PN-JUNCTION

OUTLINE Concept of energy band for p-type and n-type. Basic structure of pn junction. Pn junction at equilibrium. The biased pn junction Forward bias Reversed bias Iv characteristic For forward bias For reversed bias

The pn-Junction Some key concepts: • Electron states in a solid are grouped into ENERGY BANDS; these bands are typically separated by ENERGY GAPS in which no electron states exist. • In a semiconductor at low temperature, bands up to and including the VALENCE BAND are full of electrons; the CONDUCTION BAND (the next band up), and all higher energy bands, are empty. • Electrons in bands which are either completely empty or completely full cannot contribute to conduction, so a semiconductor at low temperature will not conduct.

• At higher temperatures, some electron will have enough thermal energy to ‘jump’ across the energy gap into the conduction band; these electrons, and the HOLES they leave behind in the valence band, can both contribute to conduction, so the semiconductor will show some conductivity.

Conduction predominantly by electrons in conduction band. • By doping the semiconductor, we can increase the numbers of electrons or holes to make n-type or p-type material: Conduction predominantly by electrons in conduction band. Conduction predominantly by holes in valence band.

Pn-junction Basic structure of the pn junction P region : doped with acceptor impurity atoms. Eg: boron. N region : doped with donor impurity atoms. Eg: phosphorus. Interface separating the n and p region is referred as metallurgical junction.

The net positively and negatively charge regions are referred as the space charge region or depleted region. All electrons and holes are swept out of the space charge region by the electric field. The electric field in the space charge region produces another force on the electrons and holes which is in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium, the diffusion force and the E-field force exactly balance each other.

The pn Junction at Equilibrium A p-n junction is formed at the interface between n-type and p-type regions in a semiconductor. Because the electron concentration is much higher in the n-type region than in the p-type, electron will tend to DIFFUSE from the n-side to the p-side. However, this leaves a net positive charge on the n-side, resulting in an electric field which tends to produce an electron flow in the opposite direction. At equilibrium, these effects exactly balance, and the net electron current is zero. In the thermal equilibrium, there are no currents exist and no external excitation is applied. A similar argument applied to the hole current.

How maximum width of the diffusion region is reach without any external voltage? Density gradient exist due to the different concentration of hole and electron. Density gradient exert diffusion force. Diffusion force on electron will push negative charge toward p-region. Diffusion force on holes will push positive charge toward n- region. Accumulation of positive charge accumulate near metallurgical junction at n- region and vice versa. Induced electric field will produce in direction from n-region to p-region. Electric field force will produce electric field force or electron will push from p-region to n-region and vice verse for hole. When electric field and diffusion force reach thermal equilibrium, no charge will move. Diffusion process will stop. The maximum width of the diffusion region is reach.

• At equilibrium:

At equilibrium: • We can think of the built-in voltage as a ‘POTENTIAL BARRIER’ which opposes the diffusion of electrons from n- to p- and holes from p- to n-.

The Biased PN Junction When pn junction is formed, depletion region is created and movement of holes and electron stops. So, the current flowing through unbiased pn junction is zero. To make the current to flow, the pn junction diode need to be biased. The pn junction is considered biased when an external voltage is applied. There are two types of biasing: Forward bias Reverse bias.

The Biased PN Junction Forward Bias Vapplied > 0 Reversed bias In forward bias the depletion region shrinks slightly in width. With this shrinking the energy required for charge carriers to cross the depletion region decreases exponentially. Therefore, as the applied voltage increases, current starts to flow across the junction. The barrier potential of the diode is the voltage at which appreciable current starts to flow through the diode. Under reverse bias the depletion region widens. This causes the electric field produced by the ions to cancel out the applied reverse bias voltage. A small leakage current, Is (saturation current) flows under reverse bias conditions. This saturation current is made up of electron-hole pairs being produced in the depletion region.

Forward bias P region (anode) is connected to positive terminal of DC supply and n region (cathode) is connected to negative terminal of DC supply. Resistance is connected in series with diode to limit current flowing through it. Positive end of supply will push holes from p side to n side. Negative end of supply push free electron from n side to p side. When supply voltage is increased, more electron and holes travel towards the junction. Holes converts negative ions into neutral atoms and electron convert positive ions into neutral atoms. Depletion region width reduces. So, the barrier potential reduced.

At one stage, the depletion region will collapses At one stage, the depletion region will collapses. So more number of electrons and holes cross the junction which produces forward currents. The current through p region is due to the movement of holes (majority carriers) and current on n side is due to the movement of free electrons (majority carriers). The current flow from anode to cathode. There will be a potential drop across diode which has opposite polarity to barrier potential, but its magnitude is same.

The pn Junction in FORWARD BIAS

• Resulting electric field: Ɛ = Ɛ0 – ƐVF • Ɛ smaller => drift does not cancel diffusion => depletion region smaller => potential barrier q(V0 - Vf) smaller ->large FORWARD CURRENT • The Fermi Levels in the two neutral regions are separated by an energy qVf where Vf is the externally applied forward voltage. • Large current by diffusion: majority carriers diffuse across barrier.

Reversed Bias P region is connected to negative terminal of DC supply and n region is connected to positive terminal of DC supply. Reverse currents flows from cathode to anode. Resistance is connected to limit reverse current. Holes are attracted forward negative terminal of power supply and electrons are attracted forwards positive terminal of power supply.

Electrons and holes are moving away from junction Electrons and holes are moving away from junction. So, depletion region increases and the barrier potential increases. The magnitude of electric field also increase. Small no of electrons will be presents in p region and small number of holes will be presents in n region. These are called minority carriers. Minority carriers (electron) in p region are attracted by positive of DC supply. These electron will cross the junction which produces reverse saturation current. The reverse saturation current depend on temperature. Avalanche breakdown occur when a sufficiently large reversed-bias voltage is applied to the pn junction. A large reversed-bias current may then be induced in the pn junction.

The pn Junction in REVERSE BIAS

• Resulting electric field: Ɛ = Ɛ0 – ƐVR • Ɛ larger => diffusion does not cancel drift => depletion region larger => potential barrier q(V0 + Vr) larger ->small REVERSE CURRENT • The Fermi Levels in the two neutral regions are separated by an energy qVr where Vr is the externally applied reverse voltage. • Small (drift) current: minority carriers are swept across potential barrier due to electric field. • Current limited by the amount of minority carriers available near depletion region.

FIGURE: A pn junction and its associated energy band diagram for (a) zero bias, (b) reverse bias, and (c) forward bias.

I-V Characteristic Ideal Current-Voltage Relationship The ideal current-voltage relationship of a pn junction is derived on the basis of four assumptions. They are.. 1. The abrupt depletion layer approximation applies. The space-charge regions have abrupt boundaries and the semiconductor is neutral outside of the depletion region. 2. The Maxwell-Boltzmann approximation applies to carrier statistics. 3. The concept of low injection applies. 4. a) The total current is constant throughout the entire pn structure. b) The individual electron and hole currents are continuous functions through the pn structure. c) The individual electron and hole currents are constant throughout the depletion region.

IV Characteristic for forward bias The current in forward biased called forward current and is designated If. At 0V (Vbias) across the diode, there is no forward current. With gradual increase of Vbias, the forward voltage and forward current increases. A resistor in series will limit the forward current in order to protect the diode from overheating and permanent damage. A portion of forward-bias voltage drops across the limiting resistor. Continuing increase of Vf causes rapid increase of forward current but only a gradual increase in voltage across diode.

IV Characteristic for reverse bias With 0V reverse voltage there is no reverse current. There is only a small current through the junction as the reverse voltage increases. At a point, reverse current shoots up with the break down of diode. The voltage called break down voltage. This is not normal mode of operation. After this point the reverse voltage remains at approximately VBR but IR increase very rapidly. Break down voltage depends on doping level, set by manufacturer.

I-V Characteristic of a p-n Junction • Behavior in both forward and reverse bias is described by the Ebers-Moll Equation: where IS is the reverse saturation current, and VT = kT/q is the thermal voltage (≈ 25 mV at room temperature) The complete V-I characteristic curve