The Basic Structures, Electronic Symbols, Working and Characteristic of Power Semiconductor Devices

1. Power Diode
2. Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)
3. Bipolar -Junction Transistor (BJT)
4. Insulated-Gate Bipolar Transistor (IGBT)
5. Thyristors (SCR, GTO, MCT)

Solid-state devices are completely made from a solid material and their flow of charges is confined within this solid material. This name “solid state” is often used to show a difference with the earlier technologies of vacuum and gas-discharge tube devices; and  also to exclude the conventional electro-mechanical devices (relays, switches, hard drives and other devices with moving parts).

The transistor by Bell Labs in 1947 was the first solid-state device to come into commercial use later in the 1960s. In this article, similar solid-state devices such as power diode, power transistor, MOSFET, thyristor and its two-transistor model, triac, gate turn-off thyristor (GTO), insulated-gate bipolar transistor (IGBT) and their characteristics (such as i-v characteristics and turn-off characteristics) is also presented. In power electronics circuitry, these switches act in saturation region and work in linear region in the analog circuitry such as in power amplifiers and linear regulators. This makes these switches highly efficient since there are lesser losses during the power processing.

1. Power Diode

I. Basic Structure

II. Symbol of Power Diode 

III. Working of Power Diode

Power diode is two terminal semiconductor device, with two terminals anode (A) and cathode (C).

If anode (A) terminal having higher potential than cathode (K) terminal , the power diode is forward biased and forward current flows from anode (A) to cathode (K). Voltage drop across diode during forward biased is 0.7 to 1V. If cathode (K) terminal having higher potential than anode(A) terminal, the power diode is reverse biased and device will not conduct and very small current flows through the device is called leakage current.

IV. VI Characteristics of Power Diode 

  

V. Reverse Recovery Characteristics of Power Diode

After the forward diode current If=0, the diode continues to conduct in the opposite direction because of the presence of stored charges in the depletion layer and the p or n-layer. The diode current flows for a reverse-recovery time t_rr. It is the time between the instant forward diode current becomes zero and the instant reverse-recovery current decays to 25 % of its reverse maximum value.

Time T_a:  Charges stored in the depletion layer removed.

Time T_b:  Charges from the semiconductor layer is removed.

Shaded area in Fig (a), represents stored charges Q_R which must be removed during reverse-recovery time t_rr.

Power loss across diode = V_f * I_f 

As shown, major power loss in the diode occurs during the period t_b. Recovery can be abrupt or smooth as shown in figure. To know it quantitatively, we can use the S – factor.

Softness factor or S-factor = T_b / T_a

S-factor is the measure of the voltage transient that occurs during the time the diode recovers.  

S-factor = 1 ⇒ low oscillatory reverse-recovery process. (Soft –recovery diode)

S-factor <1 ⇒ large oscillatory over voltage (snappy-recovery diode or fast-recovery diode).

Power diodes now exist with forward current rating of 1A to several thousand amperes with reverse-recovery voltage ratings of 50V to 5000V or more.