- PN Junction Diode Definition: A PN junction diode is defined as a semiconductor device that allows current to flow in one direction in forward bias and blocks current in reverse bias.
- Forward Bias: In forward bias, the p-type region is connected to the positive terminal and the n-type to the negative terminal, reducing the depletion layer and allowing current flow.
- Reverse Bias: In reverse bias, the p-type region is connected to the negative terminal and the n-type to the positive terminal, increasing the depletion layer and preventing current flow.
- Current Behavior: In forward bias, current flows easily once the depletion layer is reduced. In reverse bias, only a minimal current flows due to minority carriers.
- Breakdown Conditions: High reverse voltage can cause breakdowns (Zener or Avalanche), leading to a sharp increase in current, which is crucial in understanding diode operation limits.
Forward Biased PN Junction
A PN junction is forward-biased when the p-type region connects to the positive terminal of a voltage source and the n-type region connects to its negative terminal.
The external electric field opposes the junction’s built-in field. This lowers the potential barrier and narrows the depletion region, allowing majority carriers to approach and cross the junction.
Holes are injected from the p-type side into the n-type side, where they become minority carriers. Electrons are injected in the opposite direction and become minority carriers in the p-type material.
After crossing the junction, the injected carriers diffuse through the neutral regions and gradually recombine. The source continuously supplies majority carriers, which sustains the forward current.
The applied field reduces the uncovered ionised charge near the junction and therefore reduces the depletion width.
On the n-type side, the source’s negative terminal supplies electrons and drives them towards the junction. On the p-type side, the positive terminal removes electrons, which is equivalent to supplying holes. Both processes increase carrier injection as forward voltage rises.
The depletion region becomes narrower but does not disappear in a real forward-biased diode. A potential barrier remains, and the diode has a finite, nonlinear voltage-current characteristic rather than zero resistance.
Forward current rises approximately exponentially with junction voltage over the ideal operating range. A short-circuit model is only an idealised approximation and omits the forward voltage and dynamic resistance.
Reverse Biased PN Junction
A PN junction is reverse-biased when the positive terminal connects to the n-type region and the negative terminal connects to the p-type region.
With no external voltage, diffusion establishes a built-in potential and a depletion region. Values such as 0.3 V at 25oC for germanium and 0.7 V at 25oC for silicon are rough constant-drop estimates for forward-conducting diodes, not universal zero-bias junction potentials. Actual values depend on doping, current, temperature and device construction.
The reverse-bias electric field reinforces the built-in field. It raises the potential barrier and increases the depletion width, making majority-carrier injection across the junction extremely small.
The positive terminal draws majority-carrier electrons in the n-type region away from the junction. This exposes more positively ionised donors and extends the depletion region on that side.
At the same time, the negative terminal drives majority-carrier holes in the p-type region away from the junction. More negatively ionised acceptors are exposed, so the depletion region also extends into the p-type side.
The fixed ionised donors and acceptors produce a stronger junction field as reverse bias rises.
The depletion width increases approximately with the square root of the total potential across an abrupt junction. Its division between the p and n sides depends on their relative doping concentrations.
If the electric field becomes high enough, the junction enters Zener breakdown, one of the avalanche breakdowns, or a combination of the two mechanisms. Breakdown sharply increases reverse current; it does not make the depletion region disappear. An external circuit must limit current unless the device and operating point are designed for controlled breakdown.
Before breakdown, reverse voltage suppresses majority-carrier diffusion. It does not cancel the applied voltage with an equal barrier voltage. Most of the added voltage appears across the widening depletion region.
A small reverse current still flows because thermally generated electrons in the p-type semiconductor and holes in the n-type semiconductor are minority carriers. The junction field sweeps carriers that reach the depletion region across it. Surface leakage and generation within the depletion region can also contribute in real devices.
Forward Current in PN Junction
When a battery forward-biases the junction, the current is approximated by the Shockley diode equation shown below.
IS is the reverse saturation current. It varies widely with junction area, material, doping and temperature; values from 10-9 to 10-18 A are not a universal range.
VT is the thermal voltage kT/q, about 25.9 mV at 300 K.
n is the ideality factor, commonly between 1 and 2, and represents the dominant current mechanism.
The equation is an idealised model. Series resistance, high-level injection, leakage and breakdown require additional terms.
Reverse Current in PN Junction
With the n-type region at the battery’s positive terminal and the p-type region at its negative terminal, the junction is reverse-biased. The ideal diode equation predicts a small negative current iD before breakdown.
As reverse voltage becomes several thermal voltages in magnitude, iD approaches -ID. In the conventional notation used by the equation, this limiting magnitude is I0.
IS is the reverse saturation current; any range such as 10-9 to 10-18 A depends on the device and temperature.
VT is kT/q, about 25.9 mV at 300 K.
n is the ideality factor, often between 1 and 2 in practical junctions.
Real reverse current also includes generation, surface leakage and breakdown effects that the ideal equation omits.
General Specification of PN Junction
A diode data sheet specifies several limits and test-condition-dependent characteristics. Four common examples are listed below.
- Forward Voltage Drop (VF): The voltage measured at a specified forward current and junction or ambient temperature. Values such as 0.3 V for a germanium diode and 0.7 V for a silicon diode are only rough circuit-model estimates.
- Average Forward Current (IF): The permitted average forward current under stated thermal, waveform and mounting conditions. Exceeding it can overheat and damage the device.
- Peak Reverse Voltage (VR): The maximum permitted reverse voltage for the diode under the stated repetitive or non-repetitive condition. Exceeding the rating can cause breakdown and destructive current unless the device is designed and current-limited for that mode.
- Maximum Power Dissipation (P): The thermal limit on power converted to heat under specified temperature and mounting conditions. Forward conduction contributes approximately VFI, but reverse leakage, switching loss and derating can also matter.
V-I Characteristics of A PN Junction
Forward operation appears in the first quadrant. Current rises continuously and approximately exponentially rather than switching at an exact threshold. The often-used 0.3 V germanium and 0.7 V silicon values are nominal constant-drop approximations at selected currents. The curve’s local slope determines the diode’s small-signal resistance at each bias point.
Under reverse bias, the magnitude of voltage increases while majority-carrier injection remains suppressed. A small current flows from minority-carrier generation and leakage until the junction reaches breakdown.
At breakdown, a high electric field creates additional carriers by tunnelling, impact ionisation or both. Reverse current then rises sharply for a small voltage increase. Current must be limited to keep junction temperature and power within the device ratings.
- Avalanche Breakdown: Carriers accelerated by the field create electron-hole pairs through impact ionisation. Multiplication produces a rapid increase in reverse current.
- Zener Breakdown: A strong field across a narrow, heavily doped depletion region permits quantum-mechanical tunnelling. The transition is often sharp, but the actual curve depends on the device and series resistance.
Resistances of p-n Junction
Dynamic Resistance of p-n Junction
Because the I-V curve is nonlinear, a forward biased p-n junction does not have one constant resistance. Its small-signal or dynamic resistance rd equals dV/dI at the operating point, the reciprocal of the local dI/dV slope of the PN junction curve.
Large-Signal or Secant Resistance of p-n Junction
For a finite signal swing, use the straight line between the minimum and maximum operating points. The ratio of the change in voltage to the change in current is the secant resistance ΔV/ΔI over that excursion.
Other dynamic properties of a p-n junction
Transition Capacitance of PN Junction
In reverse bias, the diode behaves partly like a capacitor. The depletion region separates charge on the p-type and n-type sides and acts as the dielectric of a capacitor. Increasing reverse bias widens the depletion region, so the junction or transition capacitance decreases approximately as εA/W.
Diffusion Capacitance of PN Junction
Diffusion capacitance dominates in forward bias and is the differential change in stored injected charge divided by the change in junction voltage.
As forward current rises, more minority-carrier charge is stored in the neutral regions, so diffusion capacitance usually increases. Whether it exceeds transition capacitance depends on the bias point, frequency, carrier lifetime and device design.
Storage Time of PN Junction
When a conducting diode is abruptly driven into reverse bias, stored minority-carrier charge initially keeps it conducting. Storage time is the interval needed to remove enough of this charge for the junction to begin supporting reverse voltage.
Transition Time of PN Junction
After the reverse current reaches its peak, transition or fall time is the interval in which that current decays towards the specified leakage or reference level. It depends on carrier lifetime, junction geometry, doping and the external switching conditions.
Reverse Recovery Time of P-N Junction
Reverse recovery time is the interval required for a forward-conducting diode to regain reverse-blocking operation after current commutates through zero. A common data-sheet definition measures from the zero crossing until reverse current falls to 10% of its peak value. The result depends on prior forward current, di/dt, reverse voltage, temperature and device construction.
