Special Purpose Junction Diodes

The four devices

Four special purpose junction diodes are studied: the Zener diode, the photodiode, the solar cell, and the Light Emitting Diode (LED). The Zener diode works on the principle of junction breakdown; the other three make use of photosensitivity, an important and useful property of semiconductors.

16.3.1 Zener diode

Junction breakdown

When the reverse bias voltage of an ordinary junction diode is increased beyond a critical voltage, the reverse current increases sharply to a high value. That critical voltage is the reverse breakdown voltage, and an ordinary diode is damaged at this stage. Electrical breakdown of any material can occur by avalanche breakdown or by Zener breakdown; the syllabus discusses Zener breakdown.

Zener breakdown — mechanism

• When the reverse voltage across a p-n junction diode is increased, the electric field across the junction increases.

• This results in a force of attraction on the negatively charged electrons at the junction.

• The covalent bonds which hold the semiconductor together are broken by this force and electrons are removed from the bonds.

• These free electrons are then available for electrical conduction and result in a large current.

• As the applied voltage is increased further, the field increases and more and more electrons are removed from their covalent bonds, so the net current increases rapidly with the applied voltage.

• Zener breakdown occurs in diodes which are heavily doped, so that the depletion layer is narrow. Zener breakdown does not damage the diode. The field strength at breakdown in such a diode is about $3 \times 10^{7}\ \text{V/m}$.

Definition and symbol

A Zener diode is a heavily doped p-n junction diode designed to work in the breakdown region. Because of the heavy doping, it has a very thin depletion layer. It is used as a voltage regulator or voltage stabiliser.

Symbol in words: the ordinary diode symbol — a solid triangle pointing towards a straight bar — with the bar bent forward at the top and backward at the bottom, so that the cathode line looks like the letter Z.

I-V characteristic, described in words

• Forward bias (first quadrant): the Zener diode behaves exactly like a normal diode. Almost no current until the knee voltage, then the current rises steeply.

• Reverse bias (third quadrant): only a very small reverse saturation current flows as the reverse voltage is increased, until the reverse voltage reaches the Zener voltage $V_Z$. At $V_Z$ the reverse current suddenly increases and the curve drops almost vertically downwards.

• The important feature: once breakdown has occurred, the voltage across the diode remains constant at $V_Z$ even though the current through it increases by a large amount. This constant-voltage property is what is used in a voltage regulator.

• $V_Z$ depends on the amount of doping. A heavily doped diode has a thin depletion layer and breaks down at a lower reverse voltage; a lightly doped diode has a higher breakdown voltage. Zener diodes with breakdown voltage less than $6\ \text{V}$ operate mainly in the Zener breakdown region, those greater than $6\ \text{V}$ operate mainly in the avalanche breakdown region, but both are called Zener diodes.

Zener diode as a voltage regulator

A voltage regulator maintains a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current.

Circuit in words: a series resistance $R_S$ is connected from the positive terminal of the unregulated input supply $V_{in}$ to the cathode of the Zener diode. The Zener diode is connected in reverse bias, cathode to the positive side, with its anode returning to the negative terminal of the supply. The load resistance $R_L$ is connected in parallel with the Zener diode. The regulated output is taken across $R_L$. The requirement is $V_{in} > V_Z$, and $R_S$ limits the current flowing through the diode. Because $R_L$ is in parallel with the Zener, the voltage across $R_L$ is always equal to the Zener voltage, $V_O = V_Z$.

Working — the board's own sequence:

• When a Zener diode is operated in the breakdown region (reverse bias), the voltage across it remains almost constant even if the current through it changes by a large amount. Hence it can be used as a voltage regulator.

• A Zener diode of breakdown voltage $V_Z$ is connected in reverse bias to an input voltage source $V_{in}$, with $V_{in} > V_Z$.

• If the input voltage increases, the current through $R_S$ and the Zener diode also increases. This results in an increase in the voltage across the resistance $R_S$, but the voltage across the Zener diode does not change.

• The series resistance $R_S$ absorbs the output voltage fluctuations and maintains a constant voltage across the load resistance. Hence the output voltage remains constant irrespective of the change in the input voltage.

• If the input voltage is constant but the load resistance $R_L$ decreases, the load current $I_L$ increases. This extra current cannot come from the source, because the drop across $R_S$ does not change while the Zener is within its regulating range. The increase in $I_L$ is compensated by a decrease in the Zener current $I_Z$, keeping $V_Z$ constant.

• The voltage across the Zener diode remains constant at its breakdown voltage $V_Z$ for all values of Zener current $I_Z$, as long as the current stays in the breakdown region.

• Hence a regulated DC output voltage $V_O = V_Z$ is obtained across $R_L$ whenever the input voltage remains within a minimum and a maximum value.

Choosing the series resistance

When there is no load ($R_L$ infinite), the load current is zero and all the circuit current passes through the Zener diode, giving maximum power dissipation in the diode. Similarly a small value of $R_S$ gives a large diode current when a large $R_L$ is connected. The value of $R_S$ is therefore chosen so that the maximum power rating of the Zener diode is not exceeded when there is no load or the load is very high. The maximum power rating is

$$P_Z = I_{Z(\max)}\, V_Z$$

Voltage stabilisation is effective only when there is a minimum Zener current, and the supply voltage $V_S$ must always be greater than $V_Z$.

Specifications on a Zener diode datasheet

• Zener V-I characteristic.

• Zener voltage $V_Z$, also called the reverse voltage — the voltage at which the diode breaks down in reverse bias, and at which it is operated.

• Maximum Zener current $I_Z$ or $I_{ZM}$ — the maximum current that can flow through the diode at its rated voltage $V_Z$.

• Power rating — the maximum power that can be dissipated by the Zener diode package.

• Zener resistance $R_Z$, also called Zener impedance $Z_Z$ — the opposition offered to the current in the operating region. It behaves like a small resistance in series with the ideal Zener, so changes in $I_Z$ cause small changes in $V_Z$; this is why $V_Z$ is not strictly constant.

Applications of a Zener diode

Voltage regulator; fixed reference voltage provider in transistor biasing circuits; peak clipper or limiter in wave shaping circuits; protection of a meter against damage from accidental fluctuations.

Worked example (Example 16.2)

A $5.0\ \text{V}$ stabilised supply is to be designed from a $12\ \text{V}$ DC source using a Zener of maximum power rating $P_Z = 2.0\ \text{W}$.

• Maximum Zener current: $I_Z = P_Z / V_O = 2.0/5.0 = 0.4\ \text{A} = 400\ \text{mA}$.

• Minimum series resistance: $R_S = (V_S - V_Z)/I_Z = (12.0 - 5.0)/0.400 = 17.5\ \Omega$.

• Load current with $R_L = 1\ \text{k}\Omega$: $I_L = V_Z/R_L = 5.0/1000 = 5.0\ \text{mA}$.

• Zener current at full load: $I_Z = I_S - I_L = 400 - 5 = 395\ \text{mA}$.

16.3.2 Photodiode

Definition and symbol

A photodiode is a special type of p-n junction diode which converts light energy into electrical energy. It generates current when exposed to light and is also called a photodetector or photosensor. It operates in reverse biased mode, and only minority carrier current flows through it.

Symbol in words: the ordinary diode symbol with two small arrows pointing towards the junction, showing light falling on it. The triangle side is the anode, the bar side the cathode.

Construction

The p-n junction of a photodiode is placed inside a glass material so that only the junction is exposed to light. The other part of the diode is painted with an opaque colour or covered, so that stray light cannot reach it.

Working principle — the board's own sequence

• When a p-n junction diode is reverse biased, a reverse saturation current flows through the junction.

• The magnitude of this current is constant for a certain range of reverse bias voltages. This current is due to the minority carriers on either side of the junction — electrons are the minority carriers in the p-region and holes are the minority carriers in the n-region.

• The reverse current depends only on the concentration of the minority carriers and not on the applied voltage. This current is called the dark current, because it flows even when the photodiode is not illuminated.

• When the p-n junction is illuminated with photons whose energy is greater than the band gap of the semiconductor, electron-hole pairs are generated in the depletion region.

• The electrons and the holes are separated due to the intrinsic electric field present in the depletion region. The electrons are attracted towards the anode and the holes towards the cathode. More carriers are therefore available for conduction and the reverse current increases.

• The reverse current of a photodiode depends on the intensity of the incident light. Thus the reverse current can be controlled by controlling the concentration of the minority carriers in the junction.

The total current through a photodiode is the sum of the photocurrent and the dark current.

Characteristics, described in words

• I-V characteristic: drawn in the reverse-bias region. For each fixed intensity of illumination the curve is nearly horizontal — the reverse current is almost independent of the reverse voltage. The curves are stacked one above another, the curve for a higher illumination lying at a higher reverse current. The bottom curve, for zero illumination, is the dark current.

• Reverse current versus intensity: a graph that rises with increasing intensity of illumination and then levels off at a constant value. That constant value is called the saturation current of the photodiode.

Dark resistance

Dark resistance $R_d$ is the resistance of a photodiode when it is not illuminated, defined as

$$R_d = \dfrac{\text{Maximum reverse voltage}}{\text{Dark current}}$$

The sensitivity of the device is increased by minimising the dark current.

Advantages, disadvantages, applications

• Advantages: quick response when exposed to light; linear response, the reverse current being linearly proportional to the intensity of the incident light; high speed of operation; light weight and compact size; wide spectral response (silicon photodiodes respond from $190\ \text{nm}$ in the UV to $1100\ \text{nm}$ in the IR); relatively low cost.

• Disadvantages: properties are temperature dependent, like those of many semiconductor devices; low reverse current at low illumination levels.

• Applications: counters and switches; burglar alarm systems; detection of visible and invisible radiations; circuits requiring fast switching and high speed operation; fibre optic communication systems; optocouplers, which provide electrical isolation between two circuits; photo sensors or detectors for accurate measurement of light intensity; safety electronics such as fire and smoke detectors. The idea common to all of them is that a photodiode conducts whenever light strikes it and stops conducting the moment the light stops.

16.3.3 Solar cell (photovoltaic cell)

Principle

Solar energy is pollution free and available free of cost. Devices converting it into usable form are of two kinds: photothermal devices, which convert solar energy into heat, and photovoltaic devices, which convert solar energy into electrical energy using solar cells. Light incident on a solar cell produces both a current and a voltage, generating electric power; a solar cell thus works as a source of DC power. As the board puts it: a solar cell uses solar energy and converts it into electric energy, which is why it is also called a photovoltaic cell.

Structure

• It consists of a p-n junction.

• The n-side of the junction faces the solar radiation and is made thin, so that the incident light reaches the depletion region where the electron-hole pairs are generated.

• The p-side is relatively thick and is at the back of the cell.

• Both the p-side and the n-side are coated with a conducting material. The contact on the n-side is the front contact, and that on the p-side is the back or rear contact.

• The n-side carries an antireflection coating which allows visible light to pass through but reflects the IR (heat) radiation, protecting the cell from heat. This matters because the electronic properties of semiconductors are sensitive to temperature fluctuations. This coating also works as the electrical contact of the cell.

Material requirements: firstly it must be a photosensitive material which absorbs light and raises electrons to a higher energy state; secondly the higher energy electrons so generated must be able to be taken out of the cell into an external circuit, where they dissipate their energy and return to the cell.

Working — the four steps

• Electron-hole pairs are generated in the depletion region of the p-n junction. These are the photo-generated carriers.

• The electrons and holes are separated and collected at the cathode and the anode respectively.

• The carriers accumulate and generate a voltage across the solar cell.

• The power thus produced is dissipated (utilised) in the load resistance or the circuit connected across the cell.

The current produced is called the light-generated or photo-generated current, and it is a two-step process. First, incident photons of energy greater than the band gap are absorbed and generate electron-hole pairs. Normally such pairs would simply recombine and be lost, giving no current. In a solar cell they are spatially separated and prevented from recombining, by the intrinsic electric field of the depletion region: a light-generated electron in the p-type region that reaches the junction is swept across by the junction field into the n-type region, where it is now a majority carrier, and a light-generated hole similarly reaches the p-type region. Negative and positive charge therefore accumulate on the n-region and the p-region, and the cell acts as a voltage source. When an external circuit is connected, the light-generated carriers flow through it.

V-I characteristic, described in words

The illuminated characteristic is drawn in the fourth quadrant, because a solar cell supplies current to the load instead of drawing current from a source.

• The intersection of the curve with the current axis is the short circuit current $I_{sc}$, for the given light intensity. Here the load is short-circuited, the voltage is zero, and the power delivered is zero.

• The intersection of the curve with the voltage axis is the open circuit voltage $V_{oc}$, for the given light intensity. Here the load is open, the current is zero, and the power delivered is again zero.

• Between these two extremes there is one point on the curve at which the power delivered to the load is maximum — the maximum power point.

Criteria for selection of the material

• Band gap should be between $1.0\ \text{eV}$ and $1.8\ \text{eV}$.

• It should have high optical absorption.

• It should have good electrical conductivity.

• The material should be easily available.

Most solar cell materials have a band gap of about $1.5\ \text{eV}$: Si ($E_g = 1.1\ \text{eV}$), GaAs ($1.43\ \text{eV}$), CdTe ($1.45\ \text{eV}$), CuInSe ($1.04\ \text{eV}$). Cells for domestic and space use are mostly silicon based. Solar cells are non-polluting, need little maintenance and last long, but have a high installation cost and low efficiency.

Uses of a solar cell

• Charging batteries during the day so that the batteries can supply power at night.

• Supplying power at remote places, to equipment ranging from calculators to satellites and space stations.

• Supplying power to traffic signals and to communication stations.

• In a lux meter, to measure the intensity of light.

16.3.4 Light Emitting Diode (LED)

Definition and symbol

The Light Emitting Diode, or LED, is a diode which emits light when a large forward current passes through it.

Symbol in words: the ordinary diode symbol — triangle pointing to a bar, anode A on the triangle side, cathode K on the bar side — with two small arrows drawn pointing away from the junction, indicating the emitted light.

Construction

• The n-region is more heavily doped than the p-region of the p-n junction.

• The LED p-n junction is encased in a dome-shaped transparent case, so that light is emitted uniformly in all directions and internal reflections are minimised. The domed epoxy top acts like a lens and concentrates the light, which is why an LED looks brightest at the top.

• Metal electrodes attached on either side of the p-n junction serve as contacts for external electrical connection. The larger (longer) leg of an LED is the positive electrode or anode.

• The LED is fabricated in such a way that the light emitted is not reabsorbed into the material.

• It is ensured that the electron-hole recombination takes place at the surface, for maximum light output.

Working

• When the LED is forward biased, electrons from the semiconductor's conduction band recombine with holes from the valence band, releasing sufficient energy to produce photons, which emit monochromatic (single colour) light.

• Because the layer is thin, a reasonable number of these photons can leave the junction and emit coloured light.

• The amount of light output is directly proportional to the forward current: the higher the forward current, the higher the light output.

I-V characteristic, described in words

The characteristic is similar to the forward characteristic of an ordinary diode: negligible current until the cut-in voltage is reached, after which the current rises steeply. The LED starts conducting after its cut-in voltage is reached. The forward voltage drop of an LED is much larger than that of an ordinary diode, around $1.5\ \text{V}$ to $3.5\ \text{V}$. The current rating is only a few tens of milliampere, so a high resistance must be connected in series with an LED.

Colour and material

LEDs are fabricated using compound semiconductors made with elements such as gallium, phosphorus and arsenic. Varying the proportions changes the wavelength of the emitted light: AlGaAs gives infrared radiation, GaAsP gives red or yellow light, AlGaP gives red or green light, and ZnSe gives blue light. So the wavelength of the light emitted by an LED depends on the compound semiconductor used and the proportions of its constituent elements — that is, on the band gap of the material.

Advantages

Energy efficient (more light output per watt; up to $135$ lumens/watt); long lifetime ($50\,000$ hours or more); rugged, being solid state lights with no filament, tube or bulb to break; almost no warm-up period, emitting light within nanoseconds; excellent colour rendering, with colours that do not fade out; environment friendly, containing no mercury or other hazardous substances; controllable brightness and colour.

Disadvantages

Hazardous blue light quality, temperature dependence, voltage sensitivity, high initial cost.

Applications

Burglar alarm systems, counters, optical communication, indicator lamps in electrical equipment, display screens of cell phone handsets, LED television, vehicle head lamps, domestic and decorative illumination, street lighting, and seven segment displays used in calculators, electronic balances, watches and digital instruments.

White LED

White LEDs are made by wavelength conversion — partly or completely converting the radiation of an LED into white light. The least expensive method uses a blue LED to excite a yellow phosphor; the resulting mixture of blue and yellow light appears white. The blue LED was invented by Shuji Nakamura, who received the Nobel Prize in Physics in 2014.

Distinctions the board asks for

Standard results

• $V_O = V_Z$; $P_Z = I_{Z(\max)} V_Z$; $R_S = (V_S - V_Z)/I_Z$; $I_L = V_Z/R_L$; $I_Z = I_S - I_L$.

• Zener field strength at breakdown, heavily doped diode: about $3 \times 10^{7}\ \text{V/m}$; $V_Z < 6\ \text{V}$ is Zener breakdown, $V_Z > 6\ \text{V}$ is avalanche breakdown.

• $R_d =$ maximum reverse voltage divided by dark current; total photodiode current $=$ photocurrent $+$ dark current.

• Solar cell band gap window $1.0$ to $1.8\ \text{eV}$; Si $1.1\ \text{eV}$, GaAs $1.43\ \text{eV}$, CdTe $1.45\ \text{eV}$, CuInSe $1.04\ \text{eV}$.

• LED forward drop $1.5$ to $3.5\ \text{V}$; current rating a few tens of milliampere; light output proportional to forward current.