Tunnel diode operation is an outcome of adjusting semiconductor doping to shift valence and conduction bands.
As a result, forward biasing the tunnel diode produces tunneling current up to a maximum value until an increase in potential restricts current flow.
The negative resistance region of the I-V curve offers many intriguing properties, typically in high-frequency circuitry.
[Tunnel diode applications are vast due to the negative resistance portion of its I-V curve.]
The diode is the basis of all semiconductor devices; built from only a single junction of electron-rich/hole-poor and electron-poor/hole-rich doped regions, it acts as a current control to the system by only promoting one-way current flow (within ratings). Their usage is ubiquitous across all circuit designs, and several variants exist suited for more specific implementations. One of these descendants is the tunnel diode, which acts like a backwards-installed diode with a sinusoidal I-V curve in the forward-biased direction (pre-saturation). Tunnel diode applications include amplification, rectification, and oscillation at high frequencies.
Pros and Cons of Tunnel Diode Applications
Altering I-V Curves Through Depletion Region Contraction
Most devices exhibit a positive correlation between voltage and current – this proportionality is the basis of Ohm’s law. A tunnel diode is unique because a greater bias reduces current flow throughout the device due to a higher doping of the P– and N-type semiconductor regions. Tunneling, or the process in which an electron passes through a barrier due to the nonzero wave probability according to quantum mechanics, allows charge carriers to penetrate a classically forbidden region of a potential energy barrier it can’t surmount. Compared to a typical PN junction diode, the tunnel diode exhibits a negative resistance region while operating in the forward direction. While much of the operation otherwise follows that of a standard diode, the negative resistance significantly alters the shape of the I-V curve, producing a sinusoidal lobe from low to high voltages during forward bias.
Consider the output of the tunnel diode: much like with a standard diode, I-V behavior is similar at the edges, with an increasing current at the beginning of the forward conduction until arriving at a local maximum current pre-saturation. From this point, additional forward biasing causes the current to decrease to a local minimum before rebounding and increasing as it enters saturation with unbounded current output until dielectric breakdown and device failure. This behavior is due to heavier doping at the PN junction (the depletion region). Normally, a diode’s current flows from the P-type semiconductor to the N-type semiconductor with a forward bias; the increased doping aligns the N-type semiconductor's conduction bands with the P-type semiconductor's valence bands and reverses the polarity. Shortening the depletion region's width increases the tunneling rate during forward bias, allowing current to flow (up to the maximum tunneling current) before the increasing bias causes a gap in the energy levels, and the diode stifles forward-direction current flow.
In general, tunnel diodes are antiquated due to the manufacturing process restricting the size and compactness of the packaging – designers must carve out the board space for through-hole assembly, which can restrict automated soldering methods or require costly secondary soldering steps. The functional power output of the device is low, and various field effect transistors (FETs) have largely replaced them in this role. However, the part is cost-efficient, remains in production, and is available from many manufacturers, and microstrip tunnel diodes can bypass component issues, given ample board space. Another notable attribute is the extreme longevity of the part, with many components exhibiting continued operation for well over a half-century without failure or noticeable performance degradation.
Tunnel Diode Applications: High-Frequency Performance
The uses for tunnel diodes parallel general diode applications, with the added boon of the negative resistance furthering their utility:
- Reflection amplifier - A negative resistance device sinks DC power to source AC power (this is not a 1:1 conversion), allowing for amplification of the AC signal. A 3-port connector isolates input and output, while a negative resistor-filter network simultaneously sets the gain and cancels the reactance to prevent reflection losses.
- Rectification - Just like a “backward” PN (i.e., standard) diode, a tunnel diode can selectively pass one direction of current, transforming alternating current signals to direct current. A tunnel diode operating with a reverse bias has a square-law relationship between the input voltage and output current, leading to excellent linearity within its operating range. Because there is no differential input, the voltage doesn’t require offsetting.
- Zero-feedback oscillators - Feedback oscillators suffer from significant performance issues at high frequencies. Circuit designers can instead use the relationship between the magnitude of a tunnel diode’s negative resistance and the equivalent resistance of the tuning circuit for damping; most practical models use a negative resistance magnitude that is only slightly less than that of the tuner circuit’s equivalent resistance to “kickstart” the oscillation (as damping is negative) before the oscillations stabilize as damping goes to zero.
- Switching circuits - Switching circuits relying on hysteresis (system memory of prior states) can use tunnel diodes to reduce the number of active devices from two to one. While most transistor models are much smaller and more power-efficient than tunnel diodes, there’s the theoretical possibility of reducing board area and power consumption. From there, designers can create a variety of circuit functions like waveform shaping, timing, and flip flops for memory.
Cadence Offers Solutions for Diode Network Analysis
Tunnel diode applications are vast due to their negative resistance capabilities; while their package size and assembly integration may be lacking, they can be an inexpensive and reliable method for prototyping boards. Negative resistance provides extensive design latitude for variants of common features like amplification, oscillation, and switching. While component options often depend on practical elements like cost and availability, designers can use modeling tools to simulate performance and optimize layouts. Cadence’s PCB Design and Analysis Software suite gives designers cutting-edge tools to accelerate product development with a DFM-constraint focus. Together with the usability and power of OrCAD PCB Designer, board layouts have never been simpler.
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