EE105 – Fall 2015 Microelectronic Devices and Circuits: Diode Circuits Prof. Ming C. Wu [email protected] 511 Sutardja Dai Hall (SDH) 1 Summary ofpn Junction ! $ = N A ND Built-in potential : V0 VT ln# 2 & " ni % Under forward bias : V /VT I-V curve : I = IS (e −1) ! $ τ T Diffusion capacitance : Cd = # &I "VT % Under revserse bias : Negligible current, I = −IS C j0 Depletion capacitance : C j = V 1+ R V0 Other important parameter : ! $ 2eS 1 1 Depletion Width: W = # + &(V0 −V) q " N A ND % 2 Many Applications of Diodes LED (Light -­Emitting Diode) LED Lighting Laser Diode Solar Cells (PV) Photodiode OLED 3 How Many Diodes are in a Smart Phone? 4 How Many Diodes are in a Smart Phone? http://image-­sensors-­world.blogspot.com/2017/09/yole-­on-­iphone-­x-­3d-­innovations.html 5 How Many Diodes are in a Smart Phone? http://image-­sensors-­world.blogspot.com/2017/09/yole-­on-­iphone-­x-­3d-­innovations.html 6 Diode I-­V Curve (Forward) 0.5 2.5×10 -­11 0.4 2.×10 -­ 11 0.3 1.5×10 -­11 0.2 1.×10 -­ 11 0.1 5.×10 -­ 12 -­ 1.0 -­ 0.5 0.5 1.0 -­ 0.10 -­ 0.05 0.05 0.10 • I-­V curve at high current • I-­V curve at low current • Approximate ”turn-­on” voltage at • Soft increase at forward bias 0.7V for Si • Can see reverse saturation – There is no exact turn-­on current, I voltage S – Current keeps increasing exponentially 7 7/ 23 Zener Reference Reverse Breakdown • At sufficiently large reverse bias voltage, current starts to increase dramatically – Due to avalanche breakdown or quantum mechanical tunneling – Breakdown voltage can be designed – Sometimes used as a voltage limiter 8 8/ 23 Ideal Diode Model Example: • An ideal diode only allows current to flow in one direction – Short circuit for (~ 0.7V for Si) – Open circuit for (as well as reverse bias) > < 9 9/ 23 Exact Solution with Real Diode-­ I V 10 Half-­Wave Rectifier 11 Full-­Wave Bridge Rectifier 12 Filter to Remove Ripples What is the RC time constant in forward bias? What is RC in reverse bias? 13 Full-­Wave Bridge Rectifier with Smoothing Capacitor Where do you add capacitor? How does output waveform change? DC Power Supply 14 Photodiodes 15 23/ 23 Solar (Photovoltaic, or PV) Cells Dark Illuminated • Operating in theth 4 quadrant of the-­ I V curve à It generates power ! • Key parameters: – Open circuit voltage, VOC – Short-­circuit current,I sh – Fill factor 16 23/ 23 Peak Detector • The capacitor is charged to the peak voltage and the output is held at the peak – When input > output, diode is ON, charge capacitor to new peak – When input < output, diode is OFF. Capacitor holds output at peak • If you flip the direction of the diode, you get a negative peak detector. 17 17 / 23 Level Restorers • Diode turns on initially and charges thecapacitor to the AC voltage. – Note that once the voltage starts to drop, the diode turns off • The output voltage is therefore level shifted by the DC voltage held on the capacitor • In this case the voltage excursions are now negative and neverrise above zero! – If a load is connected, then the capacitor should be large enough to minimize droop 18 18/ 23 Level Restorers • If we now flip the direction of the diode, the current will only flow during the negative half cycle, charging the capacitor now in the opposite direction. • Then output is now lifted by the DC voltage stored on the capacitor. The voltage will now always remain positive and never go below zero! 19 18/ 23 Voltage Doubler • If we rectify the above voltages, we can generate positive or negative DC voltages of twice the magnitude. This is a voltagedoubler ! 20 29 / 23 “Superdiode” Use an op-­amp to make circuit precise 21 Small Signal Resistance 22.