P-N Junctions
Combining P-type and N-type Semiconductors
By sandwiching P-type and N-type semiconductor together into what's known as a P-N Junction, interesting electrical properties arise from the underlying atomic drama. The P-N Junction is the probably the single most important structure in electronics and is the heart of many common semiconductor components such as diodes, transistors, integrated circuits (ICs), solar cells, and more.
Atomic Behavior
When P-type and N-type semiconductors are joined, the free electrons from the N-type side diffuse nearby into the P-type side to fill the holes:
What remains are the positively and negatively charged ions along the junction boundary, since they have been stripped of the free particles that were keeping that area electrically neutral. This region is called the depletion region (along with half a dozen other names):
Junction Electric Field
The depletion region gives rise to an electric field because there's a electromagnetic force (voltage) differential between the negatively and positively charged ions:
This electric field is known as the Junction Potential and is usually between 600mV - 700mV (0.6V - 0.7V) in silicon, and 200mV - 300mV in germanium.
Anode + Cathode
Putting metal connectors on either side of the P-N Junction creates what are known as an anode (A) and a cathode (K). The anode is on the P-type side, and the cathode is on the N-type side. This effectively creates a component called a diode.
Forward Biasing
Applying a positive current to the anode and a negative current to the cathode side is a preferential bias, because the positive hole charge carriers and negative electrons are pushed towards the junction, thus shrinking the depletion zone. This voltage application, in the preferential direction of current flow is called forward biasing:
In order for charge carriers (current) to pass through the electric field, they must have greater electromotive force (EMF/voltage) than the junction potential to overcome the electric field. Meaning that it takes greater than 0.7V (in silicon) of EMF to get them through:
In the process of overcoming that junction potential, they actually lose the same amount of EMF to it. It's like holding a door open on a spring; some of the force is lost to the spring.
Voltage Drop (Vf)
The amount of voltage needed to push forward through the depletion region is known as the forward voltage (Vf), or voltage drop, because if charge carriers are pushed through this region at 1.0V, when they come out on the other side they will only have 0.3V (1.0V - 0.7V) because they dropped 0.7V in force.
Vf is like a Water Dam
An interesting consequence of voltage drop is that it provides a sort of back-pressure; any voltage above Vf is let past the junction, but the voltage measured on the side pushing will read the same as the Vf. In this way, revisiting the water analogies, we can think of a P-N junction as a dam in which the dam wall is Vf high, and any additional voltage is let past:
Voltage drop is an important concept that will come up over and over in circuit design, and as we'll examine shortly, that back-pressure can be very useful.
Reverse Biasing
Reverse biasing is when a voltage is applied in the opposite direction of preferential current flow. This causes the holes in the P-type side to be attracted to the anode and the electrons in the N-type side to be attracted to the cathode, causing the depletion region to widen:
When this happens, the area of negative and positively charged ions increases, which also increases the junction potential.
Leakage Current/Reverse Saturation Current
However, while the junction potential is actually much higher when reverse biased, some minuscule amount of current will actually flow because the polarity of the depletion region is opposite to the small amount of minority carriers that are present. Holes can flow from P-type to N-type and electrons can flow from N-type to P-type. This current is very small to the point of negligibility, usually in the range of nano-amperes (nA). A nano amp is 1 / 1,000,000,000A.
Breakdown Voltage and Avalanche Current
While a reverse biased P-N Junction (diode) will resist the overwhelming majority of current flow in the reverse direction, at a high enough voltage, known as the reverse breakdown voltage, an avalanche breakdown will occur. The electrons will then flow so quickly that they knock other electrons out of their holes, which leads them to knocking out more, and so on, causing an "avalanche" of electron flow.
Some diodes are specifically designed with this behavior in mind, and we'll examine them in the next section.
Voltage and Current Response Characteristics
Generally, the current response to forward and reverse bias currents of a P-N junction is described by the following graph:
The right half of the graph represents forward bias current, that is, positive current applied to the cathode. Until the voltage reaches the forward voltage (Vf), almost zero current makes it through, but once that threshold is reached, the P-N junction readily conducts electricity.
The left half of the graph represents a reverse bias current, that is, positive current applied to the anode. Virtually no current (nano amps) is allowed to flow through until the voltage reaches the reverse breakdown voltage and an electron avalanche occurs. If pushed at a high enough voltage, the resistance goes to almost zero, turning it into a pure conductor, and then, usually, the diode will break in an open state (no current flow).
As we'll learn in the next section, the behavior after reaching breakdown voltage can be used to do some useful things.