BySquadron Leader Zahid Mir

CS&IT Department , Superior University

PHY-BE -05 Diode Biasing

What is Biasing?

Biasing means to use a dc voltage to establish certain operating conditions for an electronic device

Diode Biasing

• Forward Bias • Reverse Bias

Forward Biasing

• It is the condition that permits current across a pn-junction.

• Negative terminal of the battery is connected to the n-region (called cathode) and the positive terminal is connected to the p-region (called anode).

Forward Bias Operation

• Negative terminal of the battery pushes the conduction electrons in the n-region towards the junction.

• Positive terminal of the battery pushes the holes in the p-region towards the junction.

Detailed Picture : Forward Bias

• As free electrons move to p-region, new free electrons are injected by the –ve battery terminal into the n-region. Thus a flow of electrons is setup in the wire connected to –ve terminal of battery.

• As holes are driven towards the junction more holes are created in the p-region by the breakage of covalent bonds. The newly created holes are driven towards the junction to keep up a continuous supply

• But the electrons so created (bond breakage) are attracted to the left by the +ve battery terminal from where they go to the –ve terminal of the battery and finally to the n-region of the crystal.

• Note – There may be a movement of both electrons and

holes inside the crystal but only free electrons move in the external circuit.

– There is also present an extremely small amount of current “Io” due to minority carriers on either side of the junction. But it is negligible as compared to forward current due to majority carriers. (Minority current depends on temperature and is independent of applied voltage).

Energy Band Diagram: Forward Bias

• Forward bias of V volts lowers the barrier potential to (V-VB) which now allows more currents to flow across the junction.

Forward V/I Characteristic Curve• Forward current rises

exponentially with the applied voltage.

• At ordinary room temperature 0.3V for Ge is required before a reasonable amount of forward starts flowing in Ge. This voltage is called threshold voltage or cut-in voltage or knee voltage.

• Note Forward voltage may increase beyond a safe limit cause it may destroy junction.

Diode Dynamic Resistance

• Unlike conductors ( which has linear resistance) the resistance of the forward-biased diode is not constant over the entire curve. Since resistance changes as you move along the curve, it is known as dynamic or ac resistance denoted by r’d

r’d = VF

IF

• Below the knee of the curve the resistance is greatest because the current increases very little for a given change in voltage. Above the knee of the curve the resistance begins to decrease as current started increasing.

Reverse Biasing

• It is the condition that prevents current across a pn-junction.

• Negative terminal of the battery is connected to the p-region and the positive terminal is connected to the n-region.

Reverse Bias Operation• In n-region, positive terminal of the

battery pulls the free electrons in the n-region away from the junction.– As electrons flow toward the +ve side of

the voltage source, additional +ve ions are created. This results in widening of depletion region.

• In p-region, electrons from the negative side of the voltage source enter as valance electrons and move from hole to hole towards the depletion region where they create additional negative ions. This also results in a widening of depletion region.

Reverse Bias Operation

• As more of the n and p-regions become depleted of majority carriers, the electric field between the +ve & -ve ions increases in strength until the potential across the depletion region equals the bias voltage, VBIAS.

Reverse Current

• There is practically no current due to majority carriers, but there is a small current (few µA) due to minority carriers.

• Thermally generated holes in n-region and electrons in p-region, are driven by the battery across the junction.

• This reverse or saturation current reaches a saturation value because it is thermally generated and increase in reverse bias voltage has no effect on it.

• Reverse current is also known as leakage current.

Reverse V/I Characteristic Curve• As reverse voltage increases from

zero, the reverse current quickly rises to its maximum or saturation value.

• Keeping temperature constant, as the reverse voltage increases, reverse current found to increase slightly. This slight increase is due to the impurities on the surface of semiconductor. This is known as Surface Leakage Current.

• Unlike the main leakage (or saturation) current, this surface leakage current is independent of temperature but depends on the magnitude of reverse voltage.

Complete V-I Characteristic Curve

Temperature Effect• Forward Bias

– As temperature increases the forward current increases for a given value of forward voltage.

– Curve 1 is at room temperature (250 C) and curve 2 is at an elevated temperature (250 C + T).

– Barrier voltage decreases as temperature increases.

• Reverse Bias– Reverse current also increases

as temperature increases.– Note Reverse current below

breakdown remains extremely small and can usually be neglected.

References

• “A Textbook of Electrical Engineering” by

B L Theraja

• “Electronic Devices”by

Thomas L. Floyd (7th Edition)