Diodes: Characteristics and Behavior

• This section will provide insight on PN semiconductors

Diodes

• Diodes are made of a small piece of semiconductor material, typically silicon
• Half of it is doped as the p region and the other half is doped as the n region
• Between the p and n regions is the pn junction with a depletion region in between
• The p region is the anode connected to a conductive terminal
• The n region is the cathode connected to a second conductive terminal

Forward Bias Condition

• To bias a diode dc voltage is applied across
• Forward bias is the condition that allows current through the pn junction
• During forward bias, voltage narrows the depletion region and creates a voltage

Reverse Bias Condition

• Reverse bias prevents current through the diode
• The extremely small current that exists in reverse bias is caused by minority carriers, known as reverse saturation current

Shockley’s Equation

• General characteristics of a semiconductor diode is defined by this equation
• The voltage in equation 1.1 is called the thermal voltage

General characteristics of a semiconductor diode

• I_D = I_s(e^(V_D/NV_T)-1)
  • I_D is the reverse saturation current
  • V_D is the applied forward-bias voltage across the diode
  • n is an ideality factor, ranges between 1 and 2
• V_T= k T_k/q
  • k is Boltzmann's constant = 1.38 × 10^-23 J/K
  • T_K is the absolute temperature in kelvins = 273 + the temperature in °C
  • q is the magnitude of electronic charge = 1.6 × 10^−19 C

Reverse Breakdown

• The reverse current can normally be neglected
• If the external reverse-bias voltage increases to a value called the breakdown voltage, the reverse current drastically increases, known as breakdown potential
• The maximum reverse-bias potential that can be applied before entering the Zener region is the peak inverse voltage, the PIV rating, aka the peak reverse voltage, or the PRV rating

Silicon vs Germanium diodes

• Silicon diodes generally have a higher peak inverse voltage and current rating
• Silicon also has wider temperature ranges than germanium diodes
• Peak inverse voltage ratings for silicon can be in the neighborhood of 1000 V
• The maximum value for germanium is closer to 400 V
• Silicon can be used for applications in which temperature may rise to about 200°C (400°F)
• Germanium has a much lower maximum rating, 100°C

Temperature Effects

• The reverse saturation current doubles in magnitude for every 10°C increase in temperature

DC or Static Resistance

• The application of a dc voltage to a circuit with a semiconductor diode provides an operating point on the characteristic curve that doesn’t change with time
• R_D = V_D/I_D

AC or Dynamic Resistance

• Varying input moves the instantaneous operating point up and down defining a specific change in current and voltage
• Q-point derived from the word quiescent, means still or unvarying
• r_d = ∆V_d/∆I_d
• The lower the Q-point (lower current or voltage) the higher the ac resistance

Average AC Resistance

• If the device carries a sufficiently large input signal it results in a swing, the resistance gets the average ac resistance
• r_av = ∆V’_d / ∆I’_d

Explore diode behavior: reverse saturation current with temperature, operating point determination, and AC resistance changes relative to the Q-point. Understand distinguishing characteristics between silicon and germanium diodes, the importance of clean room environments in manufacturing, and the impact of exceeding Peak Inverse Voltage (PIV) ratings.