Semiconductor Devices - Electronic Devices - Electronics and Communication

Zener Diode

A Zener diode is a p-n junction device specifically operated in strong reverse bias so that it conducts in the breakdown region at a well-defined voltage. The specified Zener voltage is denoted by V_Z and is given for a reference test current I_ZT. At that test point the diode exhibits a small-signal or dynamic impedance which depends on the device construction, the value of V_Z and the level of Zener current.

Two physical mechanisms produce reverse breakdown in Zener diodes. For breakdown voltages below about 6 V the Zener effect (strong electric field causing field emission and band tunnelling) dominates. For breakdown voltages above roughly 6 V the avalanche effect (impact ionisation) is the principal mechanism. Many commercial diodes use a combination of both effects near 6 V.

The Zener diode is commonly used as a voltage reference or shunt regulator by connecting it in reverse across the load so that the load voltage is held close to V_Z over a range of line or load variations. The diode has a temperature coefficient, α_Z, which is typically negative for V_Z below about 6 V and positive for higher voltages. The change in Zener voltage for a temperature change ΔT may be written

\[\Delta V_z = \alpha_Z \, V_z \, \Delta T\]

Because of its finite dynamic resistance (often written r_z or R_Z), the voltage across a Zener diode at a given current is not perfectly constant; a useful linear approximation is

\[V = V_z + I_z \, r_z\]

Zener Regulator

A simple Zener regulator uses a series resistor to limit current from a supply and a reverse-biased Zener diode as a shunt element across the load. When the diode is forward biased it behaves like a normal diode with a forward drop of approximately 0.7 V; when reverse biased beyond its Zener voltage it holds the voltage at about V_Z while excess current flows through the diode.

The load line relationship for the circuit is

\[V_s = R_s I_s + V_z\]

To ensure the Zener diode operates in its breakdown region, the supply voltage V_s must be greater than V_Z. The series resistor R_s limits the total current. For design and calculations the following relations are useful.

The series resistor required for a given supply voltage, Zener voltage, Zener current and load current is

\[R_s = \frac{V_s - V_z}{I_z + I_L}\]

The maximum Zener current allowed by power dissipation is

\[I_{Z(\text{max})} = \frac{P_{Z(\text{max})}}{V_z}\]

Because the Zener diode has an on-state resistance, increasing current produces additional voltage drop. If the operating point shifts from Q₁ (with current I₁) to Q₂ (with current I₂) the voltage change is

\[V_2 - V_1 = (I_2 - I_1) \, R_z\]

In differential form this is

\[\Delta V_z = \Delta I_z \, R_z\]

Zener diode as regulator

V-I Characteristics of Zener diode

• High-precision reference Zener diodes are available with temperature coefficients as low as 0.0005 %/°C (reported values for specially trimmed devices).Operating point in V-I Characteristics of Zener diode
• The admission of a small amount of mercury gas is a technique historically used to increase current capability in some hot-cathode gas-filled tubes (not typical for solid-state Zener diodes).
• Cold-cathode glow-discharge devices may be used as crude DC voltage regulators in a manner analogous to a Zener diode under certain conditions.

Practical design notes:

• Choose R_s so that at minimum supply voltage the Zener current remains above the knee current required for regulation and, at maximum supply voltage, the Zener current does not exceed the device dissipation limit.
• Consider the Zener dynamic resistance and temperature coefficient when tight voltage tolerance is required.
• For higher power or tighter regulation use an active regulator or a series pass transistor with the Zener used as a reference rather than a simple shunt regulator.

Optoelectronic Devices

Modern solid-state emitters, detectors and related components that interact with light are known as optoelectronic devices or optoelectronics. These devices convert between electrical and optical energy and are widely used in sensing, communication, illumination and power generation.

• A tungsten filament lamp emits predominantly in the infrared with a smaller portion in the visible; it is often used as a source of infrared radiation.
• Illumination is the measurable visible flux incident on a surface and is expressed in lumens per square foot or foot-candles (or in SI units, lux = lumens/m2).
• Irradiance is the total radiant power incident on a surface and is measured in watts per unit area (for example W/cm2).

Light Absorption and Emission

Light absorption and emission in semiconductors depend strongly on the band structure. The probability of photon absorption and the strength of spontaneous emission are determined by whether the material has a direct or indirect bandgap.

Direct Bandgap Semiconductors

In a direct bandgap semiconductor the conduction band minimum and valence band maximum occur at the same crystal momentum (same k). This allows electron-hole recombination without a phonon and therefore gives a large optical absorption coefficient and efficient light emission. Materials such as GaAs and GaAsP are examples used in LEDs and laser diodes.

Indirect Bandgap Semiconductors

In an indirect bandgap semiconductor the conduction band minimum and valence band maximum occur at different k values. Optical transitions therefore require a phonon to conserve momentum; the absorption coefficient is smaller and spontaneous emission is weak. Silicon and germanium are indirect (germanium is close to direct in some respects) and are rarely used as efficient light emitters, though they are widely used as detectors.

Photon absorption in a direct bandgap semiconductor

Indirect bandgap semiconductor assisted photon absorption

 Indirect bandgap semiconductor assisted by photon emission

Photodiode (Junction Photodetectors)

A junction photodetector (photodiode or photo-transistor) uses one or more p-n junctions biased in reverse. Photons incident near the depletion region generate electron-hole pairs; the built-in electric field separates these carriers and produces a photocurrent that adds to the reverse current. The device is often used as a fast light sensor or as a switching element in light-activated circuitry.

Symbol of Photo-diode

Note: Silicon and germanium photodetectors have peak spectral sensitivity in the infrared; their response in the visible region may be only about 40% of the infrared peak for a given device type.

• Photo-transistors provide internal current gain owing to transistor action; they typically give higher sensitivity than simple photodiodes at the cost of slower response.
• Combining a phototransistor with a conventional transistor can form a photo-Darlington amplifier with still higher sensitivity.
• Photodiodes and phototransistors are commonly used as switching devices in light-operated relays, shaft encoders, paper-tape readers, brushless DC motor commutation sensors and many other applications.

Common variations of junction photodetectors:

• PIN photodiode - an intrinsic region between p and n gives fast response and high sensitivity for high speed applications.
• Avalanche photodiode (APD) - internal gain through impact ionisation yields higher sensitivity and fast response when biased above breakdown with a controlled high voltage.
• Standard photodiode - inexpensive, moderate sensitivity and typically slower response compared with PIN or APD types.

Photovoltaic Sensors (Solar Cells)

Photovoltaic sensors (solar cells) are p-n junction devices that convert incident optical radiation into electrical energy. In open-circuit conditions they generate a voltage typically on the order of 0.5 V per junction for silicon devices; the short-circuit current is approximately proportional to illumination and can be used to measure light intensity directly. Many solar cells are connected in series and parallel to make panels for power generation.

Circuit diagram of photo voltaic cell

When a photon with energy greater than the semiconductor bandgap is absorbed, an electron is excited into the conduction band leaving a hole in the valence band. These photogenerated carriers are separated by the junction field and produce a photocurrent that flows through an external load without an external bias.

• A solar cell is self-generating and does not require an external power source to produce current when illuminated.
• The internal emf and short-circuit current of a solar cell can be measured directly, for example with a galvanometer or suitable meter.
• No practical solar cell can convert all incident solar radiation into electrical energy; conversion efficiency is limited by material properties, optical losses, recombination and thermodynamic factors.

Light Emitting Diodes (LEDs)

A light emitting diode (LED) is a specially engineered p-n junction that emits light under forward bias by spontaneous radiative recombination. The emitted wavelength depends on the semiconductor alloy and bandgap: visible LEDs are commonly made from GaAsP and related III-V compounds, infrared LEDs from GaAs, and ultraviolet LEDs from wide bandgap compounds.

Basic Structure of light Emitting Diode (LED)

LEDs are efficient, compact light sources used for indicators, displays, illumination and optical communication. When driven with very high current pulses and with appropriate device structure, some semiconductor junctions (typically GaAs or related alloys) can produce stimulated emission and operate as laser diodes. A common small-signal application is the seven-segment LED display which provides high brightness with milliwatts of input power per segment.

Summary

This chapter has described the operation, characteristics and applications of Zener diodes and simple Zener regulators, and has surveyed principal optoelectronic devices: photodiodes, photo-transistors, photovoltaic sensors (solar cells) and LEDs. Key practical points include correct selection of series resistance and power rating for Zener regulators, the distinction between direct and indirect bandgap materials for emitters and detectors, and the trade-offs among speed, sensitivity and cost for photodetector types.