2.1 Diode Operation

• Diode is a two-terminal electronic device that allows current to flow easily in one direction while presenting a high resistance in the reverse direction.
• PN junction is a border between p-type and n-type semiconductor combined, the most important region.
• Depletion Layer – the region at the junction of p- and n-type semiconductor where free electrons and holes recombine that creates pairs of oppositely charged ions on each side of the junction.
• Individually, P and N type extrinsic semiconductor acts just like a resistor. To maximize their use, N-Type and P-Type should be combine to create a DIODE.
• • N-Type and P-Type are electrically NEUTRAL.
  • P or N doesn’t mean an excess in ELECTRON or HOLE. It simply means that one ELECTRON or one HOLE doesn’t have a pair.
  • P or N refers only on Majority Carrier and not the charged.
• Depletion Region is also called:

  • DEPLETION ZONE
  • DEPLETION LAYER
  • SPACE CHARGE REGION
  • SPACE CHARGE LAYER
• The depletion region is an empty charge region. It acts like a wall between p-type and n-type semiconductor and prevents further flow of free electrons and holes. The width of depletion region depends on the amount of impurities added to the semiconductor.
• Types of Diodes: LED (Light Emitting Diode), Zener Diode, Varactor Diode, Optical Diode, Laser Diode, Schottky Diode, PIN Diode, Step-Recovery Diode, Tunnel Diode, Current Regulator Diode, Gunn Diode, Photo Diode, etc.
• Biasing:

  • Unbiased Diode
  • Forward Bias Diode
  • Reverse Bias Diode
• Barrier Potential (Threshold Voltage) - The minimum voltage required to break the electrochemical barrier between the inside and outside of the cell (depletion region). Electric field between ion is equal to the difference in potential (positive, negative), it is called the barrier potential.
• Voltage across the Depletion Region at 25°C (ambient temperature)

  • Germanium Vₜ=0.3V
  • Silicon Vₜ=0.7V
• Forward bias is the condition that allows current flows through the pn junction, from positive to negative terminal.
• The external bias voltage is designated as Vᵦᵢₐₛ. The resistor limits the forward current to a value that will not damage the diode. The negative side of Vᵦᵢₐₛ is connected to the n region of the diode and the positive side is connected to the p region. This is one requirement for forward bias. A second requirement is that the bias voltage, VBIAS, must be greater than the barrier potential (Vᵦ).
• As electrons from the n side are pushed into the depletion region, they combine with holes on the p side, effectively reducing the depletion region. This process during forward bias causes the depletion region to narrow.
• Battery push free electron in the junction

  1. Battery < Barrier Potential • No current will flow
  2. Battery = Barrier Potential • No current will flow
  3. Battery > Barrier Potential • Free electron has enough energy to pass through Depletion Region and recombines. • Current will start to flow
• Current will flow easily in Forward Bias Diode as long as Battery voltage is GREATER than barrier potential.
• Forward Bias:

  • ON Switch
  • Current will flow.
  • Depletion Region lessen.
• Ambient Temperature is the temperature outside the diode and is typically 25 degrees Celsius. Junction Temperature is the temperature inside the diode right at the junction.

  When diode is conducting: (due to heat created by recombination)

  Junction Temperature > Ambient Temperature
• As temperature increase:

  • Number of free electrons and holes increase
  • Depletion layer decrease
  • Threshold voltage decrease
• he barrier potential of a silicon diode decrease by 2mV for each degree Celsius rise, and 2.5mV decrease for Germanium.

  
• Reverse bias is the condition that essentially prevents current through the diode.
• The external bias voltage is designated as Vᵦᵢₐₛ just as it was for forward bias. The positive side of Vᵦᵢₐₛ is connected to the n region of the diode and the negative side is connected to the p region. Also note that the depletion region is shown much wider than in forward bias or equilibrium.
• • Negative terminal of the battery attracts holes. Positive terminal of the battery attracts electrons.
  • Holes and free electron flow away from junction.
  • Depletion region widens.
  • As the Reverse voltage increase, the Depletion region become wider, the Potential difference increase.
  • Stop when Potential Difference = Battery Voltage
  • Free electrons and Holes stop moving away from the junction.
• Reverse Bias:

  • OFF Switch
  • Current is approximately zero.
  • Depletion Region increase.
• Reverse Saturation Current:

  
• Basic Circuit:

  • Conventional Current
  • Electron Flow
• Convention Current: Current flow out of the positive terminal, through the circuit and into the negative terminal of the source.

  
• Electron Flow: electron flow out of the negative terminal, through the circuit and into the positive terminal of the source.

  
• Non-linear Device:

  • I vs V graph is not straight due to barrier potential.
  • When Vb is less than Vth, diode current is small, when Vb exceed Vth, diode current increase rapidly.

  
• Forward Region:

  • When Vb is less than Vth, diode current is small, when Vb exceed Vth, diode current increase rapidly.

  
• Reverse Region:

  • Negative Voltage input to a diode. Small current due to Surface Leakage, Transient and Saturation Current.

  
• Current Flowing in Reverse Bias Diode:

  1. Saturation Current
  2. Surface Leakage Current
  3. Transient Current
• Saturation Current

  • very small current
  • Due to thermal energy that continuously create a pair of free electrons and holes.
  • Called Minority-Carrier Current
  • Saturation Current is the reversed current caused by thermally produced minority carrier.
  • Saturation means can’t get more than what is produced thermally.
• Saturation Current

  • Increasing the reverse voltage (battery) will not increase saturation current since minority carrier is thermally created

  
• Surface Leakage Current

  • Small current that flows on the surface of the crystal caused by surface impurities and imperfection in the crystal structure.

  
• Transient Current

  • As reverse voltage increase, Depletion layer widens, as it adjust to its new width, current flow on the external circuit with its time dictated by the RC circuit.
  • When Depletion Layer stop to increase, transient current drop to zero.
  • It is Ignore at frequency less than 10Mhz (skin effect)

  
• General Rule:

  Current is approximately zero in Reversed Bias silicon diode (too small to notice).
• Breakdown Region:

  • Excessive Negative Voltage input to a diode.
  • Rapid Increase in Current due to avalanche.
  • Will destroy the diode.
  • Minimum reverse bias voltage that makes the diode conduct appreciably in reverse.
  • Maximum reverse bias voltage that can be applied without causing an exponential increase in the leakage current in the diode.
• Breakdown is characterized by the rapid increase of the current under reverse bias.
• Breakdown Voltage is the corresponding applied voltage, typically 50V.