Unit 2: Diodes and its Application

1. p-n Junction as a Diode

A p-n junction is formed when a p-type semiconductor (rich in holes) is metallurgically joined to an n-type semiconductor (rich in electrons). This interface creates a fundamental building block of solid-state electronics: the basic semiconductor diode.

• Depletion Region: Upon formation, electrons diffuse from the n-side to the p-side, and holes diffuse from the p-side to the n-side. This recombination leaves behind immobile positively charged donor ions on the n-side and negatively charged acceptor ions on the p-side, creating a region devoid of mobile charge carriers called the depletion layer.
• Barrier Potential: The separated immobile charges create a built-in electric field opposing further diffusion. This results in a barrier potential ( or ), approximately 0.7 V for Silicon (Si) and 0.3 V for Germanium (Ge) at room temperature.
• Biasing:
  • Forward Bias: Positive terminal connected to p-region, negative to n-region. Shrinks the depletion width, allowing significant current flow.
  • Reverse Bias: Positive terminal connected to n-region, negative to p-region. Widens the depletion width, blocking current (except for a tiny leakage current).

2. Band Structure of an Open-Circuited p-n Junction

In an open-circuited (unbiased) p-n junction, the system is in thermal equilibrium.

• Fermi Level Alignment: The fundamental rule of thermal equilibrium is that the Fermi level () must be constant across the entire structure.
• Band Bending: To maintain a constant , the conduction band () and valence band () must bend at the junction. The energy bands of the p-side are shifted upward relative to the n-side.
• Built-in Potential (): The shift in energy bands is equivalent to the built-in potential energy (). This potential hill prevents majority carriers (electrons in n-type, holes in p-type) from spontaneously crossing the junction without external energy.

3. Current Components in a p-n Diode

The total current in a p-n diode is the sum of hole current and electron current. These currents are driven by two distinct physical mechanisms:

1. Diffusion Current: The movement of majority carriers due to a concentration gradient. Holes diffuse from p to n, and electrons from n to p. This current dominates during forward bias.
2. Drift Current: The movement of minority carriers swept across the junction by the built-in electric field in the depletion region. This current dictates the reverse saturation current.

Under open-circuit conditions, the net current is zero because .

4. V-I Characteristics of a Diode

The relationship between voltage () and current () is nonlinear and is defined by the Shockley Diode Equation:

Where:

• = Reverse saturation current.
• = Applied voltage.
• = Ideality factor (1 for Ge, 2 for Si under low currents, often assumed as 1 for practical models).
• = Thermal voltage ( at room temperature, ), defined as .

Regions of the V-I Curve:

• Forward Bias (): Current rises exponentially once the applied voltage exceeds the cut-in (knee) voltage ( for Si).
• Reverse Bias (): Current is practically constant at . It is independent of applied reverse voltage until breakdown.
• Breakdown Region: If the reverse voltage is too high, the diode enters avalanche or Zener breakdown, causing a sharp, massive increase in reverse current.

5. Temperature Dependence of Diode Parameters

Semiconductor properties are highly sensitive to temperature.

• Reverse Saturation Current (): approximately doubles for every rise in temperature.
  • Formula:
• Forward Voltage Drop (): To maintain a constant forward current, the required forward voltage decreases as temperature increases. The temperature coefficient is negative.

6. Diode Resistance

Since the V-I curve is non-linear, a diode does not have a single fixed resistance.

1. Static (DC) Resistance (): The resistance at a specific DC operating point (Q-point).
   • (Secant line from origin to Q-point).
2. Dynamic (AC) Resistance (): The resistance offered to small AC signals around a DC Q-point.
   • (Tangent to the curve at the Q-point).

7. Transition and Diffusion Capacitance

A diode exhibits capacitive effects that affect high-frequency operations.

• Transition Capacitance (): Dominant in Reverse Bias. The depletion region acts as an insulator (dielectric) between two conductive regions (p and n sides). As reverse voltage increases, depletion width increases, so decreases.
  • (for step-graded junctions).
• Diffusion Capacitance (): Dominant in Forward Bias. Caused by the storage of injected minority carriers near the junction before they recombine. is directly proportional to the forward current and is typically much larger than .

8. p-n Diode Switching Times

When a diode is suddenly switched from forward bias to reverse bias, it does not stop conducting instantly.

• Storage Time (): The time required for the stored minority charges in the depletion region to be removed. During this time, the diode conducts essentially a short-circuit reverse current.
• Transition Time (): The time taken for the transition capacitance to charge to the applied reverse voltage level.
• Reverse Recovery Time (): The total time taken to turn off the diode.

9. Special Diodes

Zener Diode

• Structure: Heavily doped p-n junction, resulting in a very thin depletion layer.
• Operation: Operates in the reverse breakdown region. When reverse voltage reaches the Zener voltage (), the electric field becomes intense enough to tear electrons from covalent bonds (Zener effect).
• Application: Voltage regulation, surge protectors.

Light Emitting Diode (LED)

• Structure: Made of direct bandgap semiconductors (e.g., GaAs, GaP, GaN).
• Operation: Forward biased. When electrons and holes recombine at the junction, they release energy in the form of photons (light). The color of the light depends on the bandgap energy ().

Tunnel Diode

• Structure: Extremely heavily doped (1000 times more than standard diodes). The depletion region is incredibly thin ().
• Operation: Exhibits quantum tunneling. Electrons tunnel directly through the thin barrier at low forward voltages.
• Characteristic: Features a Negative Differential Resistance (NDR) region on its V-I curve (current drops as voltage increases).
• Application: High-frequency microwave oscillators and fast switching.

p-i-n Diode

• Structure: An intrinsic (undoped or very lightly doped) semiconductor layer is sandwiched between a p-type and n-type layer.
• Operation:
  • Reverse bias: Acts as a nearly constant capacitance.
  • Forward bias: Acts as a current-controlled variable resistor.
• Application: RF switches, phase shifters, attenuators, and photodetectors.

10. Applications: Rectifier, Clipping, and Clamper Circuits

Rectifiers

Convert alternating current (AC) to direct current (DC).

• Half-Wave Rectifier: Uses one diode. Conducts only during the positive half-cycle.
  • Efficiency: 40.6%
  • Ripple Factor: 1.21
• Full-Wave Rectifier (Center-Tapped & Bridge): Uses two or four diodes to conduct during both half-cycles.
  • Efficiency: 81.2%
  • Ripple Factor: 0.48

Clipping Circuits (Limiters)

Circuits used to clip off or remove a portion of an AC waveform without distorting the remaining part.

• Series Clippers: Diode is in series with the load. (e.g., Half-wave rectifier is a simple series clipper).
• Parallel (Shunt) Clippers: Diode is in parallel with the load.
• Biased Clippers: A DC voltage source is added in series with the diode to shift the clipping level to a specific threshold above or below zero.

Clamper Circuits (DC Restorers)

Circuits used to add a DC shift to an AC signal, pushing the entire waveform above or below a certain reference voltage without changing its shape.

• Components: Requires a capacitor, a diode, and a resistor. The capacitor charges to the peak voltage of the input and acts as a DC battery.
• Positive Clamper: Shifts the waveform upward (minimum peak touches zero or reference).
• Negative Clamper: Shifts the waveform downward (maximum peak touches zero or reference).

11. Understanding the Datasheet of Diodes 1N4001 - 1N4007

The 1N400x series is a family of widely used 1.0 Ampere general-purpose silicon rectifier diodes. Understanding their datasheet parameters is crucial for circuit design.

Key Datasheet Parameters:

1. (Maximum Repetitive Peak Reverse Voltage): The maximum reverse voltage the diode can withstand continuously. This is the main difference among the series:
   • 1N4001: 50 V
   • 1N4002: 100 V
   • 1N4003: 200 V
   • 1N4004: 400 V
   • 1N4005: 600 V
   • 1N4006: 800 V
   • 1N4007: 1000 V
2. (Average Rectified Forward Current): The maximum continuous forward current. For the entire 1N400x series, this is 1.0 A.
3. (Non-Repetitive Peak Forward Surge Current): The maximum surge current the diode can survive for a very short duration (typically a 8.3 ms single half sine-wave). For this series, it is 30 A.
4. (Maximum Forward Voltage Drop): The voltage dropped across the diode when conducting. Typically specified as 1.1 V maximum @ 1.0 A (though often closer to 0.7V-0.9V in practice).
5. (Maximum Reverse Current): The leakage current when reverse-biased at maximum . Typically 5 A at , increasing to 50 A at .