11.6: Diodes

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Diodes are two-terminal devices that have nonlinear current-voltage characteristics. The most common diodes are listed in Table \(\PageIndex{1}\).

Junction and Schottky Diodes: A junction diode has an asymmetric current-voltage characteristic, see Figure 11.5.5(a),

\[\label{eq:1}I=I_{0}\left[\text{exp}\left(\frac{qV}{nkT}\right)-1\right] \]

where \(q(= −e)\) is the absolute value of the charge of an electron, \(k\) is the Boltzmann constant \((1.37\cdot 10^{−23}\text{ J/K})\), and \(T\) is the absolute temperature (in kelvin). \(I_{0}\) is the reverse saturation current and is small, with values ranging from \(1\text{ pA}\) to \(1\text{ nA}\). The quantity \(n\) is the diode ideality factor, with \(n = 2\) for a graded junction pn junction diode, and \(n = 1.0\) when the interface between p-type and n-type semiconductor materials is abrupt. The abrupt junction is most closely realized by a Schottky diode, where a metal forms one side of the interface. When the applied voltage is sufficiently positive to cause a large current to flow, the diode is said to be forward biased. When the voltage is negative, the current flow is negligible and the diode is said to be reverse biased. In a diode, charge is separated over distance and so a diode has junction capacitance modeled as

\[\label{eq:2}C_{j}(V)=\frac{C_{j0}}{(1-(V/\phi))^{\gamma}} \]

Component	Symbol
Diode, general (including Schottky)\(^{1}\)
IMPATT diode\(^{1}\)
Gunn diode
PIN diode\(^{1,2}\)
Light emitting diode (LED)\(^{1}\)
Rectifier\(^{1}\)
Tunnel diode\(^{1}\)
Varactor diode\(^{1}\)	or
Zener diode\(^{1}\)

Table \(\PageIndex{1}\): IEEE standard symbols for diodes and a rectifier [4]. (\(^{1}\)In the direction of anode (A) to cathode (K). \(^{2}\)Use symbol for general diode unless it is essential to show the intrinsic region.)

where \(\phi\) is the built-in potential difference across the diode. This capacitance profile is shown in Figure 11.5.5(b). The built-in potential is typically \(0.6\text{ V}\) for silicon diodes and \(0.75\text{ V}\) for GaAs diodes. The doping profile can be adjusted so that \(\gamma\) can be less than the ideal \(\frac{1}{2}\) of an abrupt junction diode. Current must flow through bulk semiconductor before reaching the active region of the semiconductor diode, and so there will be a resistive voltage drop. Combining effects leads to the equivalent circuit of a pn junction or Schottky diode, shown in Figure 11.5.5(c).

Varactor Diode: A varactor diode is a pn junction diode operated in reverse bias and optimized for good performance as a tunable capacitor. A common application of a varactor diode is as the tunable element in a voltage-controlled oscillator (VCO) where the varactor, with voltage-dependent capacitance, \(C\), is part of a resonant circuit.

PIN Diode: A PIN diode is a variation on a pn junction diode with a region of intrinsic semiconductor (the I in PIN) between the p-type and n-type semiconductor regions. The properties of the PIN diode depend on whether there are carriers in the intrinsic region. The PIN diode has the current-voltage characteristics of a pn junction diode at low frequencies; however, at high frequencies it looks like a linear resistor, as carriers in the intrinsic region move slowly. When a forward DC voltage is applied to the PIN diode, the intrinsic region floods with carriers, and at microwave frequencies the PIN diode is then modeled as a low-value resistor. At high frequencies there is not enough time to remove the carriers in the intrinsic region, so even if the total voltage (DC plus RF) across the PIN diode is negative, there are carriers in the intrinsic region throughout the RF cycle. If the DC voltage is negative, carriers are removed from the intrinsic region and the diode looks like a large-value resistor at RF. The PIN diode is used as a microwave switch controlled by a DC voltage.

Zener Diode: Zener diodes are pn junction or Schottky diodes that have been specially designed to have sharp reverse breakdown characteristics. They can be used to establish a voltage reference or, used as a limiter diode, to provide protection of more sensitive circuitry. As a limiter, they are found in communication devices in a back-to-back configuration to limit the voltages that can be applied to sensitive RF circuitry.