# 电气工程代写|数字电路代写digital circuit代考|EECS151

## 电气工程代写|数字电路代写digital circuit代考|TUNNEL DIODE

The tunnel diode (also called the Esaki diode after the L. Esaki who announced the new diode in 1958) voltage-current characteristic is shown in Figure 2.39. The figure shows that the tunnel diode is an excellent conductor in the reverse direction. Figure $2.40$ is the circuit symbol for the tunnel diode.

For small forward voltages (in the order of $50 \mathrm{mV}$ in Ge), the resistance is in the order of $5 \Omega$. At the peak current $I_P$ corresponding to the voltage $V_P$, the slope of the characteristic curve is zero. As the voltage increases beyond $V_P$, the current also decreases. The tunnel diode characteristic curve in this region exhibits a negative dynamic resistance between the peak current $I_P$ and the minimum or valley current $I_P$. At the valley voltage, $V_V$, corresponding to the valley current $I_V$, the slope of the characteristic curve is again zero. Beyond $V_V$, the curve remains positive. At the peak forward voltage, $V_F$, the current again reaches the value of $I_P$.

Since it is difficult to manufacture silicon tunnel diodes with a high ratio of peak-to-valley current $I_P / I_V$, most commercially available tunnel diodes are made from germanium (Ge) or gallium arsenide (GaAs). Table $2.2$ is a summary of some of the static characteristics of these devices.

The operating characteristics of the tunnel diode are highly dependent on the load line of the circuit in which the diode is operating. Some load lines may intersect the tunnel diode characteristic curve in three places: the region between 0 and $V_P$, between $V_P$ and $V_V$, and beyond $V_V$. This multi-valued feature makes the tunnel diode useful in high speed pulse circuit design. High frequency (microwave) oscillators are often designed so that the tunnel diode is biased in its negative dynamic resistance region.

## 电气工程代写|数字电路代写digital circuit代考|SCHOTTKY BARRIER DIODE

The Schottky barrier diode (or simply Schottky diode) is a metal-semiconductor diode. The circuit symbol of the Schottky diode is shown in Figure 2.41. Metal-semiconductor diodes are formed by bonding a metal (usually aluminum or platinum) to $n$-or $p$-type silicon. Metal-semiconductor diode voltage-current characteristics are very similar to conventional $p-n$ junction diodes and can be described by the diode equation with the exception that the threshold voltage $V_\gamma$ is in the range from $0.3 \mathrm{~V}$ to $0.6 \mathrm{~V}$. The physical mechanisms of operation of the conventional $p-n$ junction diode and the metal-semiconductor diode are not the same.

The primary difference between metal-semiconductor and $p-n$ junction diodes is in the charge storage mechanism. In the Schottky diode, the current through the diode is the result of the drift of majority carriers. The Schottky diode switching time from forward to reverse bias is very short compared to the $p-n$ junction diode.

Therefore, Schottky diodes are often used in integrated circuits for high speed switching applications. The Schottky diode is casy to fabricatc on integrated circuits because of its construc tion. The low noise characteristics of the Schottky diode is ideal for the detection of low-level signals like those encountered in radio frequency electronics and radar detection applications.The photodiode converts optical energy to electric current. The circuit symbol of the photodiode is shown in Figure 2.42.

In order to make this energy conversion, the photodiode is reverse biased. Intensifying the light on the photodiode induces hole-electron pairs that increase the magnitude of the diode reverse saturation current. The useful output of the photodiode its photocurrent which, for all practical purposes, is proportional to the light intensity (in Watts) on the device. The proportionality constant is called the Responsivity, $R$, which is usually given in amperes per watt and is dependent on the wavelength of the light. Figure $2.43$ shows a photodiode characteristic curve.

If the intensity of the light on the photodiode is constant, the photodiode can be modeled as a constant current source so long as the voltage does not exceed the avalanche voltage. Naturally, the photocurrent will vary with varying input light intensity. Since the photocurrent can be very small, an electronic amplifier is used in many applications to both boost the signal level and to convert from a current to a voltage output. For example, in optical fiber communication receivers, the average intensity of a time varying infrared light on the photodiode can be significantly less than $1 \mu \mathrm{W}$. Taking a typical photodiode responsivity for fiber optic application of $0.7 \mathrm{~A} / \mathrm{W}, 1 \mu \mathrm{W}$ of light will produce $0.7 \mu \mathrm{A}$ of average current. This low level signal must be amplified by electronic amplifiers for processing by other electronic circuits to retrieve the transmitted information.

# 数字电路代考

## 电气工程代写|数字电路代写digital circuit代考|SCHOTTKY BARRIER DIODE

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