Lecture6 PN junction and diode

LECTURE 6: PN JUNCTION AND DIODE The density gradient as producing a "diffusion force" that acts on the majority carriers, called as diffusion forces • The net positive and negative charges in the n and p regions induce an electric field in the region near the junction, in the direction from the positive to the negative charge, or from the n to the p region • The electric field in the space charge region produces another force on the electrons and holes which is in the opposite direction to the diffusion force for each type of particle In thermal equilibrium, the diffusion force and the E-field force exactly balance each other Charge density Full-Wave (Bridge) Rectifier • • • Two diodes are in the conduction path Thus in the case of non-ideal diodes vo will be lower than vi by 2VC As for the half-wave rectifier a reservoir capacitor can be used In the full wave case the discharge time is T/2 and VMT ΔV ≈ 2RC Diode Clipper Circuits • These circuits clip off portions of signal voltages above or below certain limits, i.e the circuits limit the range of the output signal • Such a circuit may be used to protect the input of a CMOS logic gate against static Diode Clipper Circuits Diode Clipper Circuits • When the diode is off the output of these circuits resembles a voltage divider  RL  vo =  v i  R L + R S   Diode Clipper Circuits • • If RS [...]... two identical PN junctions connected in parallel act effectively as a single PN junction with twice the cross-sectional area, hence twice the current Diode Saturation Current IS  Dn Dp   I S = Aqni  + L N  L N n A p D   2 • • IS can vary by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations – typical range of values for Si PN diodes: 10-14... i ) −1 I-V Characteristic of a PN Junction • Current increases exponentially with applied forward bias voltage, and “saturates” at a relatively small negative current level for reverse bias voltages “Ideal diode equation: ( ) I D = I S eVD / VT − 1  Dn Dp   I S = AJ S = Aqn  + N L N L  D p   A n 2 i Parallel PN Junctions • Since the current flowing across a PN junction is proportional to its...More on the Built-In Potential (V0) Q: Why can’t we harness V0 and use the PN junction as a battery? + ? A: A built-in potential also exists at a junction between a metal and V0 a semiconductor (e.g at a contact) – If we connect the P and N regions together, there is no net voltage drop across the device: • No net current flows across the junction when the externally applied voltage is 0 V! V(x) V0 Vbn+V0+Vbp=0... piece-wise linear model Ideal Vc Rf vi v i = VM sin (ωt) vi Half Wave Rectifier • We initially consider the diode to be ideal, such that VC =0 and Rf =0 Half Wave Rectifier • The (ideal) diode conducts for vi >0 and since Rf =0 v0 ≈ v i • For vi < 0 the (ideal) diode is an open circuit (it doesn’t conduct) and v0 ≈ 0 ... constant-voltage model, only linear equations need to be solved Summary: PN- Junction Diode I-V • Under forward bias, the potential barrier is reduced, so that carriers flow (by diffusion) across the junction – – Current increases exponentially with increasing forward bias The carriers become minority carriers once they cross the junction; as they diffuse in the quasi-neutral regions, they recombine with... that negligible carriers flow across the junction – If a minority carrier enters the depletion region (by thermal generation or diffusion from the quasi-neutral regions), it will be swept across the junction by the built-in electric field “collection” of minority carriers ( ) I D = I S eVD / VT − 1 DIODE APPLICATION Diode Applications • Half wave rectifier and equivalent circuit with piece-wise linear... • PN Junction under Forward Bias A forward bias decreases the potential drop across the junction As a result, the magnitude of the electric field decreases and the width of the depletion region narrows ρ (x) qND a -b x -qNA ID V(x) V0 -b 0 a x Minority Carrier Injection under Forward Bias • The potential barrier to carrier diffusion is decreased by a forward bias; thus, carriers diffuse across the junction. .. E-field) in-between scattering events to cause electron-hole pair generation upon colliding with the lattice Constant-Voltage Diode Model • • If VD < VD,on: The diode operates as an open circuit If VD ≥ VD,on: The diode operates as a constant voltage source with value VD,on Example: Diode DC Bias Calculations IX VX = I X R1 + VD ≅ I X R1 + VT ln IS I X = 2.2mA for VX = 3V I X = 0.2mA for VX = 1V • • This... VT n e NA )e −1 Notation: − x′ / Ln J n ,diff x' Ln ≡ electron diffusion length (cm) ( ) dn p qDn ni2 qVD / VT = qDn = e − 1 e − x′ / Ln dx′ N A Ln Diode Current under Forward Bias • The current flowing across the junction is comprised of hole diffusion and electron diffusion components: • Assuming tot that the diffusion components are constant within the depletion region (i.e no p , drift current n... by orders of magnitude, depending on the diode area, semiconductor material, and net dopant concentrations – typical range of values for Si PN diodes: 10-14 to 10-17 A/µm2 In an asymmetrically doped PN junction, the term associated with the more heavily doped side is negligible: – If the P side is much more heavily doped,  Dp   I S ≅ Aqni  L N   p D 2 – If the N side is much more heavily doped, ... harness V0 and use the PN junction as a battery? + ? A: A built-in potential also exists at a junction between a metal and V0 a semiconductor (e.g at a contact) – If we connect the P and N regions... initially consider the diode to be ideal, such that VC =0 and Rf =0 Half Wave Rectifier • The (ideal) diode conducts for vi >0 and since Rf =0 v0 ≈ v i • For vi < the (ideal) diode is an open circuit... circuit (it doesn’t conduct) and v0 ≈ Half Wave Rectifier • In this simplified (ideal diode) case the input and output waveforms are as shown The diode must withstand a peak inverse voltage of