barrier by an amount ΔW, and increases the emission current. This is known as the Schottky effect or field enhanced thermionic emission. It can be modeled by a simple modification of the Richardson equation, by replacing W by (W − ΔW). This gives the equation
  • 11. ApplicationsVoltage clamping• While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes’ voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.Reverse current and discharge protection• Because of a Schottky diodes low forward voltage drop, less energy is wasted as heat making them the most efficient choice for applications sensitive to efficiency.Power supply• Schottky diodes can be used in power supply circuits in products that have both an internal battery and a mains adapter input, or similar.
  • 12. Heterojunction A heterojunction is defined as a junction formed by two semicondu ctors with different energ y bandgaps Eg, different di electric permittivities εs, different work function e φs, and different electron affinities eχ.
  • 13. Heterojunction • The difference energy between two conduction band edges and between two valen ce band edges are represente d by ∆EC and ∆EV, respectively , as ∆EC = e ( χ 2 − χ1 ) ∆EV = Eg1 + eχ1 − ( E g 2 + eχ 2 ) = ∆E g − ∆EC where ∆Eg is the difference energy bandgap of two semiconductors.
  • 14. Heterojunction• Generally, heterojunction has to be formed between semiconductors with closely matched lattice constants.• For example, the AlxGa1-xAs material is the most important material for heterojunction.• When x = 0, the bandgap of GaAs is 1.42 eV with a lattice constant of 5.6533 Å at 300 K.• When x = 1, the bandgap of AlAs is 2.17 eV with a lattice constant of 5.6605 Å.
  • 15. Heterojunction• We clearly see that the lattice constant is almost constant as x increased. The total built- in potential Vbi can be expressed by Vbi = Vb1 + Vb 2 ε 2 N 2 ( Vbi − V ) Vb1 = ε1 N1 + ε 2 N 2 ε1 N1 ( Vbi − V ) Vb 2 = ε1 N1 + ε 2 N 2where N1 and N2 are the doping concentrations in semiconductor 1 and 2, respectively.
  • 16. Heterojunction• The depletion widths x1 and x2 can be found by 2ε1ε 2 N 2 ( Vbi − V ) x1 = eN1 ( ε1 N1 + ε 2 N 2 ) 2ε1ε 2 N1 ( Vbi − V ) x2 = eN 2 ( ε1 N1 + ε 2 N 2 )
  • 17. HeterojunctionEx. Consider an ideal abrupt heterojunction with a built-in potential of 1.6 V. The impurity concent rations in semiconductor 1 and 2 are 1 x 1016 don ors/cm3 and 3 x 1019 acceptors/cm3, and the diele ctric constants are 12 and 13, respectively. Find the electrostatic potential and depletion width i n each material at thermal equilibrium.
  • 18. Heterojunction 13 × ( 3 × 1019 ) × 1.6Soln Vb1 = 12 × ( 1 × 10 = 1.6 V 16 ) + 13 × ( 3 ×10 )19 12 × ( 1× 10 ) × 1.6 16 −4 Vb 2 = = 4.9 × 10 V 12 × ( 1 × 10 ) + 13 × ( 3 × 10 ) 16 19 2 × 12 × 13 × ( 8.85 × 10 ) × 3 × 10 × 1.6 −14 19 −5 x1 = = 4.608 × 10 cm 1.6 × 10 × 1× 10 × ( 12 × 10 + 13 × 3 × 10 ) −19 16 16 19 2 × 12 × 13 × ( 8.85 × 10 ) × 1× 10 × 1.6 −14 16 −8 x2 = = 1.536 × 10 cm 1.6 × 10 × 3 × 10 × ( 12 × 10 + 13 × 3 × 10 ) −19 19 16 19Note: Most of the built-in potential is in thesemiconductor with a lower doping concentration andalso its depletion width is much wider.
  • 19. Metal-Semiconductor Junctions• The MS junction is more likely known as the Schott ky-barrier diode.• Let’s consider metal band and semiconductor band diagram before the contac t.
  • 20. Metal-Semiconductor Junctions • When the metal and semiconductor are joined, electrons from the semico nductor cross over to the metal until the Fermi leve l is aligned (Thermal equil ibrium condition). • This leaves ionized donors as fixed positive charges that produce an internal e lectric field as the case of one-sided p-n junction.
  • 21. Metal-Semiconductor Junctions• At equilibrium, equal number of electrons across the interface in opposite directions.• Hence, no net transport of charge, electron current Ie equals to zero. The built-in voltage Vbi = φm - φs.• The barrier for electrons to flow from the metal to semiconductor is given by eφb = e(φm - χs) or it is called the barrier height of MS contact.
  • 22. Metal-Semiconductor Junctions• When a voltage is applied, the barrier height remains fixed but the built-in voltage changes as increasing when reverse biased and decreasing when forward biased.
  • 23. Metal-Semiconductor Junctions• Reverse bias• Few electrons move across the interface from metal to semiconductor due to a barrier, but it is harder for elect rons in the semiconductor to move to the metal.• Hence, net electron transport is caused by electrons moving from metal to semiconductor. Electron current fl ows from right to left which is a small value.
  • 24. Metal-Semiconductor Junctions• Forward bias• Few electrons move across the interface from metal to semiconductor, but many electrons move across the inter face from semiconductor to metal due to the reduced bar rier.• Therefore, net transport of charge flows from semiconductor to metal and electron current flows from l eft to right.
  • 25. Metal-Semiconductor Junctions• Under forward bias, the electrons emitted to the metal have greater energy than that of the metal electrons by about e(φm - χs).• These electrons are called hot-carrier since their equivalent temperature is higher than that of electrons in the metal.• Therefore, sometimes, Schottky-diode is called “hot-carrier diode”.
  • 26. Metal-Semiconductor Junctions• This leads to the thermionic emission with thermionic current density under forward bias as − e( φm − χ s −VF ) / kT  J F = A**T 2e   = J s e eVF / kT − e( φm − χ s ) / kT  where J s = A**T 2e   = saturation current m* A =A. ** = effective Richardsons constant * m0
  • 27. Metal-Semiconductor Junctions• This behavior is referred to a rectification and can be described by an ideal diode equation of J = J s ( eeV / kT − 1) where V positive for forward bias and negative for reverse bias.
  • 28. Metal-Semiconductor Junctions• The space-charge region width of Schottky diode is identical to that of a one-sided p-n junction.• Therefore, under reverse bias, they can contain the charges in their depletion region and this is called Schott ky diode capacitance.
  • 29. Metal-Semiconductor JunctionsEx. A Schottky junction is formed between Au and n-type semiconductor of ND = 1016 cm-3. Area of ju nction = 10-3 cm2 and me*= 0.92 m0. Work function of gold is 4.77 eV and eχs = 4.05 eV. Find current at VF = 0.3 volts.
  • 30. Metal-Semiconductor JunctionsSoln * me A** = A* = 120 × 0.92 = 110 A/ ( cm 2 .K ) m0 − e( φm − χ s ) / kT J s = A**T 2e J = J s ( eeV / kT − 1) = ( 110 × 3002 ) × e { − ( 4.77 − 4.05) / 0.0259    } . ( e0.3 / 0.0259 − 1) = 0.897 A/(cm 2 ) I = A × J = ( 10−3 ) ( 0.897 ) = 0.897 mA
  • 31. Metal-Semiconductor JunctionsEx. Si-Schottky diode of 100 μm diameter has (1/ C2) v.s. VR slope of 3 x 1019 F-2V-1. Given εr = 11.9 fo r Si. Find NB for this semiconductor.
  • 32. Metal-Semiconductor JunctionsSoln 1 2 ( Vbi + VR ) C = ; Cj = [F/cm 2 ] 2 Cj eε s N B Area 2 slope = [cm 4 F-2 V -1 ] eε s N B 2 NB = slope × Area 2 × eε s 2 NB =  −4 2  2  19    100 × 10    3 × 10 π  −19 −12 ÷  × 1.6 × 10 × 8.85 × 10 × 11.9    2       = 6.414 × 1019 cm -3
  • 33. Ohmic contact• This contact is defined as a junction that will not add a significant parasitic impedance to the structure on which it is used and will not sufficiently change the equilibrium-carrier densities within the semiconductor to affect the device characteristics.• The I-V characteristic of ohmic contact is linear for an ideal case.
  • 34. Ohmic contact• A specific contact resistance RC is given by 1 ∂J −1 = Ω.cm 2    RC ∂V V =0• A good ohmic contact should have a small specific contact resistance about 10-6 Ω.cm2.
  • 35. Ohmic contact • When the semiconductor is heavily doped with an impurity density of 1019 c m-3 or higher, the depleti on layer of the junction becomes very thin so tha t carriers can tunnel inst ead of going over the pot ential barrier.
  • 36. THANK YOU