In real-time circuits, a diode is selected so that the speed of operation matches the application and the diode offset voltage is minimized.
If the voltage applied across a diode (forward bias voltage) is less than its offset voltage, the circuit will not work. In such a condition, the diode acts as an open switch.
When forward bias voltage applied across a diode is greater than or equal to the barrier potential, the charge carriers start to penetrate the depletion region, conducting current. The voltage at which the diode starts to carry forward current is called diode offset voltage.
A diode is a basic PN junction device
To anyone with an electrical engineering background, a diode is a familiar semiconductor device. Diodes act as one-way switches for current, and signal diodes open doors to semiconductor devices.
In this article, we will discuss the considerations that go into selecting a diode as well as the semiconductor physics behind diode offset voltage.
How to Select a Diode
When choosing a diode for an application, it is essential to review the datasheet for terms such as forward voltage drop, reverse recovery time, breakdown voltage, and forward resistance.
The first and foremost parameter in selecting a diode for a circuit application is its forward voltage drop. The diode forward voltage drop is also called diode offset voltage, knee voltage, or contact potential.
All diodes come with a definite offset voltage greater than zero. In real-time circuits, a diode is selected so that the speed of operation matches the application, and the diode offset voltage is minimized. If the voltage applied across the diode (forward bias voltage) is less than its offset voltage, the circuit will not work. In such a condition, the diode acts as an open switch.
To understand diodes more fully, let’s review the physics behind diode offset voltage.
The Semiconductor Physics of Diode Offset Voltage
The depletion region of a PN junction
A PN junction is half P-type semiconductor material and half N-type semiconductor material—also called a signal diode. The P-region consists of majority carriers as holes and some negatively charged impurity ions. There are electrons in the N-region with few positively charged impurity ions. The majority of carriers-holes and electrons are mobile ions, whereas impurity ions are immobile. Charge on P-type material equals that of N-type material, and therefore, the signal diode is electrically neutral.
For this example, we are using a diode without any external bias. As soon as the PN junction is formed, holes from the P-region diffuse into the N-region and combine with the free electrons. Similarly, the electrons from the N-region diffuse to the P-region and combine with holes. The thermal energy and the difference in the concentration of holes and electrons in the two regions is the force for this diffusion. However, after a few recombinations of holes and electrons, further diffusion stops due to a barrier set by the uncompensated ions in the P-type and N-type regions.
With each hole-electron recombination, a negatively charged acceptor ion in the P-type region and a positively charged donor ion in the N-region are left uncompensated in the immediate neighborhood of the PN junction.
The region of uncompensated charges is called the depletion region, or space-charge region, and the electric field between acceptor and donor ions is called the barrier. The physical length of the depletion region is called the width of the barrier and the potential difference from one side to the other is called barrier height.
The barrier height is the real reason behind diode offset voltage.
The Dependency of a Diode’s Operation on Offset Voltage
With no external bias, the barrier height is about 0.7 V for silicon diodes and 0.3 V for germanium diodes. As the barrier hampers further diffusion of majority carriers, a diode acts as an open switch. In this condition, there is no electric current flow through the diode.
When external forward bias is applied across the diode, holes are repelled from the positive terminal of bias voltage towards the depletion region and electrons flow from the negative terminal of the external bias. Some holes and electrons acquire energy to penetrate the depletion region, and such carriers recombine and reduce the width and height of the barrier. Subsequently, more hole-electron pairs recombine and cause movement of charge carriers in the depletion region. At this point, the diode is conducting and is a closed switch.
When the forward bias voltage applied across the diode is greater than or equal to the barrier potential, the charge carriers start to penetrate the depletion region, conducting current. The voltage at which the diode starts to carry forward current is called diode offset voltage or knee voltage.
A diode’s operation is dependent on diode offset voltage
Offset-Diode I-V Characteristics and Model
In an ideal diode, there is no offset voltage. Once the forward bias voltage is greater than zero, the ideal diode starts to conduct.
In the figures below, figure (a) shows the ideal I-V characteristics of a diode. In figure (a), Id, and Vd display forward diode current and forward bias voltage, respectively. Figure (b) shows offset diode I-V characteristics, where the diode starts to conduct forward diode current Id once the forward bias voltage Vd is greater than or equal to diode offset voltage, Vg. The offset-diode model is shown in Figure (d).
Diodes are essential elements in both analog and digital electronic circuits. When using a diode, the diode offset voltage should be expressed as a selection parameter and considered in the design to ensure the smooth operation of a circuit.