Diode Voltage Drop(Explained For Beginner’s): What, Why, How

In this article, we shall discuss the diode voltage drop, why it is caused and how we can calculate it. A diode is a semiconductor device that allows one directional current flow and restricts current flow on another side.

Diode voltage drop basically refers to the forward bias voltage drop. It occurs in a diode present in an electrical circuit when current passes through it. This forward bias voltage drop is the result of the action of the depletion region formed by the PN junction under the effect of an applied voltage.

What is diode voltage drop?

Diode voltage drop is the result of the current flow from anode to cathode. When the diode is conducting in forward bias, the potential drop across it is known as the diode voltage drop or  forward voltage drop. 

Ideally, there should not be any voltage drop across the diode when it is carrying current and operating to generate DC output voltage. In real life, a small voltage drop occurs due to the forward resistance and forward breakdown voltage. For silicon, the diode voltage drop is around 0.7 Volt. 

Diode Voltage Drop
Diode Voltage Drop

How much voltage does a diode drop?

Any diode drops a particular amount of voltage across its terminals. A diode voltage drop of 0.7 V means the voltage through the resistor or load present in the circuit is (supply voltage – 0.7) volt.

The voltage drop across different diodes are different. Typically it ranges from 0.6 to 0.7 volts for a small silicon diode. For schottky diodes, the value of the voltage drop is 0.2 Volt. For light emitting diodes or LEDs, the voltage drop ranges from 1.4-4 Volt. Germanium diodes have a voltage drop of 0.25-0.3 volt.

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Why does a diode have a voltage drop?

The diode, in forward bias, selects a suitable voltage level so that it can push the electronic charges towards the P-N junction. It can be said analogous to “lifting” each ball up to the top of the table from the floor.

The difference in energy level required to move the electronic charges across the PN junction causes the voltage drop. Also, there is some resistance in the diode responsible for a certain amount of voltage drop. The voltage drop due to resistance depends upon the allowable current flow rate at the PN junction.

How to calculate diode voltage drop?

The voltage drops of different diodes are different. For a silicon diode, it is approximately 0.7 Volt, for germanium diode it is 0.3 volt and for schottky diode it is around 0.2 Volt. LEDs have various voltage drop values. 

Now if we want to calculate the voltage drop across any other element in the circuit, we need to subtract the voltage drop of the diodes present between that element and the source from the source voltage. So the voltage drop of that element is (source voltage- sum of diode voltage drops).

How to drop voltage using diode?

Zener diodes are good for dropping voltage. However, a trivial method to drop voltage using diodes is to connect multiple diodes in series with the supply. Each diode causes a voltage drop of almost 0.7 Volt.

Diodes only allow one directional flow of electricity, but the diode will conduct electricity only, when the supply touches the threshold. The standard silicon diode threshold is 0.6 volts. … After each diode is joined in series, the voltage drops by 0.6 volts. Utilizing this technique, we can drop voltage in a circuit using diodes.

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Schottky Diode Voltage Drop

Schottky diodes, featuring a metal-semiconductor junction, typically have a V_f between 0.15V and 0.45V. This range is due to variations in metal used (e.g., platinum, chromium) and semiconductor type (e.g., n-type silicon). The lower V_f results from the absence of minority carrier injection, which is prevalent in PN junction diodes.

Applications: Their low V_f makes them ideal for high-frequency applications and power rectification, where energy efficiency is critical.

Zener Diode Voltage Drop

Zener diodes are designed to operate in reverse bias, with a stable voltage drop (V_z) that can range from 2V to over 200V. The V_z depends on the doping level; heavily doped diodes have lower V_z. Zener diodes follow the Zener breakdown mechanism up to about 5.6V, and beyond that, avalanche breakdown dominates.

Applications: They are extensively used in voltage regulation and reference applications due to their ability to maintain a constant voltage over a wide current range.

Germanium Diode Voltage Drop

Germanium diodes, with a bandgap energy of about 0.66 eV, exhibit a V_f around 0.3V. Their performance is affected at higher temperatures due to increased intrinsic carrier concentration, leading to higher leakage currents compared to silicon diodes.

Applications: Although less common, germanium diodes are used in low-voltage applications and vintage electronic equipment restoration.

Silicon Diode Voltage Drop

Silicon diodes, with a bandgap energy of about 1.1 eV, have a V_f of approximately 0.7V. This value can vary slightly with doping concentration and diode construction. Silicon diodes maintain their characteristics over a wider temperature range compared to germanium diodes.

Applications: Their reliability and stability make them suitable for a wide range of applications, from power rectification to signal processing.

Diodes in Series Voltage Drop

In a series configuration, the total forward voltage drop is the cumulative sum of individual diode drops. For example, three silicon diodes in series at identical current levels will have a total V_f of approximately 2.1V. The current passing through each diode must be identical, as different currents can lead to uneven voltage drops and potential diode failure.

Applications: Series diode configurations are used in applications requiring higher voltage drops than a single diode can provide.

Diodes in Parallel Voltage Drop

In parallel connections, the voltage drop across each diode is the same as that of a single diode. However, matching diodes is crucial as differences in V_f and I/V characteristics can lead to unequal current sharing, potentially overloading one diode while underutilizing others.

Applications: Parallel diode configurations are used to increase current handling capacity while maintaining a specific voltage drop.


How to reduce voltage with zener diode?

Zener diode is a special case of diodes which permits current to flow reverse in a certain voltage, known as zener voltage. It can also reduce the reverse voltage and work as an efficient voltage regulator.

For using a zener diode to reduce voltage, we have to connect it in parallel with the load in the circuit. The supply voltage must be higher than the zener voltage and the diode should be in reverse bias. This connection helps to reduce the reverse voltage to a specific value and act as a voltage regulator.

Diode voltage drop formula

For the sake of simplicity, forward voltage drop across a diode is assumed as 0.7 V. Now, if there is only one diode in a circuit along with a load, the voltage drop across the load is (supply voltage – 0.7) Volt. 

In case of multiple diodes in series in a circuit, the voltage drop across the load is (supply voltage – the number of diodes * 0.7). For example, in image 1, the voltage drop across the diode D1= (5-0.7) = 4.3 V. The voltage drop across the diode D2= (5-2 * 0.7) = 3.6 V. The voltage drop across the diode D3= (5- 3 * 0.7) = 2.9 V. 

diode voltage drop
Image 1

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Diode voltage drop chart

Below table depicts the voltage drop limits of different kinds of diodes.

Type of the diodeVoltage drop
Silicon diode0.6-0.7 Volt
Germanium diode0.25-0.3 Volt
Schottky diode0.15-0.45 Volt
Red LED1.7-2.2 Volt
Blue LED3.5-4 Volt
Yellow LED2.1-2.3 Volt
Green LED2.1-4 Volt
White LED3.3-4 Volt
Orange LED2.03-2.20 Volt
Violet LED2.76-4 Volt

Diode Voltage Drop vs Temperature

Silicon diodes have a negative temperature coefficient of about -2 mV/°C. This decrease in voltage drop with temperature increase is due to enhanced carrier mobility. Schottky diodes, with a lower barrier height, exhibit a smaller temperature coefficient, typically around -1 mV/°C.

Application Insight: This property is significant in temperature-sensitive applications, where diodes can act as temperature sensors or compensators in circuits.

Diode voltage drop vs current

The voltage drop across a diode increases with current in a non-linear fashion. But as the differential resistance is less, the increase is very slow. We can consider the forward voltage vs current characteristics. 

In silicon diodes, as the current increases from 1 mA to 1 A, V_f (forward voltage drop) typically varies from 0.7V to 0.8V due to increased electron flow. Germanium diodes show a V_f increase from 0.3V to 0.4V under similar conditions. The Shockley equation details the relationship: V_f = nV_t ln(I/I_s + 1). Here, n (ideality factor) ranges from 1 (ideal diode) to 2 (real-world conditions), V_t (thermal voltage) is about 26 mV at room temperature, and I_s (saturation current) is in the nanoampere range.

From the IV curve, we can see that a large amount of increase in current initially yields a negligibly small increase in voltage. Then more quickly the voltage goes up, and eventually very quickly escalates. The IV curve shows an exponential growth of voltage with current. By the time Vd crosses 0.6/0.7 V, it moves up rapidly.

When voltage drop across a PN junction diode?

When current passes through any component present in a circuit, a voltage drop occurs. Similarly, when current passes through the diode in forward bias, then there is a voltage drop, known as forward voltage drop.

The p-n junction diode can not send the current from the junction in the reverse bias for very high resistance. The p-n junction acts like an open circuit, so the voltage drop across that ideal p-n junction diode remains the same. It is equal to the battery voltage.

Also, please click to know about Organic Light-Emitting Diodes.

Diode-Connected MOSFET Voltage Drop

A diode-connected MOSFET, with the gate and source shorted, exhibits a voltage drop equal to its threshold voltage (V_th), typically in the range of 0.7V to 1V. This drop is higher due to the MOSFET’s V_gs (gate-to-source voltage) requirement. The specific V_th depends on the MOSFET type and construction, with variations observed in different technology nodes.

Applications: Diode-connected MOSFETs are used in analog circuits as voltage references and in digital circuits for logic level shifting, benefiting from the MOSFET’s high input impedance and controlled voltage drop.