What is a Diode? Characteristics and Operation

Diode

Diode is a semi conductor device that produces no current when operated in reversed biased mode and zero voltage drops when operaed in forward biased mode. Thus, a forward biased diode acts as a short circuit and reversed biased diode acts as an open circuit.

Terminal Characteristics of Junction Diodes

From Shcokley's diode equation, we have,

`I_D=I_S\left(e^{V_D/ηV_T}-1\ \right)......\left(i\right)`

where,

`V_T=\frac{KT}{q}`

where,

K=Boltzmann's constatnt

T=absolute temperature in Kelvin

q=magnitude of electronic charge

Forward Biased mode (with Vd>0)

we have, `I_D=I_S\left(e^{V_D/ηV_T}-1\ \right)......\left(i\right)`

or, `I_{D1}=I_{S\ }e^{V_{D1}/ηV_T}`

Also, `I_{D2}=I_{S\ }e^{V_{D2}/ηV_T}`

Dividing above both equation

`\frac{I_{D2}}{I_{D1}}=\ e^{\frac{V_{D2}-V_{D1}}{ηV_T}}`

`V_{D2}-V_{D1}=\ ηV_T\ln \frac{I_{D2}}{I_{D1}}`

`V_{D2}-V_{D1}=\ 2.3ηV_T\log \frac{I_{D2}}{I_{D1}}`

Physical Operation of Diode

No Biasing applied ( Vd=0 V)

In absence of an applied bias, the net flow of electron or holes (charges) in one direction is zero. Hence, no current flows if no biasing voltage is applied. 

Forward Bias Region

A forward bias condition is established by applying the positive potential to the p-type material and the negative potential to n-type material as shown in figure.

A glance at the I-V characteristics in the forward region reveals that the current is negligibly small for Vd smaller than about 0.5 V. This value is usually referred as the cut-in voltage. But for a "fully conducting diode" the voltage drop lies in a narrow range, approximately 0.6 V to 0.8 V

Reverse Bias Region

The reverse bias region of operation is entered when the diode voltage Vd is negative.

We have,

`I_D=I_S\left(e^{V_D/ηV_T}-1\ \right)......\left(i\right)`

In equation (i) if Vd is negative and a few times larger than Vt (25mV) in magnitude, the exponential term becomes negligbly small compared to unity, and the diode currents becomes 

`I_D=I_S`

Breakdown Region

The breakdown region can be easily identified on the diode I-V characteristics. The breakdown region is entered when the magnitude of the reverse voltage exceeds a threshold voltage that is specific to the particular diode, is called the "breakdown voltage". This voltage is at the"knee" of the I-V curve and is denoted Vzk.