Special Diodes UNIT 5 Thyristors and Optical Devices Lily Gupta Assistant Prof.

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4.1.2 MICROWAVE SOLID-STATE DEVICES ( SEMICONDUCTOR DIODE) Quantum Mechanic Tunneling – Tunnel diode Transferred Electron Devices – Gunn, LSA, InP and CdTe Avalanche Transit Time – IMPATT, Read, Baritt & TRAPATT Parametric Devices – Varactor diode Step Recovery Diode – PIN, Schottky Barrier Diode. Designed to minimize capacitances and transit time. NPN bipolar and N channel FETs preferred because free electrons move faster than holes Gallium Arsenide has greater electron mobility than silicon.

TUNNEL DIODE (ESAKI DIODE)

Introduction to Thyristors  Thyristors  Switch  On-state, off-state  Unilateral or bilateral  Latching  High power 5

Introduction to Thyristors  Thyristors  Sinusoidal  Firing angle  Conduction angle 6

Triggering Devices  Used to pulse switching devices  Diac  3-layer  Bi-directional conduction  Breakover voltage  Blocking region 7 Symbols

Triggering Devices 8  Unijunction Transistor (UJT)  3-terminal device  Intrinsic standoff ratio P n E B2B2 B1B1 B2B2 E B1B1 Symbol

Triggering Devices  UJT  0.5 < η < 0.9  Emitter region heavily doped  V E – B 1 = 0, p-n junction reverse biased  Increase V E – B 1, reach peak point (maximum current) 9

Triggering Devices  UJT  Continue increase, reach valley point  Further increase V E – B 1, UJT is saturated 10

Triggering Devices 11  UJT relaxation oscillator +V BB ___ __ _ v out RERE CECE +-+-

Silicon Controlled Rectifiers (SCRs)  4-layer device, p-n-p-n  Anode (A)  Cathode (K)  Gate (G)  Unidirectional 12

Silicon Controlled Rectifiers (SCRs)  High-power (I up to 2500 A, V up to 2500 V)  Phase control  Small V AK when On 13

SCRs 14

SCRs  Operation  I G = 0, no anode current  I G > I GT → regenerative feedback → high I AK  I AK < I H → turn off → I AK = 0 15

SCRs  Can cause SCR turn-on  High temperature  High ∆V/∆t (noise)  Radiation 16

SCRs  Specifications  V DRM or V RRM Peak Repetitive Off-state Voltage  I T(RMS) On-State RMS current (maximum)  I TSM Peak Non-Repetitive Surge current  I GT Gate trigger current  I L Latching current  I H Holding current 17

SCRs 18  SCR phase control

SCRs  Small R 1  Short RC time constant  SCR turns on rapidly, close to 0°  Large R 1  long RC time constant  SCR turns on slowly, close to 180° 19

SCRs  Too large R 1  SCR does not turn on 20

Triacs  3-terminal switch  Bi-directional current  Symbol  Gate trigger may be either + or – pulse 21 G MT1 or Anode (A) MT2 I

Triacs  Characteristics  Direct replacement for mechanical relays  Trigger circuit for full-wave control  4 modes  Remains on in either direction until I < I H  Blocking region, I ≈ μamps  Small voltage across Triac when On 22

Triacs  Specifications  Similar to SCR  P GM Peak Gate Power  P G(AV) Average Gate Power  V GM Peak Gate Voltage  V GT Gate trigger voltage  t gt Turn-On Time 23

Triacs 24  Phase control light dimmer

Triacs  Circuit operation  Turn-off due to small load current  Capacitor charges/discharges through load  DIAC is bi-directional  RC time constant → 0° to 180° turn on in each direction 25

Power Control Fundamentals 26  Review equations  Control  Lamp intensity  Heat from a resistive heater  Speed of a motor

Power Control Fundamentals 27

Power Control Fundamentals 28  Delayed turn-on, full- wave signal  Delayed turn-on, half- wave signal

Power Control Fundamentals 29  V and P curves for full- wave control

Introduction to Optical Devices 30  Opto-electronic devicesλ = wavelength  Current → light  Light → current  c = speed of light in a vacuum  c = 3 x 10 8 m/s

Introduction to Optical Devices  Electromagnetic spectrum  Visible (380 < λ(nm) < 750)  Infrared region (750 < λ(nm) < 1000) 31 1 Å = 1  10 –10 m = 0.1 nm

Introduction to Optical Devices  LED is a diode  When forward biased  Electron-hole recombination energy  Photons released: E = hf, h is Planck’s constant  h =  10 –34 Joules∙seconds  High energy → visible spectrum  Lower energy → IR spectrum 32

Introduction to Optical Devices  LED advantages  Low voltage  Rapid change in light output with input V change  Long life  LED output can be matched to photodetector 33

Introduction to Optical Devices  LED disadvantages  Easily damaged  Brightness dependent on temperature  Chromatic dispersion  Inefficient compared to LCDs 34

Photodetectors  R varies with light intensity  Photoresistors  Voltage or current varies with light intensity  Photodiodes  Phototransistors  Light-Activated SCRs (LASCRs) 35

Photodetectors  Photodiodes  Reverse biased  Low ambient light → very small current, I D (small leakage current)  High ambient light → increased current, I D (increase in minority carriers) 36

Photodetectors  Photodiodes  Symbol 37

Photodetectors  Phototransistor  Base open  Light on reverse-biased CB junction  Increase minority carriers  Increase I C 38

Photodetectors  Phototransistor  Usually used as a switch  Off → I C = 0  On → I C > 0 39

Photodetectors  LASCR  Light-Activated SCR or photo-SCR  Symbol  Light turns LASCR on  Open gate or resistor on gate to control sensitivity 40

Optocouplers  Couple two circuits  LED and Photodetector in single circuit  Electrical isolation  Medical equipment  High voltage circuit to digital circuit 41

Optocouplers  Use as  Linear device  Digital buffer 42

Optocouplers 43  Phototransistor optocoupler

Optocouplers 44  Current transfer ratio  0.1 < CTR < 1

Optocouplers  Operation  High diode current in input circuit yields  High diode light output which yields  High collector current in output circuit 45

Semiconductor LASERs  Light Amplification through Stimulated Emission of Radiation  Operation  Similar to LEDs  Monochromatic (same frequency)  Coherent (same phase) output  Small pulse dispersion 46

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4.1.2 MICROWAVE SOLID-STATE DEVICES ( SEMICONDUCTOR DIODE) Quantum Mechanic Tunneling – Tunnel diode Transferred Electron Devices – Gunn, LSA, InP and CdTe Avalanche Transit Time – IMPATT, Read, Baritt & TRAPATT Parametric Devices – Varactor diode Step Recovery Diode – PIN, Schottky Barrier Diode. Designed to minimize capacitances and transit time. NPN bipolar and N channel FETs preferred because free electrons move faster than holes Gallium Arsenide has greater electron mobility than silicon.

TUNNEL DIODE (ESAKI DIODE)

GUNN DIODE  Slab of N-type GaAs (gallium arsenide)  Sometimes called Gunn diode but has no junctions  Has a negative-resistance region where drift velocity decreases with increased voltage  This causes a concentration of free electrons called a domain

IMPATT DIODE

VARACTOR DIODES The variable-reactance (varactor) diode makes use of the change in capacitance of a pn junction is designed to be highly dependent on the applied reverse bias. The capacitance change results from a widening of the depletion layer as the reverse-bias voltage is increased. As variable capacitors, varactor diodes are used in tuned circuits and in voltage-controlled oscillators. Typical applications of varactor diodes are harmonic generation, frequency multiplication, parametric amplification, and electronic tuning. Multipliers are used as local oscillators, low-power transmitters, or transmitter drivers in radar, telemetry, telecommunication, and instrumentation. Lower frequencies: used as voltage-variable capacitor Microwaves: used as frequency multiplier this takes advantage of the nonlinear V-I curve of diodes Varactors are used as voltage-controlled capacitorscapacitors

PIN DIODE  P-type --- Intrinsic --- N-type  Used as switch and attenuator  Reverse biased - off  Forward biased - partly on to on depending on the bias

SCHOTTKY BARRIER DIODE

A Schottky barrier diode (SBD) consists of a rectifying metal- semiconductor barrier typically formed by deposition of a metal layer on a semiconductor. The SBD functions in a similar manner to the antiquated point contact diode and the slower-response pn-junction diode, and is used for signal mixing and detection. The point contact diode consists of a metal whisker in contact with a semiconductor, forming a rectifying junction. The SBD is more rugged and reliable than the point contact diode. The SBD's main advantage over pn diodes is the absence of minority carriers, which limit the response speed in switching applications and the high-frequency performance in mixing and detection applications. SBDs are zero-bias detectors. Frequencies to 40 GHz are available with silicon SBDs, and GaAs SBDs are used for higher-frequency applications.