Module 2-3-Diode Circuit

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) 𝑽𝑽 > 𝑽𝑽𝑶𝑶𝑶𝑶 𝑽𝑽 < 𝑽𝑽𝑶𝑶𝑶𝑶 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

𝑽𝑽𝑶𝑶𝑶𝑶 𝑰𝑰𝒊𝒊𝒊𝒊𝒊𝒊𝒊𝒊

𝑷𝑷𝒎𝒎𝒎𝒎𝒎𝒎 𝑺𝑺𝑺𝑺 • Operating𝑰𝑰 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