What is Static Resistance?

What is Static Resistance?

When connected to a DC supply, the resistance of a device or element is referred to as static resistance. Resistance is given as the ratio of voltage to current. So under DC supply, the resistance of any device would be the voltage drop across it and the current flowing through it. For a device with ‘V voltage across it and I current flowing through it, the resistance is given as-

Resistance (R) = Voltage (V) / Current (I)

V is the voltage across the device, and I is the current through it.

Diode and its VI Characteristics

A diode is a two-terminal semiconductor device that allows current to flow in only one direction. It is a nonlinear device commonly used as a switch due to its switching characteristics. An ideal diode is expected to offer zero resistance when conducting and infinite resistance when in a non-conducting state; however, non-idealities exist in everything.

Static Resistance PDF

These non-idealities offer a small (non-zero) resistance in their conducting state and very high (non-infinite) resistance when in a non-conducting state. The diode offers a static resistance when connected in forward and reverse bias with a DC supply, and the resistance offered with an AC supply is called dynamic resistance.

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VI Characteristics of Diode

The equation gives the VI characteristics of a diode-

I=I₀ exp(V_D/η V_T)

Where

• I₀ is reverse saturation current.
• V_D is the potential across the diode and is given as the sum of applied potential and the depletion region potential and is given as V_f -V_D.
• η is the ideality factor of the semiconductor material.
• V_T =0.259 m-volts is the thermal voltage and is given by the einsteins equation equal to kT/q, where k is Boltzmann's constant (k=1.23 *10^-23 J/K).

The below figure shows the graphical characteristics of a practical diode-

The above figure shows the characteristics of a practical diode. However, there are a few conclusions and points that need to be noted:

• Due to the presence of a depletion region, the diode has an initial threshold voltage that needs to be overcome before a diode can start conducting. V_T represents this threshold voltage.
• In forward bias V_F, on the right side of the vertical axis, the diode offers some resistance and has a nonlinear curve with a small portion of the linear graph.
• In reverse bias V_R on the left of the vertical axis, the diode should ideally be non-conducting, but it allows a small current to flow. If this reverse bias is increased beyond a certain value, the diode goes under breakdown, giving rise to a huge reverse current. 

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Resistance in a Diode

A diode offers a low resistance in forwarding bias and high resistance in reverse bias. Depending on the supply, these resistances are classified as Static and Dynamic resistance. The classification can be seen in below flow chart-

Forward Resistance

Diodes will not conduct after forward biasing until they reach a minimum threshold voltage level. Once the applied voltage exceeds this threshold level, the diode will conduct. This resistance is referred to as the forward resistance of the diode.

Reverse Resistance

During reverse biasing, a small current will flow through the diode. This can be attributed to the fact that when the diode functions in reverse mode, it is not completely free of charge carriers.

A diode can exhibit reverse resistance characteristics even in this state due to the flow of minority carriers through it.

Static Resistance in Forward Bias

A diode’s static resistance can be determined when connected to a DC source. For instance, suppose the circuit that includes the diode has an external DC voltage. As a result, the Q-[point or operating point of the PN junction diode characteristic curve does not change over time.

Static Resistance in Forward Bias Formula

The mathematical expression for the same is given by-

R_f =V_f/ I_f

V_f and I_f are the forward voltage and the forward current, respectively.

Static Resistance in Reverse Bias

When the diode is reverse biased with a DC supply, the resistance offered is referred to as the reverse static resistance.

Static Resistance in Reverse Bias Formula

The mathematical expression for the same is similar to that for the forward resistance and is given by

R_r = V_r / I_r

V_r and I_r are the reverse voltage and the reverse current, respectively.