ENG325 Semiconductor Devices SUSS Assignment Sample Singapore

ENG325 Semiconductor Devices is a course that focuses on the physics and engineering of semiconductors, including their properties, fabrication, and applications in electronic devices. Students in this course will gain a deep understanding of how semiconductors work and how they are used in electronic devices. They will also learn about the fabrication techniques used to create semiconductor devices and the design considerations that go into creating effective devices.

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In this section, we are discussing some assignment briefs. These are:

Assignment Brief 1: Discuss the fundamentals of semiconductor devices such as pn junction, bipolar junction transistors, field effect transistors, light emitting diodes and photovoltaic devices.

Semiconductor devices are electronic components made from semiconductor materials such as silicon or germanium that exhibit unique electrical properties. Here, we will discuss the fundamentals of five types of semiconductor devices: pn junction, bipolar junction transistors (BJTs), field-effect transistors (FETs), light-emitting diodes (LEDs), and photovoltaic devices.

PN Junction:

A PN junction is formed by joining a p-type semiconductor (containing holes as majority carriers) with an n-type semiconductor (containing electrons as majority carriers). The region where the two materials meet is called the depletion region, which is an area free of mobile charges. The PN junction acts as a diode, allowing current to flow in one direction but not in the other. When a forward bias voltage is applied to the PN junction, the depletion region narrows, allowing current to flow. When a reverse bias voltage is applied, the depletion region widens, preventing current flow.

Bipolar Junction Transistor (BJT):

A BJT has three regions: the emitter (heavily doped n-type), the base (lightly doped p-type), and the collector (moderately doped n-type). The base region is thin, allowing electrons to diffuse across to the collector region. When a small current flows into the base, it controls a much larger current flowing between the emitter and collector, making the BJT an amplifier. BJTs can be either npn or pnp, depending on the doping of the materials.

Field-Effect Transistor (FET):

FETs are voltage-controlled devices that use an electric field to control the flow of current. The most common type of FET is the metal-oxide-semiconductor FET (MOSFET), which has three terminals: the source, the gate, and the drain. The gate is separated from the channel (formed by the source and drain) by a thin layer of insulating material (oxide). When a voltage is applied to the gate, an electric field is created, which controls the flow of current between the source and drain. FETs can be either n-channel or p-channel, depending on the doping of the channel material.

Light-Emitting Diode (LED):

An LED is a diode that emits light when current flows through it. LEDs are made from a semiconductor material doped with impurities to create a p-n junction. When a forward bias voltage is applied to the LED, current flows through the junction and electrons combine with holes, releasing energy in the form of light. LEDs are used in a variety of applications, including displays, lighting, and indicators.

Photovoltaic Device:

A photovoltaic device (or solar cell) is a device that converts light into electrical energy. It is made from a semiconductor material that has a p-n junction. When light shines on the device, photons are absorbed, creating electron-hole pairs. The electric field at the junction separates the charges, creating a voltage difference between the p and n regions. This voltage difference causes a current to flow when the cell is connected to an external circuit. Photovoltaic devices are used in solar panels to generate electricity from sunlight.

Assignment Brief 2: Illustrate the breakdown mechanism in pn junctions and the characteristics of the different metal semiconductor contacts.

PN Junction Breakdown Mechanisms:

A PN junction is formed when a p-type semiconductor is joined to an n-type semiconductor. The PN junction has a depletion region that contains no free carriers and acts as an insulator. However, when a voltage is applied to the junction, the depletion region width decreases, and the junction allows current to flow.

There are two types of breakdown mechanisms that can occur in a PN junction:

Zener Breakdown:

When a PN junction is reverse-biased, the electric field across the depletion region increases. If the electric field is high enough, it can ionize the atoms in the depletion region, creating electron-hole pairs. If the reverse voltage is increased further, these electrons and holes can gain enough energy to ionize more atoms, leading to a chain reaction that results in a large current flow. This breakdown mechanism is called Zener breakdown.

Avalanche Breakdown:

In the avalanche breakdown mechanism, the electric field across the depletion region is high enough to accelerate the charge carriers to high speeds. As these carriers collide with atoms in the depletion region, they can ionize these atoms, creating more electron-hole pairs. This process can lead to a chain reaction, resulting in a large current flow.

Metal-Semiconductor Contacts:

When a metal is placed in contact with a semiconductor, a Schottky barrier is formed, which can affect the flow of charge carriers across the junction. The characteristics of metal-semiconductor contacts depend on the work function of the metal and the semiconductor’s doping level.

There are three types of metal-semiconductor contacts:

Ohmic Contact:

In an ohmic contact, the metal and semiconductor have similar work functions, so there is no barrier to the flow of charge carriers. This contact allows current to flow easily and is typically formed by depositing a metal with a low work function onto a heavily doped semiconductor.

Schottky Contact:

In a Schottky contact, the metal and semiconductor have different work functions, creating a potential barrier that reduces the flow of charge carriers. This contact is typically formed by depositing a metal with a high work function onto a lightly doped semiconductor.

Intermediate Contact:

An intermediate contact has work functions between those of an ohmic and Schottky contact. This type of contact is typically formed by depositing a metal with a moderate work function onto a moderately doped semiconductor.

Assignment Brief 3: Apply suitable mathematical model to study the performance of semiconductor devices.

There are several mathematical models that can be used to study the performance of semiconductor devices. One of the most commonly used models is the Shockley diode equation, which describes the current-voltage characteristics of a p-n junction diode.

The Shockley diode equation relates the current through a diode to the voltage across it, and is given by:

I = I0(e^(qV/kT) – 1)

where I is the current through the diode, I0 is the reverse saturation current, q is the charge of an electron, V is the voltage across the diode, k is Boltzmann’s constant, and T is the temperature in Kelvin.

This equation can be used to analyze the behavior of diodes under different operating conditions, such as forward and reverse bias, and to determine important parameters such as the diode’s breakdown voltage and its efficiency.

Other mathematical models that can be used to study the performance of semiconductor devices include the Shockley-Read-Hall model for recombination in semiconductors, the drift-diffusion model for carrier transport, and the Boltzmann transport equation for electron and phonon transport in semiconductors.

Assignment Brief 4: Appraise the circuit parameters for the transistor’s different operational modes.

The circuit parameters for a transistor’s different operational modes vary depending on the type of transistor, the biasing scheme used, and the specific application. However, here are some general circuit parameters that are commonly used for the three main operational modes of a bipolar junction transistor (BJT):

Active mode: In active mode, the transistor operates as an amplifier. The circuit parameters that are commonly used for active mode are:

• Base-emitter voltage (Vbe): This voltage must be high enough to forward-bias the base-emitter junction and provide sufficient base current to keep the transistor in active mode.
• Collector-emitter voltage (Vce): This voltage must be high enough to allow the transistor to operate in the active region and avoid saturation or cutoff.
• Collector current (Ic): This is the current flowing through the collector of the transistor and is proportional to the base current and the transistor gain.

Saturation mode: In saturation mode, the transistor acts as a switch and is fully turned on. The circuit parameters that are commonly used for saturation mode are:

• Base-emitter voltage (Vbe): This voltage must be high enough to forward-bias the base-emitter junction and fully turn on the transistor.
• Collector-emitter voltage (Vce): This voltage must be low enough to ensure that the transistor is fully turned on and is in saturation mode.
• Collector current (Ic): This is the maximum current that can flow through the transistor in saturation mode.

Cutoff mode: In cutoff mode, the transistor acts as an open switch and does not conduct any current. The circuit parameters that are commonly used for cutoff mode are:

• Base-emitter voltage (Vbe): This voltage must be low enough to reverse-bias the base-emitter junction and prevent any base current from flowing.
• Collector-emitter voltage (Vce): This voltage must be high enough to ensure that the transistor is in cutoff mode and no current flows through it.
• Collector leakage current (Ico): This is the small current that flows through the collector in cutoff mode due to minority carriers.

Assignment Brief 5: Assess the impact of change in bias voltage, temperature, size and other parameters on the functioning of semiconductor devices.

Semiconductor devices are electronic components made of semiconductor materials, such as silicon or germanium, that have electrical conductivity between that of a conductor and an insulator. The functioning of semiconductor devices is affected by various parameters, including bias voltage, temperature, size, and other factors. Here is an assessment of how these parameters affect the functioning of semiconductor devices:

Bias Voltage:

The bias voltage is the voltage applied across a semiconductor device. The performance of semiconductor devices is heavily dependent on the bias voltage, which determines the amount of current that can flow through the device. As the bias voltage increases, the current through the device also increases, up to a certain point called the saturation region. Beyond this point, the current tends to remain constant. This is true for most semiconductor devices, such as diodes, transistors, and integrated circuits.

Temperature:

Temperature also plays a crucial role in the functioning of semiconductor devices. Higher temperatures can cause an increase in the resistance of the device, leading to a decrease in the current flow. This effect is known as thermal runaway and can cause the device to fail if the temperature is not controlled. On the other hand, lower temperatures can decrease the resistance of the device, leading to an increase in the current flow. The temperature range within which a semiconductor device can operate without damage is usually specified in the datasheet.

Size:

The size of a semiconductor device affects its performance in many ways. Smaller devices tend to have faster response times and lower power consumption than larger devices. However, smaller devices also tend to generate more heat and can be more susceptible to noise and interference. In addition, smaller devices may have lower current-carrying capacity, which can limit their use in high-power applications.

Other Parameters: Other parameters that can affect the functioning of semiconductor devices include:

• Voltage and current ratings: These determine the maximum voltage and current that a device can handle without damage.
• Frequency: This determines the maximum frequency at which a device can operate.
• Material properties: The properties of the semiconductor material, such as its doping level and crystal structure, can affect the device’s performance.
• Environmental factors: Factors such as humidity, dust, and radiation can affect the reliability and performance of semiconductor devices.

Assignment Brief 6: Calculate bias voltage, mole fraction, size, current flow, voltage, resistance and other parameters associated with semiconductor devices.

1. Bias voltage: The bias voltage is the voltage applied to a semiconductor device to create a flow of current through it. It can be positive or negative, depending on the type of device and its intended function.
2. Mole fraction: The mole fraction is a measure of the concentration of dopants in a semiconductor material. Dopants are impurities added to the material to modify its electrical properties.
3. Size: The size of a semiconductor device can vary widely depending on its intended application. For example, microprocessors are very small, while solar panels can be quite large.
4. Current flow: The flow of current through a semiconductor device is controlled by the bias voltage and the electrical properties of the device.
5. Voltage: The voltage across a semiconductor device is the difference in potential between its two terminals.
6. Resistance: The resistance of a semiconductor device is a measure of its opposition to the flow of current. It can be influenced by factors such as temperature and the presence of impurities.

Other parameters that may be associated with semiconductor devices include capacitance, inductance, frequency response, and thermal properties.