Unit 6: P-N Junction Diode

P-N Junction Diode

• A P-N junction diode is a type of semiconductor device that has a junction between p-type and n-type materials.
• The p-side has an excess of holes, while the n-side has an excess of electrons.
• When the two materials are brought together, the holes from the p-side diffuse to the n-side, and the electrons from the n-side diffuse to the p-side, creating a depletion region.
• The depletion region acts as a barrier, preventing further diffusion of carriers.

I-V Characteristics

• The I-V characteristics of a p-n junction diode are extremely asymmetric.
• In forward bias, the voltage drops exponentially, making the current increase rapidly.
• In reverse bias, the current is very small until the voltage reaches the breakdown voltage, at which point the current increases rapidly.

Depletion Region

• The depletion region is the region around the junction where the concentration of charge carriers is zero.
• The width of the depletion region depends on the applied bias voltage.
• The depletion region acts as a capacitor, and its capacitance depends on the width of the depletion region.

Band Diagram

• A band diagram is a graphical representation of the energy levels of a semiconductor material.
• The band diagram can be used to visualize the behavior of electrons and holes in a p-n junction diode.
• The Fermi level is the energy level at which the probability of finding an electron is 50%.

Built-in Potential

• The built-in potential is the potential difference between the p-side and n-side of a p-n junction diode.
• The built-in potential is responsible for the depletion region and the I-V characteristics of the diode.

Avalanche Breakdown

• Avalanche breakdown occurs when the electric field in the depletion region is high enough to accelerate charge carriers to high energies.
• At high energies, charge carriers can collide with the lattice, generating new charge carriers through impact ionization.
• Avalanche breakdown can lead to a rapid increase in current and potentially damage the device.

Zener Breakdown

• Zener breakdown occurs when the electric field in the depletion region is high enough to allow charge carriers to tunnel through the potential barrier.
• Zener breakdown is a type of tunneling phenomenon that occurs at high doping levels.
• Zener breakdown is useful for voltage regulation applications.

Applications of Breakdown

• Breakdown can be useful in certain applications, such as voltage regulation and photodiodes.
• Zener diodes can be used as voltage regulators, providing a stable reference voltage.
• Avalanche photodiodes can be used as highly sensitive light detectors.

Transistors

• Transistors are semiconductor devices that can amplify or switch electronic signals.
• Transistors are the building blocks of modern electronics.
• Transistors will be discussed in more detail in the next unit.Here are the study notes for the text:

The Transistor Concept and Junction FET

First Working Transistor

• Developed at Bell Labs (USA) in 1948 by Bardeen, Brattain, and Shockley
• Won the 1956 Physics Nobel prize

The Water Tap Analogy

• Large current of water (electric current) flowing through a pipe (conductor)
• Tap (transistor) controls the flow of water (current)
• Small force on the tap (input) controls a large force at the output

Contacts, Connections, and Conventions

• Transistor is a 3-terminal device: source (S), drain (D), and gate (G)
• Source and drain are the current flow leads
• Gate is electrically isolated from the channel (no current flows from gate to channel)
• Gate influences conduction through the channel via the electric field produced by the gate voltage

Junction FET

Structure

• Central n-type channel enclosed between thin p+ gate layers
• Source and drain connected to an external circuit that applies a bias VDS
• VGS applied relative to source contact, to both p+ gate layers

Effect of VGS (Gate Bias)

• Assume VDS small
• Apply gate voltage VGS on depletion region
• VGS > 0: depletion width reduces, resistance reduces, IDS for a particular VDS is higher
• VGS < 0: depletion width increases, resistance increases, IDS for a particular VDS is lower

Effect of VDS (S-D Bias)

• Assume VGS = 0
• VDS = 0: IDS = 0, depletion region has constant width
• Small VDS: IDS vs VDS characteristic is linear
• Larger VDS: channel narrowing, IDS reduces, 'knee' in IDS vs VDS characteristic
• Very large VDS = VP: pinch-off, depletion regions almost as wide as channel

Modes of Operation

Active Mode

• IDS vs VDS characteristic is flat and current varies little with changes in VDS
• Channel acts like a constant current source
• Effect of gate bias VGS very strong in this region

Saturation and Cut-off Modes

• Saturation mode: channel has very low resistance, IDS limited by VDS
• Cut-off mode: channel has very high resistance, IDS = 0

Bipolar Junction Transistors (BJTs)

Similarities and Differences with FETs

• Both transistors with 3 terminals
• Both devices produce I vs V characteristics with a similar form
• Both devices capable of amplification and switching
• Differences: BJT has emitter, base, and collector terminals, whereas FET has source, gate, and drain terminals
• FET: source-drain current controlled by voltage applied to the gate
• BJT: emitter-collector current controlled by current flowing into the base### Bipolar Junction Transistors (BJTs)
• A BJT has two flavors: p-n-p and n-p-n, with similar operation but different polarities
• Unipolar devices, conduction occurs only with majority carriers
• BJTs have three terminals: emitter, base, and collector
• Electrical schematics: BJT has a similar symbol to FET, but the arrow appears on the emitter contact (BJT) rather than the base contact (FET)
• The direction of the arrow corresponds to the device flavor, with an inward-pointing arrow indicating p-type dominant current

Operation of BJTs

• A BJT consists of two back-to-back p-n junction diodes
• BJT operation requires a small distance between the emitter and collector (base width Wb)
• In the unbiased state, no current flows, and electrons in the emitter and collector are prevented from entering the base by a barrier
• In the active mode, the BJT can be broken into four steps:
  1. Reverse bias VBC is applied between the base and collector, widening the base-collector depletion region and increasing the E-field across the B-C junction
  2. Forward bias VBE is applied between the emitter and base, injecting electrons from the emitter into the base and holes from the base into the emitter
  3. Electrons injected into the base diffuse across the base towards the collector, and some of them make it to the base-collector junction without recombining with majority holes in the base
  4. Electrons that reach the base-collector junction are swept into the collector by the strong E-field at the reverse-biased B-C junction

Forward Active Operation

• 3-step process:
  1. Injection: EB junction is forward-biased, electrons are injected into the base, and holes are injected into the emitter
  2. Diffusion: Increased electron density in the base causes electrons to diffuse towards the collector, with some recombining with holes and annihilating
  3. Collection: Electrons that reach the base-collector junction are swept into the collector by the strong E-field

Anatomy and Fabrication of Realistic BJTs

• Realistic BJTs are built entirely in the surface layer of a Si wafer, with active p-n junctions just below the wafer surface
• Emitter, base, and collector contacts lie on the device surface, with current flow from emitter to collector via the base
• Core of the device is an n-type epitaxial layer, which is the collector, with a p-type base implanted and an n+ emitter implanted or made into an n-type ohmic contact

Heterojunction Bipolar Transistors (HBTs)

• Use different semiconductor materials for the emitter, base, and collector
• Emitter has a larger band-gap than the base and collector, increasing the barrier to hole injection from base to emitter
• Reduces parasitic hole current, increasing the emitter efficiency and hence the gain

Photodiodes

• Effectively the inverse of a light-emitting diode (LED)
• LED is a forward-biased p-n junction in which radiative recombination of electrons and holes produces light
• Photodiode is a reverse-biased p-n junction where light is absorbed to produce electrons and holes that form the output current

Operation of Photodiodes

• Incoming photons generate e-h pairs at a constant rate G per unit volume in the device
• The photocurrent is made up of three components:
  1. Depletion region: photons absorbed in the depletion region generate e-h pairs that are rapidly swept apart by the strong electric field
  2. Neutral n-type region: e-h pairs are generated at the rate G per unit volume, and holes within the hole diffusion length Lp of the edge of the depletion region can diffuse towards the depletion region
  3. Neutral p-type region: the same occurs for electrons on the neutral p-type side within the electron diffusion length Ln of the edge of the depletion region
• The total photocurrent is the sum of these three components

Maximizing the Photo-Response

• Aim to maximize the number of optically-generated electrons and holes that make it out of the semiconductor and contribute to the photocurrent
• Gain shows how much larger the photocurrent is compared to the normal current of electrons and holes flowing through the device
• The e-h pair generation rate per volume is given by the quantum efficiency and the volume of the sample

p-i-n Photodiodes

• Designed to increase the response time of the photodiode by minimizing the slow diffusion current and maximizing the prompt photocurrent
• The depletion region is significantly larger than in the p-n diode, and the electric field across it is much stronger
• Photons generate e-h pairs throughout the intrinsic region, and the electrons and holes are separated by the strong field and rapidly swept into the n- and p-type regions
• The device response is fast for two reasons:
  1. The width of the intrinsic region is much larger than the diffusion lengths in the n- and p-type materials
  2. The strong field sweeps the carriers out of the intrinsic region

Avalanche Photodiodes (APDs)

• A p-n junction operated at sufficiently high reverse-bias that avalanche multiplication occurs
• Photons create e-h pairs, which lead to a photocurrent as for the p-n photodiode
• The high reverse-bias leads to a high electric field, which accelerates the electrons and holes, causing them to collide with the lattice and create additional e-h pairs